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Band joist insulation

nkhandyman's picture

My basement maintains a temp of 56' +/- during the winter (RI). Prior to the installation of a high efficiency furnace , the basement maintained a temp of 62 +/-.

I am trying to determine if I should insulate the band joist area above the foundation. I would use rigid insulation sealed with spray foam from a can.
Do you think the temp would rise to offset the cost and effort?

The basement footprint is 24' x 50'.


It should help. IRC now (post #183066, reply #1 of 7)

It should help. IRC now requires that if the basement is not conditioned, that the basement ceiling be insulated to isolate the unconditioned basement from the conditioned living space above.

I realize you;re not new construction, but thought I'd throw that out there.

Prior to the new IRC, my local code (next door in CT) required the basement ceiling to be insulated 4' in from the rim.

There are 10 kinds of people in this world; those who understand binary and those who do not.

Does it have no insulation? (post #183066, reply #2 of 7)

Does it have no insulation? If so it's definitely worth doing. I did it myself last fall. I would thus strongly advise at least thinking about buying a $300 kit of do it yourself stuff. It can otherwise take a lot of work/time.

Assuming your basement (post #183066, reply #3 of 7)

Assuming your basement ceiling is open and not finished, insulating just that area will not help. I did finish my basement and did insulate the band joist areas just as you suggest, and I felt the temperature difference, but I suspect the improvement was in blocking out the draft.
I did this: used foam pieces to cover most of the area and spray foamed all 4 edges. I used minimal expanding cans from HD. At first I wasted a lot before I got the feel for thin bead of foam. These also have to be held upside down and it was hard to control the trigger in some tight areas. I also joined two straws to make longer applicator (heating one and forcing over the other). Bigger cans that cost $300 or more will be time and knuckle saver but I don't know if they are for thin bead application. They may be for spraying the larger area.
If you don't plan to finish the basement, I would just spray the edges of the joist bays with thin bead. Then take a caulk and block the joints of the 2x wood that joists sit on. Caulk the joint between this wood and foundation.
Meanwhile, remember that all this improvement is taking away the air supply from the furnace, if your new unit does not have its own supply. Hope this helps.

I just did the same but I (post #183066, reply #4 of 7)

I just did the same but I used a kit from tiger foam. The upside to the kit is that you can be done in about an hour. It does take some practice to put the material on correctly. While mine is not horrible looking it does have some unevenness. Everything is sealed up though. Way less drafty than before. With that said, the prep work to get to the point of being able to spray is quite a bit. You will have to clean the whole rim joist area, wipe down and get rid of any dirt. You want the foam to stick to the wood, not any dirt. Downside with the kit is that is was $335 + s/h (about $50), and you need a good quality respirator (I already had one but they are only $30), and old clothing or a tyvek type suit. Youll need goggles too. Spending the 430 bucks may be worth it for somebody not wanting to do the labor with rigid foam.

With the rigid foam approach, lots of labor. I would think that you would cut the foam at least 1/4" short around all sides so that you can foam in. getting a foam gun might be worthwhile but that is your call. I would do it like this. cut all my pieces first. Clean the rim joist as you go along. Get the right glue made for rigid, Apply the glue to rigid foam then apply to rim joist. Then screw the rigid foam to the rim joist with screws that have the extra big washers. Only need screws that will go in an inch or so to the rim. Once that is set, you can then foam it all in. I think using the screws just makes it easier. This is overkill but it will be done right this way. Price difference will be about half compared to spray foam but labor will be a lot more. You may need a compact right angle drill for the screws.

Downside either way is that if you leave spray foam or rigid exposed, technically you need a fire break of drywall. with spray foam, even though tiger foam is fire rated, it will still burn because it is an organic material. There is a paint called intumescent paint that can be applied over it if you do not plan on finishing the basement. It will provide an ignition barrier. Don't know if you can apply this paint to XPS type foam. the paint is not cheap, and by not cheap, I am thinking it will be something over $50/gal.

Will the temp rise? Maybe. You will have less drafts. If you have an unfinished basement, the concrete is a heat loss item as are the concrete walls. But keep in mind that you are also helping to lessen the stack effect in the home.


Agree with last about rigid (post #183066, reply #5 of 7)

Agree with last about rigid approach. In my case I left the 1/4" around so that I could spray great stuff in that gap and have it expand the 2", this way everything was nice and sealed up. I didn't use any screws, just cut the foam, PL300 glued it to joist, waited at least 24 hours, then great stuff. It is an awful lot of work to do right, especially if you want to do additional sealing like at the top of a hollow cinder block wall. All in all it took a great deal longer than I thought it would or than it should have.

I'd love to know if there is a paint that is a suitable fireblock and applicable to XPS. I'm otherwise looking at PL300 some slabs of drywall right onto my foam. Not sure if inspector will demand it or not but once I get framing in place it's going to be much harder to do, especially around my rim joist which I barely have any room to get to.

The walkout basement of my (post #183066, reply #6 of 7)

The walkout basement of my 1950's ranch house in Southwest Michigan had no insulation whatsoever. In the winter the basement is always very cold especially when we burn wood in our wood-burning fireplace insert and the furnace does not run for days and add heat to the basement*. As a first step towards insulating our basement, I insulated the rim joists with 2 inches of closed cell foam from one of the $300 DIY kits. I then added about 12 inches of unfaced fiberglass insulation to fill in the rest of the void especially to cover the top of the cinder block wall on which the joists rest. It has really made a huge difference in the temperature downstairs and nearly eliminated most of the sources for the air infiltration problem we were having. I did the whole basement (about 120 lineal feet of rim joists) including prep in about 4 hours. It is a great weekend job that makes a big difference.

BTW I have checked for condensation by pulling some of the fiberglass batts out and found no condensation at all.

*nowadays the basement levels off at around 54-57 degrees when we don't run the furnace for a long period time. I also added 4 inches of rigid foam board to one side of the exterior basement wall where a previous owner had added on an unheated garage for storing lawn equipment and boat stuff. This effectively has brought the thermal mass of the wall inside the thermal envelope which also helps keep things a bit warmer.

It will absolutely help. (post #183066, reply #7 of 7)

It will absolutely help. Primarily in blocking out drafts ("air infiltration") and also by reducing the rate at which heat leaves your basement. It's cheaper than ever to simply hire a spray foam contractor to do the job with closed cell foam, but if you want to save a few dollars you can do it yourself with XPS and canned foam.

Believe it or not, it's also worth it to insulate your foundation walls. A recent energy audit we had done for a big remodel project showed a payback period of only three years. Of course the client decided to upgrade the bathroom finishes instead, but that's another story....