Adding ceiling insulation...
My son's house has R-38 fiberglass batt (12") above the bedroom ceiling. There is vented attic space above that. While the R-38 may be a decent # in theory (though R-50 would probably be the current zone 6 amount), I'm thinking the air sealing capability here is slim to none (below batts is the kraft facing stapled to joist bottoms and 1/2" sheetrock.
We're considering practical ways to add insulation. One would be blown in cellulose; easy to do and reasonable in cost, but don't think this adds much air impermeability (not dense pack in this application). Is there sense in compressing the batts first, then adding the cellulose? The joists are 2x8 so the batts could easily be compressed to tops of those joists, which I know would reduce the overall R value of that batt (despite an increase per inch), -but wouldn't that compression deter some air flow, or is that too negligible to justify? Are there other/better alternatives?
Is it possible/reasonable to do rigid foam over the batts? The foam boards could be fastened downward to the josit tops, thereby also compressing the fiber batts. But, wouldn't quite a thickness of foam be needed, at considerable expense? This seems a bit like doing it above the roof decking, except for no decking! Could foam board seams be taped from above if this arrangement is otherwise feasible? Or, would it be better to foam board below current ceiling and redo sheetrock? (not a great solution due to loss of ceiling height, already low). Or, I guess foam board could go above ceiling, in between joists, below batts, which could be pulled aside and put back, -cobbled together method. The edges against the joist could be sealed with spray foam or quality tape.
Thanks for any and all advice.
Posted Wed, 02/05/2014 - 15:14
Edited Wed, 02/05/2014 - 15:42
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