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