Insulating Old House Rim Joists

When you begin your journey of home energy conservation — heat loss is one of the largest factors in your monthly utility bill. The best strategy is to tackle “low hanging fruit.” Start the projects that are DIY friendly and offer a great bang for the buck. Remember that whenever you are doing an Energy Efficiency upgrade — you are also supporting a cleaner planet and saving some dollar bills every month.

So, let’s begin with Rim Joists. This is the section of your home, (Usually in the basement or crawl space) where the first first floor meets the foundation. If you have an old home like me, these will be open wooden cavities with some form of brick, stone, or concrete below. Here is a picture of my non-insulated rim joists.


I am lucky to have a basement with 8 feet of headroom. This makes working down here quite practical. Even if you have a dirty, musty, hard to work in basement….this job is pretty straight forward.

Next, I will show you a picture of this same rim joist. However, It is taken with my handy thermal camera (More on that tech item later)!


So you are probably thinking — “Wow Luke! Thanks for that great picture of colorful blobs.” But let me break down what we see here. Notice the two big purple squares? These are the rim joists sucking energy out of my home. Basically, the temperature of this area is far below the temperature of the surrounding home. My thermal camera is allowing me to see the drastic amount of heat loss taking place on this particular day (17F outside temperature).

The next step will be to “break” the flow of energy from inside to outside. To do this we need insulation!

MarkisonFoamCuttingI was lucky enough to have some 1.5 inch rigid foam EPS segments laying about. This would give me roughly r6 of insulation value. I used a standard box cutter to slice the foam to size (9.5″x14″ in my case) and fit them into each section. Similar foam can be purchased from your local hardware store and generally comes in 4’x8′ sheets. I would recommend you purchase a thickness of 1.5-2 inches for this project.

MarkisonRimjoistfoamnosprayNext comes the process of sealing the edges. We do not want air to escape around our new foam inserts! I recommend simple spray foam insulation in a can. This material is very easy to work with and relatively inexpensive to buy. I found that a single can will insulated 10 sections. My entire basement required 3-4 cans at under $3 per can. (I noticed cans cost $4+ at hardware stores….however Walmart had a ready supply for $2.77).


After sealing the foam edges, I now have a solid air seal. The combination of rigid foam and spray foam should significantly reduce the amount of heat leaving my home. Personally, i don’t like to paying to heat my yard during the winter…I would say this was a successful project overall. Estimated time of completion was 3-4 hours. Good Luck!

Here is a thermal image of one insulated rim joist next to an un-insulated rim joist. As you can see — the right joist with foam is significantly warmer than the left rim joist. This picture alone will inspire me to insulate the rim joists on every house I’ll ever own.

Written By:

Luke Langhals

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