Insulation and vapor barriers on a cathedral ceiling
We are building an addition with a cathedral ceiling in Massachusetts. Starting from the outside, we have asphalt shingles over 7/8″ OSB on 2×12 rafters. Between the rafters is 6″ of closed cell foam then R21 fiberglass faced batting. The insulation gives us a total R value of 49, which we are satisfied with. The issue is the heat loss through the wood rafters. Attached to the rafters and running perpendicular to them are 1×4, (shown on attaced photo) to which we will eventually attach the drywall ceiling. The space between the 1x4s would seem to offer an opportunity to add 3/4″ foam board, which would increase the R value of both the insulation, but more critically of the rafters.
The issue is vapor movement. I raised the possibility of adding foam board between the lathe with my building inspector and he said no, because this would create an additional vapor barrier: 1) foam board, 2) Kraft paper facing on fiberglass insulation, and 3) closed cell insulation. Is he right? Or should I go ahead an add foam board in the gap between lathe?
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Replies
With well over half the R-value as closed cell foam on the exterior of the fiber layer there is no issues regarding dew point control at the foam/fiber boundary, unless actively humidifying the space to some crazy-high level.
The kraft facer has a much lower permeance than 3/4 of EPS or XPS when dry but becomes essentially vapor open at humidity levels that would support mold.
UN-faced 3/4" Type-II EPS has about the same vapor retardency as standard interior latex paint on wallboard. At 3/4" unfaced Type II XPS has about half the vapor permeance of latex paint on wallboard, but still isn't even a Class-II vapor retarder, let alone a vapor barrier. Only foil or plastic faced foam board would create a potential moisture trap, but the 1x4 furring is still modestly vapor permeable, not even a Class-II vapor retarder.
Friends don't let friends of the earth use XPS- it's an environmental disaster, R for R, far and away the least green insulation material in common use today:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
This is due to the extremely powerful greenhouse gas HFC blowing agents used. EPS and polyiso are blown with much more benign hydrocarbons.
If it's primarily the thermal break you're after, use 3-4" wide strips of 3/4" foil faced rigid polyisocyanurate tacked to the rafter edges, and forget the rest. The ~R5 isn't much of a thermal break on a R13-R14 2x12, but it's better than the R3 you'd get out of EPS or XPS. Foil faced polyiso is usually 1lbs density and a similar chemistry to polyurethane, so it's impact R for R is about half that of HFO blown closed cell polyurethane, comparable to fiberglass or rock wool batt.
Were you to do it over again, the CO2e hit from the 6" of closed cell foam is pretty large, even if using HFO blown goods, but at the R-value reported it's almost certainly HFC blown. In a 2x12 rafter bay 3" of HFO blown closed cell foam is sufficient for dew point control at the foam/fiber boundary with the rest of the cavity fillled with cellulose (aka "sequestered carbon"), for a total of about R50.