Spray Foam Insulation

2012 to 2017 Consumer Choice Award Winner for Insulation Contractor

Best way to insulate Rim Joists

Fiberglass doesn't stop air or moisture, stuffing batts between floor joists is a common method of insulating the rim joist in most homes, but it's a severely flawed technique.


Fiberglass works best in an enclosed space where it can trap air (between drywall/vapor barrier and the exterior sheathing). When fiberglass is used in a rim joist it cannot perform because air and moisture can move freely in and out of the building through wall penetrations like furnace, dryer and hrv ducts, water and electric lines, as well as wood joints. This causes cold floors and condensation which will eventually lead to mold and rot. Not only does air and moisture move freely around the batts, but because the insulation is not enclosed, it moves freely through the fiberglass as well.


Spray Foam Insulation does, it sticks to the surrounding surfaces. The closed cell foam expands and seals all the possible air infiltration points, stopping air and water vapor from entering or escaping. The foam expands to R7 per inch and insulates the cavity as well. Because the foam doesn't allow air to move through it (which diminishes Rvalue), it always performs at its full-rated R-value.