Insulating Basement Rim Joists

I’ve been doing a lot of reading on home insulation and one popular space I see a lot of people talking about is the rim joist area of the basement. Apparently, rim joists are either avoided, forgotten about or simply never insulated for one reason or another. I think it probably has to do with their size and location. The areas between the floor joists aren’t too large and they are located up in the corners of the basement ceiling. It’s difficult to imagine they are letting cold air in the house (or letting warm air out).

As I go through the basement here, I look for two things. First, I try to find drafts. I know they’re there. I can feel them and as Peter from Dover Projects put it, drafts in a basement can cause something called a “Stack Effect,” where the air leaving the top of the house can pull air in the basement through openings, acting similarly to a chimney. Warm air wants to rise through the chimney, thus pulling cold air from everywhere else. If anyone has had to deal with a traditional fireplace, you know how cold it gets in every room besides the one with the fire in it.

In my previous post, I mentioned that I can see daylight through a few gaps in the basement walls. Those gaps are generally around pipes and wires. Regardless of what caused the gaps, they need to be sealed.

The second thing I look for is areas that would allow cold air infiltration and condensation due to low thermal resistance. This is basically the “R-Value” you hear everyone talking about when it comes to insulation. If you have a low r-value on exterior walls, cold air will “fall” from them (kind of like old one pane windows) and moisture will condense on them. Warm, humid air settling on cold rim joists leads to mold and wood rot. Not a good thing when it comes to basements.

What’s good about the basement here at the house is that the rim joists are already insulated with fiberglass insulation. The thing is – the current insulation isn’t really up to snuff when it comes to modern standards. Even with a paper vapor barrier, warm air and moisture can seep behind the fiberglass and condensation can form on the wood. Also, I’m afraid to say, fiberglass insulation just doesn’t do all that much by itself, when house wrap, other complementary types of insulation and very thorough installation are absent. There are better ways to insulate.

My plan is to put pieces of two inch rigid foam insulation in between each and every floor joist, along the entire rim joist. After I friction fit the rigid insulation, I’ll seal the edges with painter’s caulk. This should both seal and insulate, the way it should be done.

Since I had a few extra pieces of rigid foam laying around from my previous insulation project, I started with a few areas against the rim joist last night. I finished those areas the way I just described and think they look perfect. Just the way I want. If I can get the rest of the areas done like this, I’ll be happy.

Here are some before and after shots from last night’s work.

After this is through, I’m going to tackle the walls, but I’ll leave that for another post.