Insulate Band Board / Rim Joist To Block Air Infiltration into a Basement

December 19, 2009 | by Fred (email) |

If your looking for ways to improve the energy efficiency of your home, you might turn to the area of your unfinished basement ceiling where the joists meet the sill plate (at the top of the wall) and the rim joist (or band board) at the edge of the house.

It’s in this area that most of the air infiltration into a basement takes place, and an area where you can make a significant improvement in the energy efficiency of your home.

Insulating Basement Joists at the Rim Is Important

The reason insulating in this area is important is because much of the heat loss in a home is due to convection – the loss of heat that is carried away by drafts whipping through the basement.

Since wood joists make imperfect joins, they are highly prone to air leaks. The area where the joists meet the rim will have literally thousands of gaps.

Ways to Insulate the Band Board

Insulate Band Boards and Rim Joist

There are a number of basement insulation options to consider for this job. We chose open and closed cell spray foam for our own home. We believe spray foam is the #1 way to insulate this area and will yield the best results. Unfortunately, spray foam is expensive and generally requires a contractor for the installation. There are DIY kits available but the cost is nearly as high as hiring pro help.

Here’s a few suggestions for insulating this key area of your home:

Fiberglass: The easiest method is to obtain kraft-faced fiberglass batting (the pink stuff) and stuff it tightly in between the joists. We recommend going with an R-30 insulation as opposed to traditional R-13 wall insulation, since you’ll usually have the extra space and you want to maximize the seal. StuccoHouse just added fiberglass in their home and we’re sure it will pay dividends. You can visit that post for some great pictures of this job which only takes a few hours.  You can also add to the fiberglass installation with the following:

  • Great Stuff Foam: If you want to sure up the seal, before adding fiberglass you can pick up a few cans of Great Stuff spray foam and spray it into the corners and butt joints on the rim joist, sill plate, and flooring above. Use the small straw that comes with to get into small cavities.
  • Caulk: As an alternative to Great Stuff, caulk can be used to seal the gaps. Great stuff has the advantage of finding and entering small cavities as it expands, but caulk is also a viable option. After caulking, install fiberglass.

Wet Cellulose: Wet cellulose (like NuWool) is an option only if you’re doing all the walls in the basement. We like closed cell foam better for this option since it provides its own vapor barrier which you’ll need to prevent moisture from condensating on the block/concrete wall, but wet cellulose can work.

Spray Foam: The picture in the upper right of this article shows spray foam installed between the joists. Like Great Stuff, spray foam expands to fill the available space, creating an extremely tight seal. You can watch our video on spray foam for a complete rundown on the material.

Energy Savings & Tax Credits for Insulation

Each home is different, and the energy savings will vary. If you have no insulation in the joists today, you could be wasting more than 15% of the energy in your own. Leaks cost money… a lot of money. Stop them, and you get instant savings.

As a bonus, the U.S. Government will chip in if you tackle this job in the next 12 months. In 2009, and now extended through 2010, there are energy tax credits for insulation that will provide you as much as a 30% back for doing this job.

What do you think? Have you insulated your basement?

15 Responses
  1. tom says:

    If I’m not mistaken, you used open cell foam for your above grade insulation, i thought that was mentioned in the video.

    I’m looking to do the same thing with a DIY kit since I don’t have much cubic footage to fill in.

  2. Fred says:

    Tom, we actually did the front of the house in closed cell and the back of the house in open cell foam (in the joists only). The block was closed cell all the way around.

    We didn’t clarify that in the video and probably should have. The front of the house is close to on-grade and the installer preferred closed cell in this case, just in case moisture ever did contact the insulation (say, from snow drifts).

    If we were doing the install without the desire to film it, we’d probably have closed cell done all the way around. The one advantage for us of using open cell is that on the sides of the house, access into the final edge joists was very prohibitive, so we wanted to use a foam with greater expansion properties.

  3. tom says:

    Thanks for the clarification!

  4. tom says:

    One other question regarding a comment that “Larry” left in your video post… how did you determine that your rim joists were completely moisture sealed?

    Larry had warned that spray foaming those areas without proper moisture control will speed up wood rot.

  5. Fred says:

    Hi tom,

    Larry’s comment on our post was the first we had heard of this. I think the issue is whether the joist can breath or not if it gets water inside. A few points about this:

    1. While closed cell foam is a good vapor barrier, it’s not 100% impermeable and does allow some breathing. Larry is probably right that you wouldn’t want vapor trapped. And, vapor will rise out of the block, potentially into the wood, so you need an escape path.

    2. Since our rim joist is about 1 foot above grade, the odds of moisture getting to it (say, from ground water) are less.

    3. The joist can breath to the front of the house (it isn’t buried). Since Larry’s post I’ve looked on other spray foam resources on the web and found others that are indeed foaming the band boards.

  6. David says:

    I believe great stuff foam is flammable, it is very desirable (and may be required in your area) to use a flame resistant / flame retardant insulation if it will be exposed – less of an issue if you will cover it with thick enough drywall to create the needed fire rating. Some spray foams do have flame inhibitors to meet code, but the normal great stuff foam does not.

  7. Sam says:


    We are thinking of insulating rim joist between basement and 1st floor and also 1st & second floor. We have a finished basement and ceiling are finished both basement and first floor. We are considering blown in cellulose. Is it a good option? Please advise.

  8. Fred says:

    Sam, Do you already have drywall up on the first and second floor? How were you planning to contain the cellulose in the band board? Are you talking about wet cellulose or dry cellulose?

  9. Sam says:

    Hi Fred,

    Thanks for your response. Yes, the drywall are already up on first and second floor. This is an existing home. I did not ask the vendor if it will be wet or dry cellulose but he described that cellulose will be injected to the point that it accumulates 2-3 feets from the wall so the area around floor joist/rim joist will be packed anf full. Does it make sense to you?

    By the way, I’m not knowledgeable with the construction term. what is the band board?

    Thank you


  10. Fred says:

    Sam, I’m not sure how the cellulose would get into the rim joist area of the basement based on your description. I’m also unclear as to what you mean by accumulates 2-3 feet from the wall… Perhaps you could elaborate on the proposed project?

  11. Sam says:

    Hi Fred,

    Sorry or late reply. Things have been busy crazy here in past few weeks. I will take pics and post them this weekends. Thanks again. Sam

  12. Hael says:

    Fred, regarding your post on Dec 21, have you found anything further about this? It occurred to me also that some breathing of the sill plate may be good, but on the other hand if it’s left exposed (knowing that one side of it is in contact with cold foundation) then this invites condensation, so perhaps sealing it up is better after all. And like you said, it could still technically dry out via its wood connections (and permeating through the foam). Buildingscience indicates their favorite approach is totally sealing it up. If that happens, the only place it even could get wet is through moisture in the concrete anyway; it’s likely to stay near the concrete’s moisture level, though this is lessened by its foam sill gasket.

  13. john says:

    FRED how did you get that “spray effect” to work??? When I tried Great Stuff it clumps and rolls downward to top of basement wall.

  14. Lucy says:

    If using foam from a can remember to wet area with spray bottle before applying foam. This helps the foam cure to its area and it wont run so much.
    Canned foam is bad with gravity, shake the hell out of it and keep shaking it until you are done.

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