Uninsulated studs = cold sink = much worse R-value?
This is in Minnesota.
When insulating a 2×6 stud wall (16″oc) [with closed-cell spray foam], they will fill to about 3″ thick leaving about 2.5″ of air (photo below). One concern I have is that this also leaves 2.5″ of the side of each stud exposed.
One thought is that this reduces the r-value of the stud simply based on thickness so from an r-value standpoint we now have a 3″ stud (r=3.6) rather than 5.5″ (r=6) for the approx 30% of the wall that is framing.
A bigger concern is that the sides of these studs are now effectively cold sinks. Just as a heat sink sticking out of something transfers heat from inside to outside, it seems the sides of these studs would do the same and transfer heat from the air cavity between the spray foam and the sheetrock and thus massively reduce the whole wall r-value. Doing some very rough calcs I get a reduction from r=11 whole wall to r=7 whole wall.
Is there any truth to this? Any idea how much of an impact? Enough that it’d be worth it to make sure they also build up an inch or so on the sides of the studs?
(It’s a long and irritating story why we have standard stick framing rather than double-stud or exterior foam).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part