Houseworks: The most overlooked place to add insulation

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When Canadians think about upgrading insulation, attics usually get most of the attention. Basement walls come next. But there’s a part of many houses that can leak a surprising amount of heat while also creating cold floors, drafts and mold: the rim joist area around the perimeter of the basement ceiling. 

What’s a rim joist? 

This is the band of framing where floor joists meet outside walls of the house. Rim joists are often poorly insulated or not insulated at all because it requires detailing that’s often overlooked.  

At first glance, insulating rim joists seems simple. Just cut pieces of batt insulation and push them into each joist space. Unfortunately, this easy (and almost universal) approach causes trouble. 

The challenge is vapour control. In a conventional framed wall, batt insulation works because a continuous and effective vapour barrier can be installed on the warm side. At a rim joist, though, the floor framing makes that virtually impossible. Joists, blocking, wiring, pipes and irregular corners all get in the way of creating a truly continuous seal with vapour barrier plastic. And this is a problem that can sit unnoticed for years. 

Rim joist strategies 

I’ve opened up many rim joist areas over the years and almost always found mold behind batt insulation. Sometimes a little, sometimes a lot. Warm, moist indoor air works its way through or around the batts, reaches the cold wood at the rim joist and condenses. Once moisture is trapped there, mold quickly follows. 

All this is why my preferred solution is three inches of closed-cell spray polyurethane foam applied to the rim joist itself. This does two jobs at once: it insulates, and it air-seals. Because the foam itself is highly resistant to air movement and moisture vapour, warm indoor air never gets to the cold wood surface behind it, eliminating the condensation that triggers mold growth. 

The problem is scale. Rim joists are often a relatively small job, and hiring a professional spray-foam contractor for a few dozen joist spaces costs way too much. 

There are two practical DIY alternatives. 

The first is a DIY spray-foam kit designed for spraying larger surfaces. This isn’t the same thing as the small aerosol cans sold for filling cracks around windows and doors. These kits deliver closed-cell foam much like professionally applied material, just on a smaller scale that’s homeowner-friendly. They can make sense where you have enough rim joist area to justify the cost but not enough to bring in a contractor. 

An economical DIY option 

The second approach is cheaper and especially practical for small jobs. Cut pieces of two-inch-thick extruded polystyrene foam board so each piece fits loosely between the joists, leaving roughly a half-inch gap around all four edges. Use a dab or two of hot-melt glue to hold each piece in place temporarily. Don’t use the cheaper white, beady expanded polystyrene foam since moisture-laden air can pass through it, forming condensation on the cold side. 

Next, use canned spray foam to fill the gap around the perimeter of each foam-board piece. The rigid foam provides most of the insulation, while the canned foam seals the edges and prevents indoor air from sneaking behind the board. 

As long as you’re careful, this creates a remarkably effective air-and-insulation barrier with materials that are easy to find and easy to work with. Measure each joist space individually before cutting because framing is rarely perfectly uniform. Also remember that exposed foam may require a code-approved thermal or ignition barrier, depending on where they are installed, so check local requirements before getting started. 

Rim joists aren’t glamorous, and that’s probably why they’re overlooked. But if you have a cold basement, chilly floors or unexplained drafts around the perimeter of the house, this small area deserves a close look. It may be one of the most cost-effective insulation upgrades you can make. 

Steve Maxwell uses foam insulation whenever he can. Visit him online at BaileyLineRoad.com for insights and know-how on home improvement, renovations and hands-on living.