Permeability of rigid insulation in my wall and roof systems
I’m planning a timber-framed cathedral-ceiling addition to my house in climate zone 4A. My planned wall system (inside to out) was originally:
– Drywall with latex paint
– 2×6 wall with blown-in cellulose
– R-9 Zip R sheathing as the only air barrier
– Cellular PVC board and batten siding
In my previous GBA post about Zip R’s inability to handle the seismic shear loads in walls with a lot of openings, several responses urged me to add a rain screen behind the siding (because more water gets past vertical siding). So I decided to switch over to regular Zip (the primary air barrier), plus 1 1/2″ of rigid continuous exterior insulation with joints taped (a secondary air barrier). The insulation will be held on by horizontal wooden battens on strips of corrugated plastic to provide drainage/ventilation.
Naturally, I welcome any critiques of my present plan, but I have a specific question:
1) Given that Zip has a real-world permeability of about 2-3 perms (thanks Martin, per https://www.greenbuildingadvisor.com/question/zip-or-zip-r), should I be specing a rigid insulation which is relatively permeable in hopes of better drying to the exterior? If so, I’d welcome suggestions.
That brings me to my vented roof system, which was originally (from inside out):
– Timber rafters 4′ O.C.
– 2X6 tongue and groove decking
– Two layers of 4” of faced polyiso insulation (to maximize R value), with seams in each layer offset and taped to serve as the only air barrier. This will have a perm rating of less than 1, according to various sources.
– First layer of 2X4 strapping at 4’ O.C. at rafters
– Second layer of 2X4 strapping, 2’ O.C., perpendicular to the first. The resulting lattice will help vent an intersecting gable on on side (thanks Rob Myers, per https://www.greenbuildingadvisor.com/article/a-timber-frame-house-for-a-cold-climate-part-3 ).
– Plywood sheathing
– Standing seam metal roof
My builder had me to add a layer of #30 roofing felt (5 perms) on top of the T&G for rain protection during construction (duh!). Having now added a second air barrier to both wall and roof, I will make sure that each is connected to the other at the wall/roof intersection. Again, I welcome any critiques, but my question is about insulation permeability:
2) Is the relative impermeability of the polyiso ok, since there is good inward drying potential through the felt and T&G? If not, I’d welcome suggestions.
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Replies
tonybones,
The roofing lay up as described sounds like there will be total of 3" venting under the sheathing with the two 2x4 layers placed as a grid. Have you discussed the vent covers at the eaves? I would look for fine screen vents or small hole metal panels to ensure ember resistance. Not sure from CZ4a where that puts you, but fires have been happening in the east a lot more than usual.
The wall change sounds better for keeping things rigid, but I would review the cavity insulation in light of local humidity and AC needs. I think an Intello type membrane might be a good idea here.
The roof permeability is not of concern since the T&G decking will dry to interior. The final sheathing under the metal roof will have a massive air gap and venting if I read the description correctly. I would change the tar paper to a synthetic roofing material to avoid smell over time. I would strongly suggest high temperature roofing material on the sheathing under the metal roof.
Since you are offsetting and taping the two layers of polyiso I would guess the "dew" problem of the large gap between sheathing bottom and foam top will resolve by and condensated heading down to the eaves. Might want to review how this transition is sealed off so condensate doesn't trickle down the open edge of the polyiso which can take up water though maybe not much here.
One other major detail to attend to would be the T&G sealing at the wall roof intersect on the gable end. In colder climates each T&G pairing creates a cold air leak and a condensation point. In CZ4a the amount of time you see this problem is shorter, but still there. With a longer AC season the reverse can hold true. Warm humid exterior air can condense when the T&G gap imports humid air into the room envelope. The point where this occurs will tend to be closer to the interior side of the gap. You may get staining and aging effects at the interior side of the intersect during the summer and possible moisture build up leading to rot nearer the outside of the wall intersect in winter.
In any case, it is best to seal the gap so air in the room stays put. This T&G gap issue is also why I don't recommend the tar paper layer.