June 21, 2018: Follow my progress as I build the first-ever net-zero home in Spring Green, Wisconsin and I’ll keep you up-to-date and send you Open House invites!
Saturday February 15 from 10 am to 5 pm: Stop by today and see what we’ve done! Drywall is up and mudding the seams is “in process”. Over the next few months, finishes—from paint to cabinets to counters—will be installed, all vetted for their ultra-low or non-toxic properties. Ask me anything you want about the construction so far—like how to source healthy materials, what makes it super-insulated, what makes it low-carbon footprint, and what makes it zero net-energy.
A beautiful, sparkly snow fell on Sunday. It shut down solar production for the day and the next and the next. That’s the downside of a roof-mount install. A ground-mount (on a rack) could be swept clean. My low pitch roof (3:12) doesn’t help. Snow would slide off a steeper roof faster.
How do I know when the PV (photovoltaic) system is working? The easiest way is to look at the meter on the side of the garage. If the arrow is pointing toward the street, I’m sending kWh’s to the grid, and getting paid. If the arrow is pointing toward the house, I’m buying from the utility. Of course, it constantly varies depending on sun and clouds and whatever equipment I might be running. I’m paid 3 cents/kWh for my production. When I buy, it’s at 12 cents/kWh.
There’s also an app for that. I can look at my phone to find out exactly how much the panels are producing, hour by hour. Last week a clear day with no snow netted me 31 kWh of electricity. That’s equivalent to running an efficient refrigerator for 19 days. It’s also equivalent to the greenhouse gas emissions of driving a car 53 miles.
December 16-January 16: Nothing is more motivating on a building project than knowing you have put 100% into a task, that it is done, and you are now free move on to something new. As if it were Christmas, I tore open 4 boxes of plastic MemBrain vapor retarder and began hanging it like a curtain on my walls. Of course, it quickly became another labor of love and laborious. Each electrical wall outlet and switch, each wire and pipe, each window opening had to be carefully lapped around and caulked before I could confidently call in the drywallers and the insulators.
I explained the necessity and benefits in my previous posts “The Whys & Hows of Vapor Retarders” and “Hanging the Lid”, written after I’d draped and drywalled the ceiling.
Just as before, the vapor retarder hung long and the drywall stopped short at the edges. Running along the ceiling perimeter and under each window was a 6” gap—just enough room to stick in the insulation hose.
My crew from Accurate-Airtight Exteriors arrived with a box truck fitted with a hopper and assortment of hoses, and a trailer full of bales. They were eager to see how my house was coming along. Last time they were here was in June, when it took them 3 days to pack the ceiling. This trip, they’d need 4 to pack the walls. A lot of their work is retrofit and crawling around in attics and basements. My house was going to be a nice break in the routine, easy—even a little boring.
My walls are 11-3/4” thick. The north wall is 13′-6” tall and 56′ long, with few windows. The bales were slit open in the garage and slowly fed into the hopper. The hoses were draped through a window and down through each frame cavity. Cellulose—non-toxic, biodegradable, and 100% a byproduct of the paper mills up in the Fox Valley—trickled out. It was going to be a slog. Wallace Kennedy and son Nicholas took it in stride, and were a delight to have around.
There are many ways to build a super-insulated wall. My goal was “double Code minimum”—about R-38 to R-45. Here are the basic options I considered:
FRAME WALL WITH EXTERIOR INSULATION
THICK FRAME WALL
All are interesting and have their advantages. To decide, I turned to my most trusted source for practical advice and solid building science, Green Building Adviser. What method—for my situation—would be the most build-able, affordable, least toxic, and have the lowest carbon footprint? I settled on a 2×4 double-stud wall with dense pack (not loose fill) cellulose, for an R-45. I’d be happy to talk with you in more depth about the options, pro & con.
The house was, of course, noticeably warmer by the time my crew left. On sunny days, I can turn off the heat and coast on passive solar energy. Next step: mudding & taping the drywall.
December 10, 2019: Finally, it was time to find out how well I’d done. In the last few days, the electrician had made a lot of holes. Had I followed up with each one? Did they all get hit with caulk or tape?The energy rater who is shepherding me through the process of getting the house certified through Wisconsin’s Focus on Energy’s New Home Program was arriving with his blower door kit. This was going to be my second of three tests—and the most important. The drywallers were due in a week, and this was my last chance to make things right. I really, really wanted a good number. My goal was a “1”.
An airtight building shell is essential for the kind of house I’m building, and a blower door test is the only way to know for sure you have it. Jim Kjorlie of Kjorlie Design Services has tested hundreds of homes in the Madison area. The best result he’s seen is 1.2 ACH (air changes per hour)—literally how many times the volume of air in the home is changed out with fresh air from leaks in the building shell.
Wisconsin’s Uniform Dwelling Code allows a new home to be as leaky as 7.0 ACH, though studies have shown that the average new home here tests out at 4.0 ACH. Some neighboring states mandate 3.0 ACH. Some really meticulous builders are hitting 0.5 ACH or less. Is all that caulking and taping worth the extra cost? I’d argue that a well-sealed home insures against more than high utility bills: it insures against moisture, mold, mildew, and insect problems. Even so, out of curiosity I ran the numbers on my house through a modeling program called REM Design:
Poem Home @4.0 ACH = $908/year heating costs @0.12/kWh
Poem Home @0.6 ACH = $630/year heating costs @0.12/kWh
YEARLY SAVINGS = $278
Jim set up his equipment by propping open the front door and replacing it with a fan attached to an adjustable shroud. The fan slowly depressurizes the house to simulate a 20 mph wind bearing down on all sides. Drafts that normally aren’t felt are suddenly amplified. My friends Eric and Lew stopped by to see how it’s done, and thanks to their sleuthing, we found two major leaks.
Cold air from the mini-split was pouring out the narrow gap between the unit and the drywall. We later found out that the installer hadn’t sealed or insulated the line set connecting the outdoor unit to the indoor unit. I can stuff this with fiberglass.
The second leak was at the fixed panel of the patio door. A steady draft was easily detected by waving a hand over the crack between the frame and the sash. I can caulk this.
So how did I do? We got a 1.0. Yeah! “Don’t celebrate yet, Amber” is what Jim didn’t say but implied. He’ll be back for a final test when the house is complete.
October 14 to January 7: Now we punch holes. After all the careful air-sealing, after all the work and worry with caulk guns and rolls of tape and spray-foam—it was time to make intentional holes, lots of them.
