Floor above unconditioned space
Firstly thanks for the feedback on my previous roof assembly post. This question relates to the floor of the conditioned area (ceiling of a garage/carport). Thankfully no ducts, just wires and plumbing in a few locations.
By way of background, its climate 4C (near Vancouver, Canada) and a small building used as a holiday home, not fully occupied in Winter. The occupied conditioned space will sit above both an unconditioned garage and cantilevered over a carport.
Designers build up from conditioned side down:
– ¾ OSB T&G
– Poly sheet
– 9.5” I Joists on 16” centers
– R22 Roxul Comfortbatt in cavity (not clear how they keep the insulation ‘up’ against OSB with only about 6” spec’d)
– Air gap, ventilated above carport area
– Carport ‘ceiling’ will have T&G Pine, garage ceiling will be finished with ply or OSB
This build up raises some questions for me having read the Do I Need a Vapor Retarder and How to Insulate a Cold Floor :
1) Vapour barrier – House designer says they normally lay down a sheet of poly tight over the joists prior to laying the OSB subfloor. My reading here suggests this is not needed, OSB is sufficient. Assuming we can rely on OSB, I guess we should tape the seams and try to seal the locations under the joists?
If code forces our hand towards a ‘sheet’ barrier, would you suggest a more variable vapour barrier like Membrain ?
2) Air Barrier – Seems there is mixed info on OSB as an air barrier. I’m not targeting Passive standards by any means here, as such can I assume the OSB is sufficient to meet reasonable air tightness standards?
3) Related to the above, Martin suggests a thin layer of rigid foam directly under the subfloor, sealed with expanding foam. Is this required or a “nice to have”? Are the benefits the improved air barrier characteristics in addition to boosting the R Value?
4) Our exterior walls are planned to have R5 or R6 Roxul Comfortboard, would not insulating the underside of the joists (be it rigid foam like the article or Comfortboard) be a material loss in your views (purely budget restrictions).
5) The joists appear to only have about 6” in them based on the R22 Roxul spec and code is actually R28. I wonder if we are better off with R31 9.5” Fiberglass which is almost the same price as far as I can see.
6) If we fully fill the joists and follow Martin’s cross section, there appears to be no need for a ventilation gap above the carport soffit. However, if we can’t afford to fully fill, and therefore don’t insulate the underside (depending on your input as well), should we ventilate the remaining space?
Thanks very much.
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