Exterior Air-Sealing

December 10-21, 2019: While my crew was busy framing the eaves and laying down roof sheathing, I tackled the exterior air-sealing.

Stopping air from whistling through random cracks is an essential step to meet my net-zero goals. Taking the time to caulk and tape each and every joint adds up to energy savings. It also makes a home more comfortable by eliminating drafts, cold spots, and uneven indoor temperatures. I can be confident that my house will last for the long haul because moisture-laden and dust-laden air can’t enter the wall and roof cavities. My insulation and framing will stay dry, which means mold, mildew, and wood decay can’t take hold. And a not-to-be-taken-lightly side benefit: carpenter ants, cluster flies, Asian beetles, and mice are stopped cold.

With all these benefits, you’d think exterior air-sealing would be standard practice in residential construction—but you’d be wrong. Sheathing seams are left untaped. Builders assure themselves that a layer of housewrap is enough to ward off wind and rain. But from what I’ve observed, it’s often hastily tacked up. Seams are sloppy and mechanical penetrations are haphazardly sealed. Siding covers all sins and buyer beware!

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Several visitors to my building site have been surprised to see my use of plywood sheathing—not the more common OSB (oriented strand board). Plywood is a reliable air-barrier and can tolerate some saturation. OSB has been found to leak air when pressurized during blower-door testing (more about this in a later post), and is more vulnerable to decay when repeatedly wetted. A good alternative to stock OSB is Huber’s ZIP sheathing which has a factory applied coating that replaces housewrap.

Plywood scores worse for efficient use of wood resources (it requires larger diameter trees and generates more waste), but it uses less glue than OSB. It’s also more expensive: I chose 5/8” 5-ply fir, a significant upgrade from code-minimum 1/2” OSB. My decision to use plywood came down to its durability, and the advice of many building scientists who say it’s the “least risky” choice for thick double stud walls like mine. When packed with 12” of insulation, the plywood sheathing will stay cold—much colder than in a thin wall that rapidly loses heat to the exterior. The plywood can become a “condensing surface” (like window glass) if the drywall is breached and moist indoor air finds a convective pathway.

My first step was to caulk seams wider than 1/4” and tape narrower seams. I chose the same caulk (ChemLink M-1) I used for the under-slab vapor barrier where it seals to the top of the foundation wall. It’s a waterproof, non-shrinking, low VOC sealant that can withstand joint movement in excess of 35%. It was very easy to work with and had no apparent odor. The tape is 3M’s All Weather Flashing Tape 8067, selected for its good track record.

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cCaulk and tape are more reliable than foam products because they remain flexible. They hold up to thermal expansion, structural settling, lumber shrinkage, and wind stresses. Foam is brittle. It can pull away, leaving hairline cracks that link outdoor air to indoor air.

At the bottom of the wall, I folded up and taped the under-slab vapor barrier that I had let run long (see blog post “Sills & Slabs”). Now married to the plywood sheathing, it completes the continuous air barrier at this critical juncture.

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At the gap between the truss tails and the wall sheathing, I used DuPont’s FlexWrap EZ, a flashing tape that takes a curve. Later, I’ll use it for mechanical penetrations. On the inside, a “vent chute” will complete the air-barrier at the top of the wall (more on this in a later post).

1
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5My next step was to roll out the housewrap. A few green builders swear by old-style tar paper (asphalt impregnated felt), but most agree that synthetic products tear less and hold up better under repeated wetting. They are also lighter and easier to install. Known to the trade as “Weather Resistive Barriers”, a good quality housewrap is wind-resistant, waterproof, and vapor open.

I chose DuPont’s Drainwrap which has a slightly crinkled surface to facilitate water drainage behind the siding. With help from friend Bob Rowen, we got a nice tight fit around corners and secured it with caps. Simple stapling wouldn’t hold up in my windy location. We kept clear of window and door openings.

20181220_125303The caulk/tape/housewrap method takes patience and many steps. There are other ways to air seal including sheet foam, spray foam, fluid-applied membranes and peel-and-stick sheets—all more expensive in material but may come with a savings in labor. What I’ve chosen is simple, readily available, and easy to execute or adapt when future changes are made.

When spring comes around we’ll install a combination of vertical wood and horizontal cement board siding. Siding is the first line of defense against rain, wind, and snow. Any water that sneaks past will hit the housewrap and drain down. If the housewrap is breached, the plywood can take the blow. Each layer deflects water and wind, and does so while staying vapor-permeable. Any moisture that’s absorbed can eventually evaporate. The wall can dry to the exterior when the sun comes out and warms surfaces. Or it can dry to the interior when the indoor air is warmer and drier than the outdoor air (more on this in a later post).

I hope what I’ve described here makes sense to you whether you’re a fellow builder or a homeowner. Please let me know if you have any questions or insights of your own on the fascinating topic of air-sealing!

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