ELECTRICAL My electrician got started by extending the service from the panel in the garage to the far corners of the house. I got to help, drilling holes and running wire. It was all new to me, and interesting. Bob Rowen of Rowen Electric insisted on straight runs, artful bends, and minimal slack.
The installation required a few extra steps and probably more wire compared to a conventional home, but in several ways is more environmentally friendly. Most critical is the extra attention paid to air-sealing. Each line coming in and going out is sealed with flexible silicon caulk or flexible tape. Electrical boxes have integral gaskets that compress when drywall is attached. And the vapor retarder was taped to the gasket, providing both a continuous air-seal and vapor-seal.
In other posts, I’ve extolled the virtues of a slab-on-grade home, but here I have to admit that it made running mechanicals harder. In a conventional home, you run freely through wall and ceiling cavities but on this job site they are “Sacred Space” reserved for insulation. You also run through the attic (none here), the basement (none here), and the floor cavities (none here). Instead, I built a chase on top of the loft floor, tucked along the back wall and out of sight from the living room below.
The electrical runs are attached to the loft flooring, covered by a plywood lid that can be flipped up for service at any time in the future. The plumbing pipes are above. Future plans call for a cushioned bench built over top and running the length.
LIGHTING Everything is LED (light emitting diode)—-no incandescent, halogen, or CFL (compact fluorescent lights). LED’s use about 10% of the electricity of incandescents and about half that of CFL’s. LED’s don’t contain the mercury that CFL’s do—and supposedly no hazardous chemicals—but do contain small amounts of heavy metals that can be recycled. I have yet to find a recycler. Do you know one?
LED’s are not only cheap to use, they are cheap to buy. The first bulb I ever saw—just a few years ago–was proudly displayed by a friend who dropped something like $50 on it. For the garage, I chose simple porcelain sockets fitted with 25 cent LED bulbs that are expected to last 22 years, at a cost of $1.20/year. Thank goodness for those “early adopters” who purchase by purchase spur technology along—-but I’m not one of them.Outside, I chose “dark sky friendly” light fixtures. Any fixture that casts light downward instead of upward or outward is good. To be sure, you can look for a compliance label when shopping. For everything you need to know, check out International Dark Sky Association.
A personal pet peeve and (as I learn more), harmful to human and animal health are LED bulbs that cast an eerie, unnatural brightness. It’s the same “blue light” our phone screens give off that we know disrupt our circadian rhythms. When used in outdoor lighting, they brighten the night sky more than other light sources do and effect the survival strategies of nocturnal animals.It doesn’t have to be this way. Look for “warm white” bulbs in the 2700-3000 Kelvin range and a CRI (color rendering index) of at least 90. Notice the difference between my house and my nearest neighbor.
HEATING & COOLING Building an all-electric, fossil-fuel free home in our northern climate would have been a laughable proposition just a few years ago, but is now possible with the latest “mini-splits”. You’ve probably seen these devices installed in a room addition, office, or small apartment. The indoor part looks like an oversized through-the-wall air conditioner, and the outdoor part looks like a rectangular version of an air-conditioning compressor. The technical name is Air Source Heat Pump (ASHP). I chose the simplest version: single-zone and ductless.
These super-efficient appliances are also super-affordable. Where a conventional house might spend upwards of $20,000 on a heating and cooling system, mine came in under $5000. Of course, this is possible because my house is wearing a thick down parka on top of a wool sweater and a turtleneck—not a sporty spring jacket.
A conventional furnace is overkill for a small, super-insulated house. Ductwork is cumbersome, expensive, often poorly sized, and can harbor dust and allergens. With an open floor plan, and for families who can tolerate some temperature differences from room to room— a single mini-split can work. Daily modifications may be needed, like closing and opening doors for better circulation and closing and opening curtains for better temperature control.The Mitsubishi unit went in mid-October. By mid-November outdoor temperatures were dropping and snow was flying. Reluctantly, I turned it on. The ceiling was insulated, but not the walls. We were still wiring and willing to work chilly, but the water was on and the plumbers were coming. As it turned out, 2 months would go by before wall insulation went in. I dreaded looking at my electric bill. Good news: those two months of heat were “paid for” by my solar production in July.
More good news: since I broke ground and tapped into the grid (November 2018) until now, I’ve produced more electricity that I’ve consumed. So far, my construction project is “energy net positive” by 2782 kWh. That’s a “carbon dioxide equivalent” of the carbon sequestered by 2.6 acres of forest/year OR the greenhouse gas emissions from 4881 miles of driving.
VENTILATION People ask me, “Isn’t it true you can build a house too tight?” In a word, no. A conventional house gets its fresh air through cracks in the windows and electrical boxes and random places. That “fresh air” is pulled through dusty crevices and frame cavities and in most homes, formaldehyde-laced fiberglass batts. Along the way, it’s depositing moisture—creating a breeding ground for mold and mildew.
A super air-tight house shuts down random leaks and instead relies on a mechanical system that brings in filtered fresh air. I looked for a system that could dial up or dial down depending atmospheric conditions and how many people and plants and cooks I have over.
Condensation on window panes is the bane of Northern Homeowners and a sign of too much moisture. Stuffiness is unpleasant and a reminder that “indoor air pollution” really is a thing. Damp, stagnant air is more likely to harbor and spread disease. Effective ventilation that dries and dilutes indoor air can limit bacteria, viruses, dust mites, and mold—and prevent allergies and asthma according to the American Lung Association.
Finding the right appliance turned out to be the biggest headache on my “build a house or bust to-do list”. The answer is what’s called an HRV or ERV (heat-recovery or energy-recovery ventilator). Most HRV’s on the market are sized for larger homes or commercial spaces and require complicated ductwork and expert commissioning. They’re not yet common. Contractors I contacted seemed unfamiliar with them or unable to answer my questions. I poured over manufacturers specs and called distributors who could point to a specific unit but not installation details. I finally settled an innovative, ductless, through-the-wall product called Lunos from 475.com My electrician gave his tacit assent.
The units look simple and unobtrusive, were easy to install, and have the best heat recovery efficiency and noise ratings I found. Working in synchrony, each Lunos (1 in each bedroom, 1 in the living room, 1 in the loft) push stale indoor air out through a ceramic core, heating it up. After 70 seconds, the fans reverse direction. Fresh air is pulled through the ceramic core—and a filter—to deliver a stream of clean, warm air to the room.
