Let me squeeze in a quick note on insulation and air movement, something that is not much talked about.
Imaging you are skiing in the Rockies on a bitterly cold and blustery day. No problem, because you have a very cozy skiing suit. What if the skiing suit has a couple of thin vertical slits on the front and the back? Now you are freezing your behind off, because of the cold air infiltration on the front and air exiting on the back.
Although many would classify the ski suit example as common sense, we don’t make the connection to our homes and insulation.
An exterior stud frame wall with batt insulation is an exact analogy to the ski suit with slits. There is plenty of air infiltration potential between the batts and studs, unless the exterior wall has an air barrier.
See also: Thermal control in buildings (buildingscience.com)
The listed R-value of insulation does not take into account air infiltration, which can severely compromise its effectiveness. The bottom line is that any insulation is only as good as it is airtight.
This principle really stuck with me and it didn’t take much convincing that I probably should think about the air tightness of the basement floor insulation.
Solution: Taping all the seams of the foam boards.
That sounds easy, except that I had a tough time finding the right tape. Don’t use the pervasive duct tape! You need sheathing tape, which appears to be much less common. I finally found it tucked away in far corner at one of the typical home improvement stores.
Here are two product examples of sheathing tape you find on the web: