Air sealing behind T&G wainscot
Hi,
I am in my fourth year building a large home in the mountains of Colorado, Zone 5A or 6A depending on which map you look at. The house is a hydronically heated 4″ insulated concrete slab. The exterior walls envelope from the outside in is:
Hardi Plank, 1.5 in rainscreen, house wrap, two layers of 1.75″ Poly iso properly lapped and taped, another layer of house wrap, 7/16″ OSB, 2×6″ studs, R21 faced fiberglass batt (properly detailed and installed), drywall or drywall with white pine T&G wainscot.
The wainscot only covers 48″ from the floor, then drywall to the ceiling. Only sections of the exterior wall have wainscoting, not the whole exterior wall.
My question is about the best way to do the interior vapor barrier on the exterior wall when there is wainscoting.
I already purchased faced R21 fiberglass insulation for the exterior walls. I intend to install it correctly. My calculations indicate that with only 3″ of Poly iso, there are times when the OSB will hit condensing temps. To prevent condensation on the OSB, I need a plan that isn’t too expensive.
If I install the fiberglass and then tape every seam, tear, cut, etc. with Siga Majrex, ProClima Intello, or 3M Air and Vapor Barrier 3015NP for the entire wall (not just the bottom wainscot portion). Will that be sufficient, or do I have to start looking at peeling all the facing off the fiberglass in those walls and installing a smart vapor barrier?
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Replies
You mention that you have two layers of 1.75-in polyiso on the exterior, which equals 3.5-in total. You don't mention who manufactured it, but if we assume an R-6 per inch, you have R-21 of continuous exterior insulation. You also mention the cavity insulation is R-21 faced fiberglass batt. The ration of continuous exterior insulation to cavity insulation is 50%, which works even in climate zone 8. For climate zone 5, the percentage of continuous exterior insulation is 27%, climate zone 5 is 36%. You can move to a class III vapor retarder. The faced fiberglass should be a class II vapor retarder. Your wall should be safe.
Your question title mentions air sealing. Don't confuse a vapor retarder with an air barrier, they are two different control layers, though sometimes they are the function of one product. If the Polyiso insulation, or either of the WRBs was detailed as an air barrier, your assembly should be good. You may want to conduct a mid-build blower door test before you close the walls. If you're not achieving the air tightness goal, and the reason is the detailing of the wall air control, you may want to consider an interior air barrier to lower air leakage.
The easiest way to air seal would be to drywall the entire wall, then put the wainscoting over the drywall, which is the usual way to do it anyway. The drywall gives you your air barrier in the usual way in this case.
I agree with Randy regarding the vapor retarder (and you don't want to use a vapor BARRIER here). My preference is to use Certainteed MemBrain over the entire wall, then unfaced batts (I usually use mineral wool), but the kraft facer on faced fiberglass batts will work too.
Bill
mhenson,
As Randy said, you don't need a class 2 vapour-retarder. Unless you are detailing the membrane as an air-barrier (I would suggest using the sheathing instead), you only need painted drywall on the interior.