After months of researching radiant heat in my new basement and garage decided to go with XPS 2 inches under the concrete slabs and around the perimeter. I'm in SD and have a lot of clay in my building area. The concrete guys showed up today to grade the basement and said that the floor slab was an integral part of the wall integrity and didn't like the 2 inches of xps that will be between the slab floor and the concrete walls. Now I'm worried about my walls moving and/or cracking. I used 25 psi foam - what are others thoughts on how much trouble I'll have with concrete in order to enjoy my in floor heat? All replies much appreciated. Thanks, SDP
xps will weaken my basement wall? Help!
- Thread starter SDPrairie
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cvalentine
Member
Yes, you may have some problems, but first, a couple questions. How deep is you footing from ground surface (grade), and how deep below grade will the slab be? How high is the water table? Is the footing directly on the clay? You have the inside of the footing wall insulated, and none on the outside? Is the slab going to be directly over the foam?
nehog
Well-known member
I may be wrong, but you'd want that 2" XPS on the outside of the walls, not between the slab and the walls. Basically fill, XPS, wall then slab.
OK got more info. The footings are 6-7 feet deep. The spouse and I can't remember whther they poured on clay or put some gravel down but think they poured on clay. It's been a wet summer and the water table is about at three feet. When studying up on insulating the basement most people agreed that putting the insulation around the putside of the basement walls rather than inside was best from a thermal standpoint. In practice protecting the insulation both in the ground and for the distance between the ground and where the wood frame sits and not have the house look like it had a trailer skirt meant practice trumped theory. It is common to put xps on the inside of a basement walls around here with drywall over, so we thought we would insulate on the inside walls from the slab to the ceiling. There will also be 2 inches of xps under the slab. The question becomes how to tie the perimeter of the slab into the walls and not have 4 inches of concrete on concrete where the slab meets the wall. There is about a 4-6 inch "lip/ledge", or foundation, of concrete running around the base of the wall. Unfortunetly there is about 40 ft along one wall that only has a 1.5 inch ledge. The concrete will be poured to cover this ledge in four inches of concrete. Could we put at least 1 inch of 25 psi along this ledge? Thanks much for the answers.
cvalentine
Member
Check with the designer, if your floor is structural, then it has to be tied directly to the stem wall. Basement slabs are usually non-structural, and if your slab is, putting insulation between the stem wall and slab should be fine. The contractor will want to use the lip of the footing to cast the slab to, to reduce the risk of slab settlement. 25psi seems low to me, so you might want to check to make sure it won't be crushed.
If it is structural, then your only option is outside insulation. The XPS insulation is also typically buried on the outside of the foundation because you don't want to risk freezing under the foundation footing when you're in a frost-heave susceptible soil with high groundwater. It might not be a issue in a warm climate like yours
The insulation goes from the base of the footing, up the stem wall, to above ground, where it's covered with a flashing so it looks good and keeps some of the water out. Installed this way, the outside insulation extends above and below the thermal gap when the slab is tied to the wall, so the gap is insulated by overlapping inside/outside insulation.
If it is structural, then your only option is outside insulation. The XPS insulation is also typically buried on the outside of the foundation because you don't want to risk freezing under the foundation footing when you're in a frost-heave susceptible soil with high groundwater. It might not be a issue in a warm climate like yours
The insulation goes from the base of the footing, up the stem wall, to above ground, where it's covered with a flashing so it looks good and keeps some of the water out. Installed this way, the outside insulation extends above and below the thermal gap when the slab is tied to the wall, so the gap is insulated by overlapping inside/outside insulation.Thanks for the reply. It was the concrete guy that that recommended against the XPS between the stem wall and slab. I will ask him specifically whether the floor is non-structual or not. There is a difference in stem width around the perimeter. 5 inches along one 60 foot wall and only 1.5" along the other 60 foot wall - uneven pour or some other reason I don't know. Will th at make a difference?
Did you mean 250 PSI foam?
Diesel Dan
Well-known member
My brother lives around Cicero, NY and in his relatively new home they still used a "floating slab". Meaning there is a 2-3" gap between the slab and walls that runs to the sump crock. Makes no sense to not keep the water out of the basement in the first place. He wanted to tile the outside of the basement wall and not do the floating slab but the contractor simply said "this is how WE do it here".