March 29-April 14: My favorite thing so far is the 12 foot wide patio door we installed on a glorious spring day. It’s what’s going to open the living room up to my garden and help heat the house in winter. I chose a Marvin aluminum clad wood slider, with a heavy-duty fiberglass sill. Thank goodness for my crew of Very. Large. Men.

I prepped the sill by securing the under-slab vapor barrier to the edge foam with caulk and measuring out a length of 5” EPDM sill gasket. The patio door sill will sit directly on these materials, completing a continuous thermal-barrier, air-barrier, and vapor-barrier to the exterior.

edge foam with caulk

EPDM gasket

Mark of Bear Paw Design & Construction and “the boys” hustled the thing into place.

  Here’s Mark finessing the gasket, bathed in sunlight.  “Passive Solar” was the buzzword of the 70’s that got me excited about architecture and back to school to take a drafting class. Since then, the idea that south-facing windows can serve as heating appliances has been disparaged. Those early, hippie homes with their wall of factory-seconds windows skimped on insulation and leaked like a sieve. They overheated during the day, and without thermal curtains lost all that heat at night. We now know that thick layers of insulation and a double-down on air-sealing are far more important than trying to capture solar energy.

Many high-performance builders shooting for net-zero are building conventional-looking homes without regard for window orientation. Even so, I’m inclined to cling to my roots. My design follows the recommendations of Dan Chiras and his seminal book The Solar House which balances “solar glazing” to “thermal mass”. During the winter when the sun is low in the southern sky, sunlight will stream through my south-facing windows and patio door and its heat energy will be absorbed by my concrete slab. There will be some lag. Some days, it will be cloudy. Other days, the house will get too hot. Thermal mass is slow—it’s slow to absorb heat and it’s slow to release heat.

Here’s where I divide company with the mainstream—I don’t need and I don’t want to live in a thermostat-controlled environment that never, ever varies. I’ll live in a house that responds to our flight across the sky and I’ll save energy by accepting indoor air temperatures that climb to 80 degrees on sunny afternoons and drop to 60 degrees overnight. So it’s slippers for me.

One of the most-asked questions I get is if the slab is heated, meaning with PEX hot-water tubing run through or under the slab. Heated slabs are wonderful. They’re warm to the touch and can carry a well-insulated house through most of the winter with supplemental heat turned on only for the coldest days. But because thermal mass works slow, the PEX system really should be left on continually—leaving very little absorptive capacity for incoming solar. And because it’s slow it can’t respond to a sudden change of weather. Most homes need a backup heating appliance that ideally also supplies air-conditioning. So while wonderful, heated slabs are more expensive than unheated slabs because you’re investing in two—not one—mechanical systems.

To satisfy my curiosity, I ran my design through modeling software REM-Design. Per recommendation, my “solar glazing” is 12% of the area of my “thermal mass”. When I remove the slab, annual heating costs increase by 3.5% and surprisingly—cooling costs rise by 9%. Southern Wisconsin is hot in the summer, but has enough temperature differential between daytime and nighttime (about 20 degrees) to power up the “thermal flywheel” effect. While overhangs will mostly shade my south windows from the direct rays of the sun, August heat will warm the slab during the day. If I keep the windows open and there’s a breeze, that absorbed heat will flush out and by morning, I’ll feel a nice cool underfoot.

Besides the cooling effects of the slab, windows placed for cross-ventilation in each room will keep the air-conditioning off most of the summer. Casements or the patio door can be cracked open a little or a lot to funnel prevailing Southern breezes out North-facing awning windows. High windows enhance the “stack effect”, naturally drawing warm air up and out, all hurried along by the sloped ceiling. The speed of the air can be increased even more when the area of opened window on the windward side is small compared to the area of opened window on the leeward side. Awning windows are perfect for passive ventilation because they can be left open all summer long without any worry about rain.

I chose Marvin Integrity All-Ultrex windows for their rot-proof, maintenance-free finish and long-term durability. They’re less expensive than Marvin’s wood aluminum clad windows but more expensive than common vinyl windows. They’re made from pultruded fiberglass, which takes 39% less energy to manufacture than vinyl. And because they’re 60% glass (silica sand), the frames expand and contract at nearly the same rate as the glazing, making the units less likely to break seal.

Window glazing comes in a dizzying combination of features that effect energy performance, and I gave them all an equal run through REM-Design. Turns out, the most commonly available is also the most cost-effective. I chose “dual-pane, low E2 with argon” which is rated for year-round comfort in Northern and North-Central states. The equivalent R-value is 3.4, the same as one inch of cellulose or fiberglass insulation—not too impressive. There are several (mostly Canadian and European) window manufacturers that make up to R-10 windows, but they are far outside my budget.

Other high-performance builders in our climate zone upgrade to triple-panes and low E1 coatings where south-facing. If I did the same, I could save $59/year in operating costs. However, the package would cost me $2,785 extra or about 28% more. Even when amortized over the expected service life of the windows (30 years), the triple-pane windows never pay for themselves.

The argument then comes down to comfort. It’s true that dual-pane windows are colder and more prone to condensation problems. If you’re sitting next to a cold window in the evening, you’re going to feel a chill as your warm body radiates heat to the cool glass. And you might experience a waterfall-like spill of air and be tempted to turn up the thermostat. Curtains or thermal shades are the answer here, or to paraphrase Frank Lloyd Wright: “move your chair”.

I saved money and resources and kept to budget in another way: my home doesn’t really have many windows. If my design answers the need for natural light, views, and cross ventilation—it works. If the windows are well-proportioned and well-placed to create a harmonious whole with other architectural elements—I have style. But the biggest savings of all: the house is small.

flexible sill flashing

flexible corner flashing

Installing the windows was a satisfying step for me. I chose DuPont’s Tyvek system of flashing products. While it can sometimes seem like you’re wrapping a Christmas present, tape and Tyvek are stronger and long-term more reliable than old-school caulk and tar paper. The products had a quality feel and were easy to use.

I used FlexWrap NF, StraightFlash, and FlexWrap EZ—all butyl rubber flashings that are solvent- and VOC-free. They can be installed in cold weather on cold surfaces, are self-healing, and don’t off-gas. There’s no waste except for the peel-off paper backings. With windows in, the house is warmer and a better place to be.