To meet the high CFM ratings required by Code for year-round bathroom ventilation, we installed ultra-quiet Panasonic fans above each shower.
The Code is silent on kitchen range hood ventilation. I plan to install a high quality ductless hood with carbon filter. For the laundry, I plan to install a ductless condensing dryer. So altogether, we avoided 2 holes (range hood, dryer) but punched 8 big holes (4 Lunos, 2 bath fans, 1 plumbing vent, 1 line set sleeve for the ASHP), and dozens of small holes through the thermal, vapor, and air barrier that is the wall. There are none through the roof.
Is it really possible to survive winter without burning fossil fuels? I’ll keep you posted on how everything mechanical goes once the house is up and running and occupied. If you have any questions or can recommend something, please let me know!
September 30-October 4: Gutters are a maintenance headache and generally, pretty ugly. I decided to try “in-ground gutters”.
My excavation crew laid down a 12″ deep layer of washed river stone in a landscape-fabric-lined-trench under the eaves. The trench prevents erosion at grade and splash-back on the siding. Rainwater flows freely through the gaps in the stone, seeps through the fabric, and drains away into the native sand below. The fabric prevents the underlying sand from working its way up through the river stone and clogging the works.
If I had a basement to keep dry and/or heavy clay soils, I’d need a more robust solution. It would include plastic draintile (slots up) laid within the river stone and run downslope to daylight. This strategy would serve the same function as downspouts with extenders—to get the water away, fast.
Ringing a house with stone like this is fairly common, and usually done to make mowing easy. Most installations include black plastic bulb-style landscape edging but I decided to take my chances without. It seemed like another maintenance issue (they have a tendency to pop up) and I visualized a “naturalized” edge where stone and grass meet. So far, I’m pleased with the look but worried about future weeds until a friend gave me a tip: “hit ’em with a blower torch”.
August 5-October 4: The loft is the most unique feature of my design and so far, the most fun to build. I ordered a 4×8 Douglas Fir beam from the lumber yard, and it arrived in pristine condition—28 feet of straight, clean, perfectly milled wood. I called in Mike & Nino, the two strong guys from Wood & Stone to set it in pockets they had framed out a few weeks earlier.
While sturdy on its own, the beam would need support to carry the weight of the loft. My design called for threaded rods, similar to what you’d find around here supporting the hay mow of a “hung barn”. Just like a farmer who wants a wide open milking parlor, I wanted a large living room free from posts. I turned to my friend Bob Rowen, a master electrician who also happens to be good at solving any sort of mechanical problem. He tinkered with wood scraps and presented me with a mock up that we then took to a local machinist to weld up from steel rods and plates. Thinking ahead, Bob built two wood boxes reinforced with steel channel and installed them between pairs of trusses. At the design stage, I had coordinated with the truss manufacturer to factor in the point load on these trusses, which came with an engineered pattern of webbing & steel plating, along with an extra-wide 2×6 bottom chord.
My worries about drilling the holes and having them come out straight were found-less—Bob devises a jig for everything he does and has a clear road map in his head for each step to be taken. The beam was hoisted up and down several times with block & tackle as we tested fit.
When the last bolt was turned tight, I masked the area with paper and tape and spray painted the steel matte black (note: the spray paint came straight from the hardware store, not vetted for low VOC or other chemical emissions). We think the rods turned out pretty handsome, with their extra-wide bottom plates and over-scaled double nuts.
Road trips to Timbergreen Farm became our pleasant distraction over the next week. Just a few miles up the road, consulting forester Jim Birkemeir runs a milling operation and solar kiln. His stash of lumber from the Driftless includes white oak, red oak, black oak, hickory, black walnut, ash, cherry, and other hardwoods but I had my heart set on pine. Humble, easy-to-work-with, aromatic pine. He had just the thing: rough planks harvested from a stately White that had stood strong in the Village for a hundred years. He agreed to mill it for me.
Our first haul included 2×8’s for ledgers and 2×6’s for floor joists. We sorted for warp, wane, and knots. We accepted some scant thickness or width and agreed that skips (saw marks) added character. Pieces that had pronounced staining or dark streaks were pronounced “uglies” and went in the mechanical room. Working with minimally processed wood takes a different mindset. You have to love the fresh smell and slight stickiness, the lack of conformity, and the limits of band saws. The payoff is rustic charm and knowing that this is about as low-carbon footprint for a building material as you can get.
People are surprised to learn that you can build with “unstamped” wood—wood that doesn’t come from a lumberyard and doesn’t carry a grade stamp. The Wisconsin Uniform Dwelling Code allows it, but downgrades it to #3 (most framing lumber is #2). My joist spans are short (under 10′), and the design load is low (“attic with storage”—not “habitable space”).
Just as we did for interior walls, we installed vapor retarder and drywall before attaching the 2×8 ledgers in place. We lag-screwed the ledgers to the inner 2×4 wall and set the 2×6 floor joists temporarily with scraps of plywood.
The original plan was to support the floor joists on a 2×2 nailer but a mock-up looked clunky. I also considered decorative joist hangers, but they looked busy. Bob had a better idea. Why not use an angle iron? He ordered it up from a local shop (raw steel for the kitchen and stainless for the bathrooms) and we spent several days laying out a pattern of holes and patiently drilling them out. I helped, then set up an ad hoc assembly line to spray paint the dozens of washers and bolts we’d need to attach the angle iron to the ledger and the joists to the angle iron.
Finally, each joist was top screwed into the ledger, blocking, or the beam via a concealed pocket hole.
Our next foray to Timbergreen was to pick up full 1” thick pine flooring, milled from the same tree. Jim had neat piles ready for us which we sorted for width and quality. As before, the best stuff went to the kitchen. Never mind the discoloration from where straps held bundles together in the kiln and unevenness from thickness differences and skips. My job was a hands & knees operation up in the loft—running the air nailer—while Bob manned the saw below.
The loft is a storage loft. By Code, it’s an “attic”. It doesn’t qualify as habitable space, because the ceiling is less than 7′ high. The advantage is that I don’t need to install a guardrail, and I don’t need a proper stair to it. In lieu of a basement, I at least have some space for junk but am still forced to downsize. The disadvantage is that it’s not really convenient, and you can’t really stand up.
The loft is also a mechanical chase way. The electrical runs are now in place along the back wall and plumbing will come next week. Tucked along the back wall, the chase isn’t visible from the living room.
