Attic air barrier/vapor retarder
I’d like to get some feedback on this detail (attached). Background: <700 SF seasonal home in Climate Zone 6. 2×6 at 24″ walls are insulated with dense pack cellulose. Rafters are 2x12s at 16″oc (snow country) and ceiling is 2×8 T&G douglas fir laid across 2×10 (actual) bottom chord ~44″ apart which are exposed to the living area below. It is 1-1/2 story (just lower level and loft) and I am venting the roof. Heat is radiant slab from tankless boiler augmented with a wood stove when we are there (the only penetration in the ceiling besides the hatch, both sealed well). There is no air conditioning and never will be (just mountain air). To get a min R-49 and a vapor retarder at the rafter bays we are using closed cell spray foam and I have installed SmartBaffles in each bay giving 2″ of venting (love SmartBaffle by the way, super easy, installed myself). The spray foam goes all the way to the top wall plate. I have used Henry Blueskin VP as my air barrier on the exterior of the walls (with Homeslicker over that for a drainage plane). The air barrier membrane ties in to the spray foam at the bottom of the rafter bays. Above the attic floor however, I am switching to Roxul which will be applied continuously over the floor and over the top of the spray foam (which stops at the solid blocking in each bay). Since the attic plank flooring is not air tight I wanted a AIR BARRIER and also felt it would not hurt for it to double as a vapor retarder. So I am using 4 mil poly which is lapped and sealed and yes, will be meticulously sealed over the top of the blocking in the rafter bays so it can be as continuous as possible with the spray foam below. Here is where I am looking for feedback: My insulation contractor suggests I spray 1″ of foam over the planking instead of the poly. He thinks it will cause moisture problems but I did not get a lot of reasoning as to why. (We are installing the poly, not him.) I am resisting spray foam in the attic for a number of reasons: 1.) I am only using spray foam where nothing else will do the job as well (sloped rafter bays). While spray foam has its benefits I do not think it is the be-all to end all (I will defer that discussion to others already held in GBA). 2.) Spray foam is irreversible and we may do other things in the future with our attic space. At least with poly and batt I can remove it without damaging the planking. 3.) Spray foam is expensive (Also, I have never used this contractor before and have inspected enough bad spray foam jobs to know it can be screwed up. I don’t want to increase that potential). 4.) Spray foam is flammable and I do not want all of that over my family’s head…our chimney passes through the attic and while it is insulated & separated from the insulation I would rather not increase the combustible materials around it. I would have to spend more money covering it with a thermal barrier. 5.) What would the moisture problem be? The 1.5″ thick wood planking has hygric buffering capacity. The poly has a perm rating of 0.08, while 1″ of foam would be more than 1.0 perms (We would need at least 1.5″ or 2″). The surface temp of the poly should be warmer than the air hitting it as it has R-38 on top of it and I can always add more insulation. I don’t see what the spray foam would do differently (or better)? Thoughts? Comments?
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