I have a 40 yr old single story modular home set on a block foundation. I used a thermal temperature gun to verify a temperature difference. the insulated wall is approximately 1 to 2 degrees cooler than the room ambient. I consider that normal with 6” exterior stud walls. Problem is I’m finding at the baseboards the temperature to be 10 to 12 degrees colder than room ambient. (Typically when outside temp. is below freezing) This is attributed to cold air transferring into the home.

The interior floor joists are well insulated in the basement. I removed and inspected a few locations but find no gaps to exterior. Typical interior insulation as current is as shown in this photo.

enter image description here] Fiberglass batt with foil backing between floor joists.[1]

Investigating the exterior, i find there is a 1/2” gap between the rim joist and the original exterior lap siding. You will note in the photo a metal siding was applied over the original vertical lap siding.

enter image description here] Gap/ typical[2]

In the photo, you can see the gap which had what appears to be a foam rubber type insulation. Appears this foam rubber insulation has fallen out or disintegrated over time. Even vermin of some type or field mice could be the cause.

I have the original construction drawings and included two copies of the detail. It does not indicate any type of insulation within this area. Yet the gap between the rim joist or so called Sill Band exists all around the house perimeter on exterior.

enter image description here

enter image description here

My question is could I use a spray foam to fill in this gap? I was thinking maybe Great-Stuff window and Door spray foam would be a good choice. If an alternate would be advised, please advise? Especially since the spray foam would be difficult to remove after the application. Yet may be the best deterrent to mice, etc.

I do not foresee moisture being trapped, as metal siding would provide the shedding any rainfall, etc.

  1. Follow up for clarification: Appreciate any thoughts. I am not concerned with risks for cleanup using spray foam. The picture provided is the only side of house you can actually see the gap. It is on the one side with garage doors. (Garage is part of the basement.) The rest of the house siding is only about 6” above grade. Would be very difficult to manually pack the other 3 sides of the house. Would almost need a mirror and go by feel to find the gap. That is why I am considering spray foam.

Would spray foam be a concern for trapping moisture between the rim joist and exterior sheeting? Also I have used spray foam before and worked well to stop rodents. Not bullet proof, but certainly is a good deterrent.

  • You really ought to give those spray foam products a flame test before you think about using them in a home. The cheapo ones you named are basically napalm. In that location it would dramatically accelerate a fire. Commented Nov 7, 2022 at 7:50

1 Answer 1


I think spray foam will be a bit of a mess applied this way, you'll get drips, and it's incredibly difficult to remove the residue if it runs down the wall.

I would not try to seal this up air tight, I'd just rather make it less drafty but still permeable. You may see a small difference in insulation but if it winds up trapping water or vapor somewhere unexpected, it won't be worth it.

I think I'd pack this space with mineral wool insulation and put some kind of flashing or trim over it. Drip edge might work for that, it's inexpensive, readily available, and easy to apply. The rock wool won't trap water or vapor, wick water, rot or grow mold.

I think mineral wool would be at least as much of a rodent barrier as spray foam, which isn't hard for rodents to get through, but you could use stainless steel wool instead of mineral wool if you wanted a filler that's really rodent resistant.

  • 1
    I added clarification note #1. Appreciate your thoughts.
    – user97590
    Commented Feb 25, 2019 at 14:07
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    @batsplatsterson this stainless steel wool sparked my interest since I almost daily punch holes in people's houses (hvac) I looked into it on google. Excluder and Elephant Brand amazon.ca/COARSE-Stainless-Steel-Wool-Roll/dp/B00EDLQCI6 came up it's very expensive. I like this stuff as customers are always like "can mice get in there?" most of the time I core holes with a pretty tight tolerance but it doesn't always work out. I don't mind dropping a couple of hundred bucks on something I deem worthwhile. I just want to make sure it's the same stuff you are referring to.
    – Joe Fala
    Commented Mar 28, 2019 at 3:05
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    @JoeFala - That one might be listed wrong or something, it's available much cheaper. There are some marketed specifically for pests such as "Xcluder" and "Pest Plug" but you can use regular coarse pads such as amazon.com/Stainless-Steel-Wool-Coarse-Manufacturing/dp/… too. Commented Mar 28, 2019 at 10:24
  • 1
    Perfect! My mind is actually blown.
    – Joe Fala
    Commented Mar 28, 2019 at 12:28

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