The idea of the loft took shape in the design phase as I settled on a shed roof (facing south) for the PV solar panels. But even if I kept the slope as low as possible—3:12 for a metal roof is pretty much the limit—I’d still have a lot of space above the bathrooms. Frank Lloyd Wright did it, but I didn’t want a 13′-6” high ceiling in my bathroom. I like vaulted space but I also like cozy. Having a wood trellis-like or pergola-like structure overhead was appealing, and having a low entryway give way to a grand space as you turned the corner into the living room seemed like a better way to channel FLLW.
The raw and rustic nature of the loft will animate the more contemporary forms and materials I’ll use elsewhere in the house. It will bring my affinity to nature indoors, and restore my spirit especially over the long and drab Wisconsin winters. Research shows that nature-connected design makes people healthier. It improves our emotional state and reduces our blood pressure, heart rate, and stress level. It increases social interactions and creativity. But architect Frank Lloyd Wright said it better:
“Wood is the most humanly intimate of all materials, and the most kindly to man.”
July 18-October 10: People seem to like big, open, vaulted spaces and many a visitor has “oohhed” and “aahhed” over the barn-like quality of my Poem Home. I almost hated to call in the crew to build out the interior rooms, but it was finally time. The first step was to install the missing strip of drywall at the ceiling. That’s the 12” gap down the length of the building where the insulation crew accessed the ceiling cavity just a few weeks earlier. Once sealed up, top plates were screwed through the drywall into blocking between the roof trusses and studs were dropped down to bottom plates anchored in the slab. At exterior walls, the framing was held 3/4” shy and the last stud left loose, so I could slip in drywall later.
This isn’t the usual sequence. Conventional practice has all the framing done before drywall shows up. But for a super-airtight house like mine, you take extra steps to shut down air movement. In the same way that you want your down parka to cinch up at the wrists and waist when you venture out in sub-zero weather, you want your exterior walls to block drafts—-whether those drafts come from outside or from the volume of air inside the building. Insulation works best in a dead-air cavity—-in my case 12” of it sandwiched between plywood and drywall.
To maximize space for insulation, and to save on the cost of wood, the interior walls abut “ladder blocking” instead of doubled-up or tripled-up studs. The crew from Wood & Stone got the interior walls up in just 2 days. Fitting sections of vapor retarder and drywall between the ladder blocking and the new walls was my job, and that took longer.
Here’s are plan-view sketches that explain why I took this extra step. In conventional construction, interior walls are attached directly to the exterior frame. The first sketch shows that when built this way, air can pass from an opening in an interior wall (like an electrical outlet) into the exterior wall. Even when packed with insulation, air can move through and carry with it warmth and moisture—wasting energy and risking condensation within the colder outside wall. In the second sketch, a continuous vapor retarder and drywall is in place before the interior wall is permanently attached, blocking air movement.
In this house, the drywall serves an additional function. Just as I did for the ceiling, I need to have drywall in place before the insulation crew shows up next month to blow the walls. Conventional batt insulation (like pink fiberglass), can simply be placed in an open cavity but blown-in insulation like cellulose needs an enclosed cavity. One way to do that is to staple a fabric across the wall studs. But my crew asked for drywall, promising it would make for a better job and save money. I’ll need to leave them a 6” gap at the top of each wall and at the bottom of each window so they can access each bay with their tubes.
For the plumber, I built a half wall inboard of a section of drywall at the laundry so his pipes don’t use up space in the exterior wall better left for insulation. This also eliminates any risk of freezing and makes the plumbing accessible for future repairs or replacement.
Next, I installed 3/4” plywood blocking between studs wherever grab bars might be needed in the bathrooms. The blocking runs horizontally behind the toilet and continuously through the walk-in shower. Plywood also covers the 6 foot high shower wall on both sides, for vertical and/or horizontal applications.
It was Bob Rowen’s idea to secure the shower wall with a section of square stock. He sketched out a 5 foot long bar with metal plates and had a local shop weld it up out of stainless steel. It’s bolted through the end studs and to the loft floor above with T-nuts.
While the walls went up fast, the details took time. Fitted out, the rooms have each taken on their own shape, and they feel right. The size and proportion of the bedrooms is pleasing, and the bathrooms work. The “great room” stills feels big, and with the loft in place (see next post)—still feels like a barn. And that’s a good thing.
Sunday November 24th from 10 am to 5 pm: Turns out, we got slammed by freezing temps early this year. But not before this stunning rock retaining wall got in the ground, thanks to my friend Lew Lama and his crew at Wood & Stone. Come see it and explore it in all its sublime charm. It all started when a dump truck off-loaded a few boulders from a farm in Ridgeway. Then, smaller stones of mixed provenance snagged on the cheap as overage from Lew’s other projects. These were sorted & stacked as bottom layer, middle layer and cap layer. Loads of crushed limestone, washed river stone, and assorted fill material stood at the ready.
The wall rose steadily, battered back and keyed together, to reach the string line set to house grade. Mike & Nino set the biggest, baddest bolder at the Southeast corner.
When filled with topsoil, I’ll have a level vegetable garden right outside the door. Please join me on Sunday to talk nice about local materials, craftsmanship, and energy-efficient building.
Saturday October 19 from 10 am to 5 pm: Please join me at the building site to talk about how we can transform our homes from energy-wasters to energy-producers. See how our first model home plans to solve the problem of cold, drafty rooms and high monthly fuel bills, and if it’s a sunny day watch the electrical meter “spin backwards”.
And check out our latest project: “in-ground gutters”. Instead of conventional gutters and downspouts, I had my excavation crew lay down a 12″ deep layer of washed river stone in a trench under the eaves. The trench prevents erosion at grade and splash-back on the siding. Rainwater flows freely through the gaps in the stone and seeps through the sand bed below. If I had a basement and/or heavy clay soils, plastic draintile run downslope to a drywell or daylight would be in order. This strategy would serve the same function as downspouts with extenders—getting the water away, fast. My site is easy: no clay, no rocks, no standing water, no muddy boots. The Village is built on the ancient bed of the Wisconsin River. Life is a beach!
Saturday September 28 from 10 am to 6 pm: Can you stop by (maybe after Farmers Market and brunch at the General Store) and see what we’ve been up to at Spring Green’s first net-zero energy house?
The loft is taking shape and now instead of pointing up and waving my arms in the air and struggling to find the words to explain my design, you’ll see for yourself how it might work. We’re building it with rough sawn lumber that came from a towering White Pine at the corner of Winstead Street & Monroe. Do you remember it?
My friend Jim Birkemeier of Timbergreen Farm felled it, hauled it home just north of town, milled it into planks, stacked it in the solar kiln, let it dry, ripped it into boards, ran it through the saw to make tongue & groove flooring, and stacked it neatly for me to pick up.
This is about as local and low-carbon footprint as you can get! Have you hugged a tree today?
June 1-September 13: Working with wood “for show” is rewarding and I spent many pleasant days sealing 1×4 pine boards for the soffit and 1×8 pine boards for siding. By the time my crew showed up, I had an impressive stack.
We started on the underside of the roof overhang, and hit on what might be an original idea—spacing the 1×4’s apart the width of a nail to create an integral vented soffit. The corners turn like a woven basket.
Most modern homes or remodeled homes have unremarkable “punched tin” soffit panels or metal strips. So unremarkable that I bet most people (not counting architectural snobs like me) take no notice. Look at a historic home and you’ll see boards—often with a interesting pattern or molding added.
Code requires roof venting where fluffy, air-permeable insulation like fiberglass or cellulose is used. Impermeable sheet foam or spray foam doesn’t need venting. The idea is that if warm moist air from the living space penetrates the attic or roof cavity, it can escape. The slots in my soffit will supply more than enough air flow through the 3” deep vent chute we built back in April (see blog post “Vent Chute”).
I shopped around for siding. Cedar is naturally decay-resistant but pricey. Rough-sawn wood in any species looks great and holds a finish better but also costs more. I looked at locally milled wood and visited several Amish farms. I trolled Craigslist and called up area lumber yards. The best deal I found was from Cedar Direct, just a few miles down the road. They import white pine from forests in British Columbia that are certified SFI (Sustainable Forestry Initiative) or from Central Oregon forests that are “selectively harvested”—not clear-cut. The price was right and the quality was good. Delivery was DIY.
I sealed the the boards with PolyWhey, a water-based sealer derived from whey—a byproduct of cheese making—from Vermont Natural Coatings. Whey is a natural bonding agent that displaces the toxic ingredients found in more common polyurethane. The manufacturer claims their product contains up to 45% renewable ingredients and is made in a plant using 50% renewable energy. When applied, it creates a non-toxic waterproof barrier, and protects again UV degradation, mold, and mildew. I found it easy to brush on, easy to clean up, and had virtually no odor. The matte finish lets the grain and coloration of the pine show through without a plastic-y look.
PolyWhey is ultra low VOC (volatile organic compounds) and meets “CA Prop 65”—the toughest environmental standard now in force in California. The standard disallows any product that contains any of the 900 chemicals that have been linked to cancer, birth defects, or other reproductive harm. PolyWhey also complies as Red List Free—a worst-in-class list of materials found in the building industry.
I ordered the sealer in 5 gallon plastic buckets instead of 1 gallon plastic containers. It wasn’t less expensive this way but I knew I could always use another bucket. Plastic containers are, in theory, recyclable—though as many of us are learning, it’s not so easy to find a facility that takes them or may take them but landfill them.
Leftover product isn’t classified as hazardous waste, and can be dried and discarded—but it’s not clear where. Because I sealed the boards before the crew chop-sawed them to fit, I have quite a few cut-offs. I contacted the manufacturer about burning or landfill. The rep admitted there isn’t any official recommendation, but said he personally would without hesitation burn them in a camp fire. So thats what I’m going to do.
I hope the fresh appearance of my pine lasts a long time, but of course no one is going to hand me a guarantee. The USDA publication “Build Green: Wood Can Last for Centuries” points out that when wood is installed properly, it doesn’t deteriorate. Decay can be prevented. The culprit is fungi who attack the cell walls of wood in the presence of moisture, air, and favorable temperatures. On siding, we can’t control air or temperature, but we can control moisture.
My wood siding counts as a sustainable choice if I can keep it dry and on the house for a long time. I can’t control the needs or whims of a future owner, but here’s what I can do as the original builder:
But why go through all the work and worry when I could chose vinyl, steel, aluminum, wood composites, stucco, or brick? All have positive attributes, and promise low-maintenance. Vinyl is the least expensive and most common siding here in the Midwest, and many in the green building community argue its environmental impact isn’t so bad, even though it depletes fossil fuel reserves and uses a slew of chemical additives. It sheds water well, is inherently “back-vented” to allow the wall behind to drain and dry, can last 40 years, can contain some recycled content, and can be recycled (though most tear-offs still land in a dumpster). Here’s a good article: Pro/Con: Vinyl is Green. And here’s another take on it: The Seven Deadly Sins of Vinyl.
Wood is abundant, renewable, low-tech, and requires far less energy to make than metal or cement-based siding products. It can be a source of local jobs and spur on a more regionally-based economy (more on this in a future blog post). Its disposal doesn’t place a burden on future generations. What will our post-post-post industrial society look like in 2060 when today’s vinyl siding or steel is slated for recycling?
Each homeowner has their own preferences, place value on certain architectural features but not others, and bet on different products. Each building material we use has environmental costs associated with their material extraction, manufacture, transport, and disposal—but there is no difinitive source that tells us its durability or carbon footprint “score”. There are just too many variables.
Green building usually means reducing operating costs by adding layers of insulation and products like high-performance windows. But today, more attention is being paid to the carbon footprint of the products themselves—especially as climate scientists warn of an ever narrowing window of opportunity to reduce our carbon emissions. Does it still make sense to build an uber energy-efficient home if the materials used cause more climate disruption today than the cost to heat and cool the home over these next, crucial years?
Alas, the best choice is to not build at all. Refuse, Reduce, Reuse, Recycle. More on this in a future blog post.
June 26-July 12: My crew was impressed with how sturdy and how easy it was to work with the cement board siding I ordered through my local lumber yard. My design called for lap siding on the garage and as a “skirt” to wrap the base of the house. I chose James Hardie’s extra-thick “Artisan” series in a 7 inch reveal with smooth texture for its strong character and robust shadow lines.
The crew did a great job planning the joints to reduce visual distraction and material waste. Installer-friendly features include an integral tongue & groove for a tight butt joint and excellent rain-shedding ability. The boards are nailed “blind” and “off stud” to avoid tear-out at vulnerable edges. Alignment is a sure thing with a galvanized steel “joiner” from Simplicity Tools placed under each joint. Corners are finished with another steel accessory, for a look reminiscent of a mid-century rancher. However, the installation left a worrisome gap at the bottom, a place bees might like to nest. I made the hole inhospitable by packing it with inexpensive stainless steel “scrubbies”. A rough-sawn cedar sill caps off the skirt.
