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