©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
I have a client in Zone 3 whose home is suffering from extensive air infiltration through the diagonally run dimensioned 1x subfloor that is covered with T&G hardwood flooring. The homeowner shows very little interest in encapsulating the vented crawl space and adding a dehu, and I cannot convince them to use SPF in their crawl space floor cavities either. They have read an article that suggests the installation of rigid foam board @ the bottoms of the floor joists. The floor joist cavities in the home currently have R-19 fiberglass batts installed in them that appear to be effectively in contact with the 1x subfloor material. I have reservations regarding the application of rigid foam board at the bottoms of the floor joists due to the air space void that will be created between the rigid foam board and the kraft paper faced fiberglass batt insulation that currently occupies the floor joist cavities.
I have seen and have installed rigid foam board within floor joist cavities in the past, but the rigid foam board was friction fit into the cavities to be in contact with the subfloor material. After its installation into the cavities, all of the edges of the rigid foam board were sealed with caulk or spray foam in order to eliminate any opportunity for air leakage to bypass the rigid foam board. Fiberglass batt insulation without a kraft paper vapor retarder was then installed in the floor joist cavities in order to increase the R-value provided. All of the air barrier and insulation components were in contact with each other, and there were no air spaces between them. I am confident that this application will work, but it is very time-consuming. Not to mention that the floor joist cavities in this homes crawl space have a lot of diagonal cross bracing within them, and removal of the bracing in order to friction fit install rigid foam board into the cavities would be a real pain and increase the time involved.
Any thoughts on the installation of rigid foam board @ the bottoms of the floor joists? Does the air space void that will be created between the rigid foam board and the kraft paper faced fiberglass batt insulation create the opportunity for an issue to occur?