Fiber cement sidings “green” credentials are debatable. It’s mostly cement, with cellulose fiber added as a binder. Mixed in, but not disclosed on the packaging or the Safety Data Sheet, are James Hardie’s proprietary ingredients. Cement is simply crushed rock—abundant and benign. But processing it and forming it into something you can nail onto your house burns up a lot of fossil fuels. It’s estimated that cement plants account for 5% of the global emissions of carbon dioxide, the main cause of global warming.
To their credit, James Hardie claims that 75% of the products raw materials are locally sourced, including the portland cement, cellulose pulp, sand, and water. These raw materials are low in toxicity, and the siding poses no health concerns in ordinary handling. However when cut, drilled, or crushed the dust is an inhalation hazard. My crew used a proprietary James Hardie saw blade designed to minimize dust and set up outside, away from people and buildings.
Like most manufacturers, James Hardie is cagey about disposal. Their Saftey Data Sheet says to dispose of in a “secure landfill, or in a way that won’t expose others to dust”. I talked with a representative, and in the end decided to toss my cut-offs on site. They’ll be fill for the driveway, displacing the amount of gravel to be hauled in.Using a high embodied-energy material can be justified if you don’t use much of it and you design for a long service life. I can’t predict what a future owner might do, but here’s what I can do:
I love to paint. Sure it can be messy and a pain, but in the category of work I find it enjoyable. Most James Hardie products come with a factory finish, but in the thickness and width I chose it only came primed. That gave me the chance to pick my own hue and to try out a best-in-class eco-friendly paint.
I chose ECOS Paints in a matte gray, and it went on smoothly. Coverage was good, though there was some objectionable odor. Clean up was a breeze. Life expectancy is 15-20 years.
The paint can be purchased directly from the manufacturer, or through a distributor like Green Building Supply. The reviews at GBS were compelling. The paint is non-toxic and has zero VOC’s (volatile organic compounds). ECOS was the first and remains the only paint manufacturer to meet the strict labeling of both “DECLARE” and “Red List Free”.
DECLARE is a disclosure statement with more transparency than the more common Material Safety Data Sheet (MSDS)—it requires listing ALL ingredients, right on the label. Red List Free means the product doesn’t contain ANY of the thousands of known hazardous chemicals. Leftover paint and the empty containers aren’t classified as hazardous waste, so can be left to dry then tossed in an approved landfill.
The house is taking on a handsome look, and I hope you’ll stop by and see for yourself. You’re welcome anytime!
Saturday August 10 from 10 am to 6 pm: Stop by and see what we’ve accomplished in the last couple of weeks. The slotted wood soffit is in, looking smart and doing its job to keep the insulation and roof framing dry. Horizontal cement board siding skirts the house and wraps the garage, now sealed with “Red-list Free”, low VOC, made-in-America paint. Inside, rooms are framed out and waiting for mechanicals. Learn how double-stud walls and other energy-efficient details will make this Spring Green’s first net-zero energy home!
Saturday July 6 from 10 am to 6 pm: The house is taking on a handsome look as wood siding and soffit go up piece by piece. When we fire up the saw, we’re running on clean energy—the solar panels are now hooked up to the grid. Inside, you’ll still see the skeletal structure of the double stud walls and the barn-like open space. Wait till next month, and each room will be defined by their stud walls. I’m just back from the Midwest Renewable Energy Fair where I gave a talk on our progress and plans for a new vision of suburbia. Come see what we’re doing, ask me questions, and tell me what you’re looking for in a modern home!
Friday-Sunday June 21-23: Have you been wondering what you can do to reduce your carbon footprint? Would you like to help build a straw-bale wall? Get a good look at the latest electric cars? Learn how to raise chickens? Can you picture yourself eating pizza from a outdoor wood-fired oven? Listening to live music while lounging in the grass? If so, you can’t go wrong at one of my all-time favorite events: the Energy Fair in Custer, Wisconsin.
I’ll be giving a talk about building Spring Green’s first Net-Zero Energy house on Sunday at 10 am called “Follow the Build-Part 1”. Here’s the link to my presentation:
I hope to see you there (or at the beer tent).
June 3-5: At last I was ready for insulation. For good reason, the type of insulation, where to install it, and how to install it is of obsessive interest to green builders like me. Other building materials might be the same, but insulation is where we veer off. I chose dense-pack cellulose because there’s no other insulation that takes less energy to produce, uses more recycled content, and is less toxic to people and animals. Not counting direct-from-the-earth materials like straw bale, straw-clay, cordwood, log, and cob.
Yes, I did consider straw. But for my subdivision house geared to taking super-insulation and net-zero mainstream, I’ve heeled close to convention. All I’ve done is make deeper cavities and prioritized initial cost over operating cost.
There aren’t many contractors set up for this kind of work. Wallace Kennedy and sons of Accurate-Airtight Exteriors arrived from Madison to install what turned out to be the most number of bags ever for a dense-pack job. Wallace packed 238 bags in my 22 inch deep truss cavity, for a total R-80. That’s double code minimum.
The insulation comes from Champion Insulation in nearby Fond du Lac. It’s made from wood fiber paper stock and claims 85% recycled content. By weight, it’s 84% cellulose, 14% boric acid, and 2% starch—all benign materials that pose no threat to the environment. It doesn’t cause skin irritation like fiberglass, and has little or no smell—though breathing the dust should be avoided.
Cellulose reduces sound transmission, and when dense-packed it resists air movement, fire, pests, and vapor diffusion. Because it’s hygroscopic, it can take on and release moisture from surrounding materials like sheathing. This protective characteristic makes it especially favorable in double-stud walls that lack a layer of exterior sheet foam. The added borate aids in resisting mold and insects.
Cellulose is a “low embodied-energy” material. That’s the energy it takes to source its ingredients, manufacture and transport the product, and dispose of the product at the end of its useful life. Side-by-side comparisons with other insulation types are hard to come by. Cotton, wool, and cork are more energy-intensive and cause some pollution, while fiberglass, mineral wool, and all the foam insulations pose significant risks.
