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There's a gap between some of my floorboards and the baseboard:

overview

The gap isn't just cosmetic. I used a wooden skewer to probe the hole and was able to insert it around 6 inches in:

skewer

This is an exterior wall, so I believe that means this gap is open to the stud cavity of our exterior walls. This is a 125-year-old balloon-framed house, so even though we have blown-in cellulose insulation in the walls I'd like to better seal it to reduce air leakage.

This need is compounded because I'm planning to install baseboard hot water radiators along this wall. That will require me to remove the wooden baseboard, which will presumably make the gap even bigger. I'd like to install something to seal it better before I attach the baseboard radiators.

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4 Answers 4

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Expanding foam would do a fantastic job especially if you are already removing the baseboard anyway it should be no problem to fit the straw back there.

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  • Is there any concern around having foam right behind a radiator? My system typically heats to 140F but could go higher, theoretically up to 180F.
    – Shimon Rura
    Commented Jan 24, 2018 at 22:24
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    Probably not, but if you're worried some are rated for use as fire blocks between stories. 3M states theirs is heat resistant to 240F for example.
    – Matthew Gauthier
    Commented Jan 25, 2018 at 7:31
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According to the manufacturers of various spray foam products, NEVER use near a heat source, as the product a) is very flammable and b) degrades under heat and produces toxic fumes. I made the mistake of using a spray foam near my baseboard radiators and discovered the product turned brown and brittle, i.e., burned.

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Is this house raised on wood joists or on a concrete slab?

I am guessing wood joists and your issue is bigger than what you think. I believe you have some settling in your foundation and or issues with joists.

You might need to jack some points up or replace some supports.

I will add this picture to demonstrate. Your baseboards are against the wall and lengthwise - they do not sink, your floor boards are sinking. So in your situation what can cause the floor boards to sink (rise and fall)..take a look at example 'A' & 'D'. I provided this as an answer because you want to seal it up .. well there are two options: To fix the cause of this gap or to patch around it and hope the cause does not become worse.

Balloon Framing

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  • The house is balloon framed studs over a parged fieldstone foundation. It is old and has had some settling over 125 years, but is currently structurally sound. This gap has probably existed for decades and may date to whenever this current top layer of flooring was installed (maybe it replaced something that was tighter under the baseboard).
    – Shimon Rura
    Commented Jan 24, 2018 at 17:45
  • That's not the cause of this gap. This gap is an artifact of the way the floor was installed or modified. This is an exterior wall on the 2nd floor, so the floor joints are nailed into studs and held up by a ledger. The joists here run perpendicular to the baseboard. For this part of the floor to sag there would have to be widespread failure of the attachment between the joists, studs, and rib band. That would be a huge obvious problem and doesn't apply.
    – Shimon Rura
    Commented Jan 24, 2018 at 19:27
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use clear caulking or add shoe molding

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    Welcome to DIY.SE! This isn't a very good answer. Can you elaborate so the person who asked this can know more of what you're describing?
    – Machavity
    Commented May 1, 2019 at 19:43

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