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Insulate Basement Rim Joists

In just a couple of hours, you can seal and insulate your rim joists, which are major sources of heat loss in many homes. This project will help lower your heating costs and save you money. Insulating the rim joists is one of the best things you can do to make your home more energy efficient. And it’s easy, too, so anyone can do it.

By the DIY experts of The Family Handyman Magazine

Insulate Basement Rim Joists

In just a couple of hours, you can seal and insulate your rim joists, which are major sources of heat loss in many homes. This project will help lower your heating costs and save you money. Insulating the rim joists is one of the best things you can do to make your home more energy efficient. And it’s easy, too, so anyone can do it.

By the DIY experts of The Family Handyman Magazine

Insulate the joists

Properly insulating and air-sealing rim joists takes patience, so most builders simply stuff in some fiberglass and walk away. If you have an unfinished basement, you can properly insulate the rim joists in two or three hours. (This will also block tiny passages where spiders and other insects enter your basement!)  

Call your local building inspections department before you begin this project. The inspector may require you to cover the new insulation with drywall (as a fire block) or leave some areas uncovered to allow for termite inspections. You can insulate second-floor rim joists following the same steps shown here if you happen to tear out a ceiling during remodeling.

Rigid foam is the best insulation for rim joists. We chose 2-in.-thick (R-10) “extruded polystyrene.” Don’t use “expanded polystyrene,” which is a less effective air and moisture barrier. Cut the foam into 8-ft.-long strips 1/8 in. less than the height of the rim joist. A table saw is the fastest way to “rip” these strips, but you can also use a circular saw.

Then cut the strips to length to fit between the joists, again cutting them 1/8 in. short (Photo 1). A heavy-duty box cutter is the best knife for making short cuts and trimming foam; the long blade slices cleanly through the foam (a utility knife blade is too short). Use long sections of foam to cover the rim joists that are parallel to the floor joists (Photo 2). Don’t worry about cutting the foam for a tight fit around pipes, cables or other obstructions; you can seal large gaps with expanding foam sealant later.

It’s important to create an airtight seal around each section of foam using caulk or expanding foam (Photo 2). Otherwise, moist inside air could condense on the cold rim joist. The resulting dampness can lead to mold and rot. If you have a solid concrete foundation, also run a bead of caulk where the sill plate meets the concrete. If you have a concrete block foundation, also seal the openings on top with expanding foam. Stuff a wad of fiberglass insulation into each opening to support the foam as it hardens (see Figure A in the Horizontal Callout).

Place rigid roam against the rim joists,
then caulk along the insulation.

Figure A: Insulated Rim Joists

Airtight insulation reduces heat loss through the rim joist. Fiberglass insulation and expanding foam seal the open top of hollow concrete blocks.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • Tape measure
    • Circular saw
    • Caulk gun
    • Stepladder

Use a box cutter to cut the insulation. You can cut the insulation strips with a circular saw or table saw.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Acrylic caulk
    • Expanding foam

Comments from DIY Community Members

Share what's on your mind and see what other DIYers are thinking about.

1 - 16 of 16 comments
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February 05, 4:46 PM [GMT -5]

This is a great project for "do it your selfers". It is actually a little harder and more time consuming than most sites tell you. I used a table saw and mitre saw for cutting the foam. Thet were great time savers. I started by cleaning out the old fiberglass and vacuuming the joist area. I siliconed the rim edges, cut and fitted the insulation board and rather snugly at that. Don't forget there are plate bolts and nuts to work around also. Then I followed up with spray foam. This was the hardest part. It is messy and you need to be a bit of a contortionist to get to all the areas around pipes, heat runs and wiring. It was well worth the effort. I was surprised at how much air comes in through even a nail hole. All in all I highly recommend this as a way to stop air inflatration as well as insulate.

December 24, 8:49 AM [GMT -5]

Did anyone check their area building codes before starting the project? If so, what code are you under?

November 11, 1:02 PM [GMT -5]

what issue was this article?