I ran the vapor retarder long and instructed the drywallers to skip over a 12” gap down the center of the ceiling for access to each truss bay. It took the crew 3 (monotonous) days to complete the job. Wallace stayed on hose, while the younger men fed the blower machine. Their final step was trimming & taping up the vapor retarder over the gap, making it ready for the 12” strip of drywall that will complete the airtight lid. I’m grateful for their conscientious work!
May 24-29: Time was running out. I wanted to have things looking nice for my June 2nd Open House, but I could tell after just a few hours of scrubbing that getting the slab ready to seal was going to be a long and grueling ordeal.
During the planning process, I made the decision to go with a “raw” concrete look: no stains, no stamping, no pattern cuts. No grinding or polishing to reveal the aggregate. No faux finish and no visual tricks. Above all: honesty of material. A simple matte finish would save money that could be put towards fabulous area rugs. Plain with a touch of luxury would suit my style.
I’m not one to rush out and buy expensive equipment or take a chance on a rental that may or may not get the job done. I also don’t like to hire out if it looks like something I can do. All it is, is work. Stubborn stick-to-itiveness and shear parsimoniousness has made for many a long day.
The slab was well-cured after 6 months and it was finally up to temperature. Over the last several weeks, I watched as my borrowed infrared thermometer read out 40, then 50, then 60 degrees. I had my plumber install a hose bib and the Village hook up a meter. I broke down and bought a shop vac and a stiff push broom. I watched a couple of YouTube videos about how to apply muriatic acid (it micro-etches concrete and prepares it to “grab onto” a sealer), and invested in a spray bottle, rubber gloves and a mask. I was good to go.
By sections, I scrubbed the slab 3 or 4 times (just water, no soap). When the water finally vacuumed up mostly clear, I applied the prescribed dilution of muriatic acid and watched it bubble & mist. I rinsed 2 or 3 times then let it dry for a few days before brushing on a test patch of sealer where the cabinets will go.
I chose ECOS Paint’s “Concrete Sealer”, a zero VOC, no odor, water based acrylic product that was easy to use. It carries the “Red List Free” label from Living Future Institute for not containing any of the worst-in-class materials prevalent in the building industry. I hoped to save time by rolling it on with an 18” wide paint roller but quickly saw it left tiny bubbles. I switched to brushing it out on hands and knees.
Two coats got me an interesting finish that I kind of liked but told a friend that “10 out of 10 people will find this unacceptable”. My prediction was way off: at my Open House, many people commented on it and likened it to a natural stone floor. The finish seals out water (I tested it), but took to the concrete in a mottled way. Some areas are shiny, others are dull. The look is growing on me! What do YOU think?
May 20: Sometimes, conventional wisdom should be questioned. In this case, it’s the conventional sequence of residential construction. Today, the drywall crew arrived to “hang the lid”—builder speak for installing ceiling drywall.
In my quest for a super air-tight house, I decided to eliminate all electrical and mechanical penetrations through the ceiling and to insulate it and drywall it before framing out interior partition walls. While it takes separate set-ups for each crew and some flexibility in scheduling the next phase, it does make the actual job easier and faster.
Drywall is relatively cheap and it’s common to add a few sheets in case of a miss-cut, but I wanted to reduce waste. I also wanted to try out a product called Insta-Back that eliminates the need to cut drywall back to the nearest truss. I came up with a plan that saved 4 sheets of drywall, reduced the number of seams to be taped, sped up installation, and left me with a manageable pile of scraps at the end.
My plan called for cut sheets at the perimeter only and full size sheets in the field, run past their truss supports. All lengthwise seams are factory. Butt seams are joined by Prest-on’s “Insta-Back” drywall clips that promised a “bump-free” joint with a 1-2 degree taper, similar to a factory joint.
Adam Esch of Esch Drywall appreciated the wide open space to roll his scaffold and did a great job. But I could tell he wasn’t too impressed with the clips. Later, I went back through with a level and determined that of 20 butt seams, only 2 had the requisite taper, while most simply held their own by laying flat. I’ll have 3 or 4 bad joints to deal with. At this point, I’m not sure if the problem can be blamed on the Insta-Backs, or the adjacent trusses. For the walls, I may try another product.
I chose 5/8” thick USG’s “EcoSmart” panels for their long list of green credentials and green certifications. The upcharge was under $20. The panels are significantly lighter and use less water in the manufacturing process. The ingredients are so benign, I tossed scraps at the edge of my lot to decay into the soil.
We left a 12” gap down the center of the ceiling at the request of the insulator. He’ll use the gap to snake his hose into the truss cavities. Later, we’ll fill the gap with 1/2” thick drywall for a smooth connection to the factory seams. Here’s hoping drywall mudding and taping will go well, because I’ll be tackling it by myself!
May 18-19: The latest buzzwords are “smart & sustainable” and I’m saying them but thinking ruefully of all the things builders and industry get wrong and have to walk back in say, a couple of decades. It was on my mind while on a ladder while wrestling with a 50 foot long roll of so-called “smart” vapor retarder.
Today’s homes are much more vulnerable to mold and decay than older homes because they’re slow to dry out if they get wet. We pack wall and ceiling cavities with insulation, and cover surfaces with materials that trap or retain moisture. When driving rain sneaks past cracks & crevices and soaks in, a day or two of sunshine isn’t enough to make things right.
Inside, a family of four can create 2-3 gallons of water vapor a day when cooking, bathing, and washing—and all that moisture can find an exit through a hundred sloppy construction holes.
An older home is like a wood box. A new home is like a wood box lined with sponges imperfectly wrapped in plastic.
Since the 1970’s, building codes in cold climates have required a vapor retarder installed on the “warm side” of a wall or ceiling. It can be old-school kraft paper that comes with fiberglass batts, but is usually 6 mil polyethylene stapled to the face of the studs and covered with drywall. Poly is a Class I Vapor Barrier, practically impervious to water vapor. Kraft paper, once maligned as too loosy-goosy, is now considered “smart”. It’s a Class II Vapor Retarder, which means it can stop some but not all vapor.
The problem is that with the advent of air-conditioning the “warm side” of the wall in summer is on the outside. How this obvious truth could be overlooked (and why Wisconsin’s building code is still stuck in the past) is hard to fathom, but the building industry has stepped in with a new product—and this is how I ended up on a ladder, wrestling.
CertainTeed’s MemBrain is a 2 mil polymide film that alters its physical structure (!!!) when the relative humidity changes—shifting from Class II to Class III. Water vapor can pass through when humidity is crazy high (60%), but stays “shut down” when humidity is in the normal range. So in winter, MemBrain works like a normal vapor retarder in that it stops warm, moisture-laden inside air from entering a wall or ceiling cavity and condensing on the cold surface of the sheathing. However, should the cavity become seriously saturated, MemBrain will “open up” and allow drying to the inside.
