I’m in the process of gluing 2″ extruded foam to my basement walls with vertical glue beads. I have an interior perimeter drain (meaning there is about a 1/4″ gap at the edge of all walls/slab joint for water to drain into gravel for eventual dissipation/pick-up by sump pump.
My question is what should I do at the bottom of these? I’m sealing the heck out of them along all lines and at the top near the joist bays. Should I seal the bottom of this foam to my concrete floor? I guess not in case I ever get some water on the floor as it would have nowhere to go. That appears to be what insofast company says to do, though.
Or should I seal the bottom edge of the foam against the wall so that it’s “airtight” against the wall on all edges? Not sure about hydrostatic, pressure, though, so I’m leaning simply toward leaving that bottom edge unsealed entirely.
Replies
The correct solution depends on the potential for water in your basement. Water entering through the walls, water entering somewhere on the floor or water entering through the slab/foundation joint.
I saw a good detail used by Building Science Corp on a Deep Energy Retrofit project.
Dimple mat over the old concrete floor rolled up onto the walls by 6 inches or so. Foam board on walls as you did (or spray foam. Then a couple inches of XPS over the dimple mat on the concrete floor and a new concrete slab poured atop.
The idea was to let the water keep the water beneath/behind the insulation on the floor and walls.
Since it doesn't sound like you'll be treating the floor with insulation then I think you could either seal the inside edge of the foam to the concrete floor if you think the slab won't leak. Or leave the joint opened if you think water may come through the slab.
The main reason to seal the joint between the bottom of the foam and the slab is to prevent moisture laden air inside the basement from contacting the cold concrete walls and condensing. The foam board is the air barrier and sealing the joints and perimeter completes the seal.
Thanks for the reply. My slab is pretty dry based on what I've seen with some sheets taped down and my sump pretty much never runs, so if I seal the bottom edge to the slab I'll have perfect wall drainage but zero in the room.I've not yet decided what I'll use for the floor, either some kind of a mat or dricor squares.Technically it will be impossible to seal up the walls perfectly because there is a room I'm leaving unfinished and as I break from the new to the old, regardless of what I do around the perimeter drain gap between the old and new I'll still get air mixing.Since I'm sealing the top and edges for the foam insulation, would I not be almost wasting that effort if I don't at least try my best to seal the bottom of the foam in some manner?The one benefit to my approach is that as I build the stud wall 1/4" or so away from the rigid insulation, that entire gap behind the stud wall and rigid should be somewhat treated by the rest of the space and my dehumidifier should be working on the gap although air movement won't be huge.Buildingscience pics clearly show a sealing. However, Black & Decker's 2009 basement book which says to use foam specifically says to not caulk the bottom edge!
Edited 11/23/2009 2:48 pm ET by Skoorb
I think you're safe either sealing or not sealing the bottom edge since you are sealing the top. BSC shows lots of details sealing the bottom edge and I'd trust them more so than B&D - they've been doing a lot of Deep Energy Retrofits the past number of years and have tested a lot of different approaches.