Vapor barrier between foam and sheetrock
Bob_the_cartoonist
| Posted in Energy, Heating & Insulation on
I’m having my two story addition insulated with open cell sprayed foam. The new walls are 2×6 as is the vaulted second floor ceiling. The walls’ exteriors are 3/8″ plywood sheathing, tar paper and cedar clapboards. The downstairs walls will be sheetrocked, the upstairs walls will be 3/4 tongue and grooved knotty pine. MY QUESTION: should I put up an interior vapor barrier before the sheetrock and pine? The foam insulator guy said,”No, not in the Kansas City area. Only if you live in the South, where the humidity is greater”. Opinions from you Masters???
Replies
For KC, you are in a mixed climate. You really shouldn't have a vapor BARRIER, but the open cell foam does have some permeability to it. All you need on the wall covered with drywall is a vapor retarder primer, or even just a couple of coats of latex paint. The pine paneling is more porous, due to all the joints and cracks. You could put a half inch DW layer, painted, under the pine. Or you might consider using Certainteed's "MemBrain" smart vapor retarder under the pine.
Any way you handle it, avoid the poly sheet and plastic covered wallpaper, and during the humid summers you get there don't run the A/C to an inside temperature below the outside dew point.
The foam guy was only partly right about his comment on a vapor barrier in the deep south. There it should be on the outside, under the sheathing, on the predominantly humid side of the wall, if used at all. Plywood and OSB, when dry, are decent vapor retarders themselves. Good air sealing of the outside against infiltration of humid air is important.
The Building Sciences web site (http://www.buildingscience.com/) has a lot of good info on building envelopes for various climates.