In summer, with air-conditioning running full tilt, MemBrain will stop the warm, moisture-laden outside air from entering the house. But should the cavity become overloaded, it has a chance to diffuse its vapor to the dry air of the interior. Picture a downpour followed by sunny skies. Wood, brick, stone or cement board siding becomes saturated. Solar heat drives the moisture into the building cavity. If traditional poly is used behind the drywall, that moisture will condense and saturate the insulation. With MemBrain, the moisture can pass through.
But many building scientists and high-performance builders say a vapor retarder isn’t needed, except in extremely moist situations like pool rooms and greenhouses, or in homes up north where air-conditioning isn’t used. They argue that vapor diffusion through drywall is minuscule compared to bulk vapor drive through sloppy holes. What’s needed instead is an interior air barrier. Drywall is fine, as long as it’s well-sealed.
Martin Holladay of Green Building Advisor makes an exception for double-stud walls like mine. Because super-thick insulation keeps exterior sheathing extra-cold in winter, the chance for moisture accumulation is greater than in a conventional 2×6 wall. He recommends a vapor-open sheathing like fiberboard or exterior-grade drywall in addition to siding installed on a ventilated rainscreen. Both measures speed drying to the exterior. The other option is to slow moisture diffusion from its source by installing a smart vapor retarder.
My sheathing is plywood, which is more forgiving of moisture accumulation than industry-standard OSB (oriented strand board), but less so than fiberboard. I’ll be installing my siding on DuPont’s DrainWrap, a crinkled version of Tyvek that accelerates drying, but not nearly as well as a dedicated rainscreen. Holladay would say that in my case, a smart vapor retarder is cheap insurance.
While my wall is moisture-forgiving, I’ll bet most of the homes in my neighborhood (built over the last 20 years), are a “moisture sandwich”. Typically, the walls are 2×6 frame with fiberglass insulation and a poly vapor barrier under the drywall with 1 inch of foam over the exterior sheathing. Sheet foam is a good solution for increasing the R-value of a conventionally framed wall, and it really reduces “thermal bridging” but it comes at the cost of trapping moisture. Both foil-faced polyiso and pink or blue XPS are Class I vapor barriers on par with poly. If this wall gets wet, it will likely stay wet for a long time.
My vapor-variable retarder might be cheap insurance, but it’s not cheap. I tried hard to find kraft paper (not attached to a fiberglass batt) that was labeled a Class II vapor retarder, but no luck. I looked at vapor retarder paint, allowed with special permission in Wisconsin, but only found the usual toxic mixtures.
Installing MemBrain wasn’t really too hard. It was more durable than I imagined it might be, and held up to hanging and stretching without tearing. I lapped seams by 12 inches and sealed the perimeter with ChemLink’s DuraSil, a non-toxic, low odor silicone adhesive. In this way I achieved both a continuous vapor retarder AND a continuous air barrier.
Some type of vapor-variable retarder is the only option I’d consider for a smart and sustainable home—given the current level of building science and available technology. But it’s not a hill I want to die on: what will progressive builders be doing in 20 or 50 years? Will my so-called high-performance assembly look antiquated?
May 15: There’s a lot of confusion out there about how tight is too tight. Some builders say a house “needs to breath”, and that sealing every gap is a waste of time. This is how I see it: we all want fresh air. Some of us with allergies need filtered fresh air. What we don’t want is “fresh air” filtered through the building materials in our walls and ceilings. If we leave gaps we are:
1. Wasting energy (losing heat in winter, coolness in summer).
2. Allowing moisture-laden air a route into walls and ceilings, where it can condense on cold surfaces, creating a breeding ground for mold and mildew.
3. Allowing dirty or polluted air a chance to dump dust and allergens into walls and ceilings, or directly into the indoor air.
4. Creating a pathway for insects, or worse—rodents.
In the summer, I can open windows. But in the winter, I’ll need a mechanical system to bring in fresh air and exhaust stale air. Later, I’ll explain the quiet, energy-efficient ducted heat-recovery ventilator (HRV) that will be installed.
Just as I did for exterior air-sealing, I took an enthusiastic approach to interior air-sealing. I used canned spray foam to seal window and door rough openings. It was easy and fast. I’m a little worried about reports I’ve read that foam can crack and separate from it’s wood substrate over time. A bead of caulk or a run of tape over the foam would be prudent, but I haven’t found a way to do that in my particular situation. Where spray foam wasn’t appropriate, I used caulk or tape.
Foam in all its guises is one of the worst building materials for carbon footprint. I chose a low-pressure polyurethane foam called Handi-Foam. It’s a Green Guard Gold product certified for low chemical emissions and is non-toxic when cured.
The electrician has run a few pipes. I sealed those with DuPont’s FlexWrap EZ, a butyl-based peel & stick tape that’s a breeze to install.
With air-sealing complete, the moment of truth arrived—my first Blower Door Test. I’ve arranged for energy auditor Jim Kjorlie of Kjorlie Design Services to come out and test my house three times: before insulation, after insulation and before drywall, and finally upon completion. With his help, I hope to be certified through Focus on Energy’s New Home Program.
Blower Door tests aren’t too common around here, but in neighboring states where stricter energy codes have been adopted, they’re required. Jim set up a large fan in the front door, turned it on, and brought it up to speed. The idea is to depressurize the air to simulate a 20 mph wind bearing down on all sides of the house. Drafts can then be hunted down. Gauges and gizmos spit out numbers—and you either “pass” or “fail”. Here’s how I did, expressed in air changes per hour (ACH)—literally how many times the volume of air in the home is changed out with fresh air each hour:
Wisconsin Code (2009 IECC national code) =7.0 ACH
Focus on Energy New Home Program =3.8 ACH
Test #1 =2.6 ACH
It’s good, but I was hoping for better, like 1.0. Jim went around with a “smoke stick” that detects leaks by showing a trail. Tell-tale puffs shot out at the base and top of the patio door—the usual places, he said. He suspects that the fiberboard vent chute—which comprises 40% of the interior surface area is much more air permeable than the plywood sheathing on the walls. He suggested I find air-perm ratings for both materials, but so far I haven’t found any definitive information.
I’m stoked to bring that number down, so stay tuned for Test #2!