Currently fitting some new skirting board, I have the opportunity to fill some gaps that exist between the original floorboards and the brick wall. (1930s build). I'm considering squirting some expanding foam in this gap (mainly for insulation, but also mice) before fitting the new skirting.

My question is whether I should. Is there's a good reason to leave this gap and not fill it in? Possibly the gap is there for a purpose. Ventilation, damp etc?

gap between wall and floor boards

Not easy to see in the image, but may help to describe. That's a modern laminate floor on top of the old boards. The large gap is one of several all around the room where a brick has been left out probably deliberately. It's not this I am considering filling, but the thin (1 inch or so) gap indicated by the yellow arrows.

Below the floor boards is a void of about half a metre until you hit soil, which seems to be dry. There's no cellar and I have no access down there without pulling up the floor. None of the walls are external, except one used to be before an extension was added.

Fill it or leave it?

  • Are you talking about the "Great Stuff" foam? It expands quite significantly. For hardwood floors that gap exists for expansion/contraction of the flooring and of course because it would be a real pain to get flooring perfectly up against a wall for little to no reward. Consider what will happen when the laminate floor needs to be replaced; the expanding foam will have molded itself around the flooring and will be annoying to pull out let alone cut new flooring and jamming it into the foam. As for sealing it in general, I see no reason not to.
    – MonkeyZeus
    Apr 21, 2021 at 12:18
  • The foam is the type that expands significantly yes. I've used it before and so I know I'll have a job keeping it away from the laminate. The plan was to keep it no higher than the level of the old boards. Happy to consider other materials that are easier to control though.
    – Tim
    Apr 21, 2021 at 12:40
  • If your main concern is to prevent rodents/bug from living in the channel then consider just foaming off the corners of the room and obviously that square opening in your photo. For better control you can consider low expanding foam which is used around windows.
    – MonkeyZeus
    Apr 21, 2021 at 12:43
  • 1
    As for the gap from the wall to the subfloor you can push foam backer rod into the void and seal it off with silicone/caulk.
    – MonkeyZeus
    Apr 21, 2021 at 12:46
  • Window & door foam would prevent buckling & most messes, but it doesn't go very far. I'm not sure how any of it does against rodents, though.
    – isherwood
    Apr 21, 2021 at 13:55

2 Answers 2


It will do no harm to fill the gap. You may find that there are vermin proof/resistant foams on the market. If you use an aerosol dispensed foam, you can place the tip of the dispensing straw to stick a little onto a vertical surface below the floor and let that cure. Then using the cured foam as a shelf to hold the next application of foam to fill the remaining gap.

It will help reduce drafts too.


I had a issue with air from a vent was causing air to rise and cause damp, I had all the floor filled with foam and backed with wood, prevented damp and a draft.

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