Our exterior front door sits on what was once the house's original front porch. Those old floor wood planks extend under the front door's threshold, but there is a noticeable gap between the planks and the threshold. You can see sunlight through it (photos attached). What might be best way to seal that gap to prevent drafts and critters from crawling in? Spray foam?
If the sill is wood AND the exterior mating floor is wood, use "WoodEpox". The stuff is terrific for exterior work. Mask off the face of the sill and the mating floor surface. Also, you can use JB Steel two-part epoxy to seal it. Prep surfaces well using hand and/or light machine sanding. Brush/vacuum the sanded areas before proceeding. I just did our original 93-year-old covered back porch using both sealants. They actually restore areas that are in need of restoration, even the 7/8" tongue and groove oak floor! Paint as needed afterwards.
I would use urethane caulk.
Use painters tape to mask off the floor boards and the threshold and then apply caulk with a caulk gun, Just enough not to much.
Then dip you finger in Denatured alcohol and gently smooth it out by running your finger along the bead a couple of times to remove excess and make smooth.
Remove tape repeat with the moistened finger, carefully and lightly, just enough to smooth out the lines left from pealing the tape off.
You will still have the seams between the boards but the main gap between threshold and boards will be sealed. You may try to force some quad into those seams, Tape them off first, the quad is difficult to clean off.
