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In my new-to-me home the baseboard moulding was originally put in before the plywood subfloor. We are installing engineered hardwood so I pulled up the baseboards to provide the expansion gap with intention of having new baseboards added on top of the expansion gap and floor. Would it be a good idea to fill in that extra gap between the subfloor and the wall for any functional reason (i.e. heat loss) or should it not matter?

The new flooring is all being put above grade in a 1960-built, two-story cape home with a full basement. So I don't think we should be worried about heat loss because of that gap but I'm not sure.

Some ASCII art to illustrate what it would look like from a profile view with the gap left in (W=wall; B=new baseboard; F=new floor; S=subfloor).

W
W
WBB
W FFFFFF
W SSSSSS

Thanks.

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  • Is the basement heated and part of the HVAC envelope or is it an unheated, uninsulated crawl space/root cellar type of basement? If the former, then it won't matter much, if the latter, then you'll want to seal it.
    – FreeMan
    Commented Feb 4, 2022 at 12:37
  • The basement is finished and we have an heating oil furnace. The basement itself does not have any of the hydronic baseboards but I think it is somewhat insulated as it stays surprisingly warm down there even when Massachusetts hit several continuous days of 10-20 degree weather. Sounds like I might be okay without extending the subfloor on top floor at least since that is just above the heated first floor.
    – Thien
    Commented Feb 4, 2022 at 14:21
  • Is there not existing/original subfloor that extends under the bottom plate of the wall? This would be typical. If so, then there is no "gross" air gap at the edge of the floor between the basement and 1st floor.
    – blarg
    Commented Feb 4, 2022 at 16:31
  • Yes it is frustratingly unusual - the baseboard moulding and door trims were all installed BEFORE the subfloor as you can see where they cut the subfloor around the baseboard. Giving them benefit of doubt there was a plywood delay in 1959/60 and they just did other things including trim. Or the other take is that the builders just did things in a messed up order and ignored the whole functionality of moulding and trim.
    – Thien
    Commented Feb 5, 2022 at 3:08

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For aesthetic reasons and maybe for cleaning ease (dust/ debris will fall into the gap), you will want to "cover" the gap with baseboard as you illustrated. In doing so, make sure the baseboard is only attached to the wall, not the floor, to allow if to expand/contract freely.

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  • Hi. I edited my post to be more specific about concern about any functional issues such as with heat loss. Thanks
    – Thien
    Commented Feb 4, 2022 at 12:33

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