November 04, 7:35 AM [GMT -5]

I did this on my modular home that sits atop a poured concrete foundation. It took about 8 hours to do the whole perimeter, but I had to pull out the old fiberglass, and work with a lot of electrical and plumbing obstacles. I used 2-in Polyisocyanurate which has an R-13 insulation value.

The end result: my basement's temperature used to be 20 degrees less than upstairs, and now it's only 12! Also, the ambient air feels a lot more comfortable. Upstairs, the exterior walls and flooring were warmer to the touch.

You probably don't have to go as overkill as I did with 2-in Polyiso, but the rim joists are the thinnest part of your home's exterior wall. This project is a must-do!

January 16, 7:13 PM [GMT -5]

While finishing my basement last year, I insulated the rim joists just as they describe here. Did it during winter and noticed a huge difference! My basement has poured walls which are rarely perfectly flat, so where there were small gaps between the rim joists and the sill plate, the cold air just rushed in. Builder just had fiberglass stuffed in, which we all know does not stop air infiltration. It is winter now and my basement is just as warm as the first floor of the house.

As far as cutting the blue board, I took a putty knife and sharpened one edge on my grinder. It would cut through 2" of polystyrene board like a hot knife through butter. (okay, maybe not that easy, but you get the picture). Had to resharpen periodically, but this worked great!

Can't say enough about the benefits I saw by insulating the rim joists. Did the entire perimeter of the house. Easy to do and well worth it!!

January 16, 6:25 PM [GMT -5]

I see no reason NOT to use polyisocyanurate boards instead, but I'd be sure I used foil-covered versions. It will be a cost vs insulation issue. 2" polyiso yields an R of about 14, instead of the "pink" R-10, but it MIGHT cost more, too.

January 16, 5:48 PM [GMT -5]

I had this done about 18 mos ago, but rather than cut individual chunks of foam I had a contractor in and sprayfoamed it. Cost was fairly reasonbable (1800sq/ft basement) and it has made a huge difference. We also had them air seal the attic while they were here. Foamed all penetrations under the blown in insulation (pot lights, wires in top plates, etc). We have seen approx 15% saving in gas/elec cost over the past two winters...money well spent.

You could also get a DIY foam kit - I thought about that but we were finishing the basement at the same time so I had them sprayfoam the rim joists, basement walls and air-seal the attic all at the same time....more than I wanted to do via the kits

January 16, 1:44 PM [GMT -5]

I have engineered floor joists, so cutting the 2" foam to fit will be tedious. Any ideas to speed this up? Also, is not exposed foam board a fire hazard, does it not have to be covered by a noncombustible covering, like sheet-rock?

December 11, 11:55 AM [GMT -5]

Ive done one side of my basement, (1950 Cape) with fiberglass insulation. I was about to do the other side in the same fashion until I read this page. I used 2 pieces of unfaced insulation 7x15. Is this not good enough? Will it work? Will I develop mold? Ive read another website posting that said I may get mold but I cant see how. I would of course like to do this once. I am in New England so it does get cold up here and the perimeters of the interior of the living space gets cold due to no insulation. Any help would be appreciated.

July 11, 10:42 AM [GMT -5]

Is this also appropriate for crawl spaces and if so, what about foundation vents?

March 07, 12:36 AM [GMT -5]

I'm working on this project now. While I cutting the foam board with the box cutter (as recommended), I found it a bit tedious (and somewhat difficult) to get a good straight cut. If you've got a miter saw sitting around, then put away the box cutter and use some POWER.

January 04, 10:02 PM [GMT -5]

ISO, if you mean polyisocyanurate, can be used, however, as the project is to seal, not insulate and the heat loss is not radiant, ISO is an expensive alternative to to the suggestions here.

November 16, 1:10 PM [GMT -5]

Do you require a vapour barrier once you have installed this ?

October 18, 3:21 PM [GMT -5]

I need to do this.

August 05, 7:45 AM [GMT -5]

Wasn't this article in our magazine a short time back. It's on my "Honey Ya BETTER Do" list this summer in preparation for the sever cold winters here in central Michigan.

 

 

August 03, 3:15 PM [GMT -5]

why not use iso board instead of plain polystyrene?

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