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enter image description hereI have a detached mother-in-law suite being finished. The exterior walls have three 2x6s stacked. I believe the bottom is the sill plate. The anchor bolts go through there. Then there is a second layer. Each anchor bolt has a ~3-inch gap around it in this level. Then a third layer on which the wall is built.

The gaps in the second layer with the anchor bolts go through to the exterior siding. This seems like a likely ingress for rodents, not to mention drafts.

Is there any issue with me filling these gaps with moderately expanding foam? There are also some gaps under the sill plate where it interfaces with the concrete. I assume I can fill this with steel wool or something else to keep mice out?

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  • If that's reasonably well sealed off on the outside by the sheathing you should be OK. A bit of insulation (expanding foam is fine, just cut off any that expands past the sill plate) won't hurt to keep it from being cold.
    – FreeMan
    Feb 5 at 19:15
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Get yourself a can of pestblocking spray foam and feel free to fill in that gap. You can just shave off any excess with a putty knife or oscillating tool. If you don't have pestblocking foam then honestly any foam should be fine; even the high-expanding big gap filler.

enter image description here


If you want something that feels more permanent then cut an appropriate piece of dimensional lumber (2x4, 2x6, 2x8, or whatever) and screw it in place from the top using 2 1/2 or 3 inch screws.

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I'm really not sure why there are three bottom plates. Even if the treated board is a sill, doubled bottoms are not common, in my experience. However, I can't think of a reason why you couldn't insulate that as you describe.

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    Think second board was used just as spacer so they would not need to notch/place holes in third board for anchors bolts, if they did that for every bolt.
    – crip659
    Feb 5 at 22:07

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