Basement insulation strategies
Now that I’ve got my above grade strategy worked out I’m starting to look at my basement and foundation strategy. Climate Zone 6.
I’ve listened to the excellent podcast featuring Martin here:
And read these two links as well as other articles on basement insulation:
The prescriptive code IRC 2015 Table N1102.1.2 states that for climate zone 6 the basement wall shall be and R-value of 15/19. Where 15/19 mean R-15 continuous insulation on the interior or exterior of the home or R-19 cavity insulation at the interior of the basement wall. 15/19 shall be permitted to be met with R-13 cavity insulation on the interior basement wall plus R-5 continuous insulation on the interior or exterior of the home.
My insulation guy is suggesting we use a RES-Check analysis to only use a continuous blanket drape which gives about an R-11 and then put studs on top of that to hang the dry wall and not even filling the stud bays. That might pass RES-Check and inspection but I’m not sure how you go about air sealing the basement then?
I’d always thought the best way to do it would be to use rigid foam insulation like described here at GBA.
I still have a few questions regarding a rigid foam strategy.
1) You don’t want to use a polyethelene vapor barrier so the concrete has a chance to dry to the inside. However using rigid foam especially a foil faced Polyiso will prevent drying to the inside. Does it make sense to use 1″ of XPS instead and then 2×4 blown in stud cavity to allow for interior drying?
2) In going with a combined approach of rigid foam and blown in stud cavity is there a ratio rule of thumb between rigid foam and blown in insulation like in roofs and walls (51% and 36%)? It seems like there should be because of the hot moist summer air cooling on the inside of the foam and potentially causing moisture issues in the stud cavity?
3) A foundation wall it is unlikely to be very smooth or dead straight. Foam board is flat and very rigid. How difficult and costly is it to attach the foam board to the concrete wall?
4) Is air sealing the basement wall just as important as air sealing the above grade stick frame?
5) How are the portions of basement wall that are above grade insulated? If I’m planning an R-30 exterior foamed stick frame above grade and per the code the basement requires R-18 then the part of the basement wall above grade exposed to the snow and all the elements is the weak part of my design. How do people typically handle that issue?
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