How To Insulate Rim Joists
We’ve written a number of articles on insulating basements including; insulating basement walls, insulating basement floors and basement insulation options. In this article I want to focus on rim joist insulation and the options that are available.
Rim Joist Spray Foam
Spray foam insulation is the best way to insulate a rim joist in my opinion. However, spray foam insulation comes at a serious price point and it is best left to professional installation. Spray foam does a great job not only insulating the rim joist but it also seals all the possible air infiltration points.
Rim joists are typically riddled with nails from siding, trim, decks and porches. The foam also helps encapsulate the nails and screws which helps eliminate any condensation problems from the cold fasteners.
Foam Board Rim Joist Insulation
If you’re looking for a good way to insulate your rim joists that’s easy to do and relatively inexpensive then foam board insulation is your best option. When I insulate rim joists I like to use a minimum of 2 inches of foil faced polyisocyanurate foam board insulation.
Installation of the foil faced polyiso is very easy. All you need to do is cut (you can use a utility knife or hand saw) the foam to fit the space between floor joists, the bottom of sub-floor and the top of sill plate. Typically we will cut the foam about 1 inch smaller so it can be fit into place easily.
In the adjacent photo you can see a piece of the insulation that was cut to fit between I-Joists. The insulation doesn’t have to be cut perfectly because of the next step.
Once the insulation is in place we use a can of spray foam (like Great Stuff) to seal the edges of the foam board to the framing. As you can see above in the photo the spray foam fills the voids and also helps hold the products in place.
Fiberglass Rim Joist Insulation
The last method for insulating rim joists is to use fiberglass insulation. Over the years I’ve come to dislike this method. In fact, our new home was built this way (this photo comes from my own home) for the simple fact that I had too much going on to focus on this detail.
I’ve recently started replacing all the rim joist insulation in my home with the foam board that is shown above. Fiberglass insulation doesn’t work very well in this application. As you can see from the photo it’s very difficult to get the fiberglass insulation installed evenly around all the framing members.
I’ve also seen this application result in mold problems. As I mentioned early nails are very common along the rim joist. Those nails are always very cold and act as moisture condensation magnets! This application is also very hard to seal up and create vapor barriers. The bottom line is it doesn’t work well and I’d recommend you replace your fiberglass rim joist insulation with some type of foam product.
Rim Joists at Bay Windows / Deck Cantilevers
Many readers have asked about special situations where the deck cantilevers out past the foundation (typical at bay windows, fire places, and deck overhangs). If you have a situation like that, then I recommend reading my article: How To Insulate Bay Window Floors.