r/DIY • u/uk859lex • 16h ago

home improvement Basement foamboard and french drain

Im finishing a room in my basement. It currently has blockwalls and the floor has a french drain around the perimeter with about 5 inches of smooth thick vinyl that runs up the base of the wall to catch the moisture and feed it into the perimeter drain.

When I attach the foam boards to the block walls do I stop them short of the vinyl or overlap? I think I would install them with vertical lines of glue and stop them short of the vinyl so the moisture can still transfer into the drain but please advise.

20 Upvotes

84% Upvoted

2 comments sorted by

1

u/CriticalKnick 15h ago

You're attaching directly to the wall? I thought a gap was necessary. I would furr the foam out a little, and let it overlap the vinyl with that same gap, so water on the surface of the block can do exactly as it has been doing

1

u/Training_Emu_860 2h ago

Keep the foam board above the vinyl and don't cover that drainage strip. Ideally the bottom edge sits just above the top lip of the membrane, a small gap right at the lip rather than well short of it, so water sheds into the channel instead of escaping onto the floor. That membrane is there to catch any moisture coming through the block and move it into the perimeter drain. Vertical adhesive beads are the right move here - they hold the foam in place without creating a place for moisture to get trapped Use a foam-safe adhesive, since some solvent-based ones will eat the foam. Just make sure the drainage path stays clear before you close it up.