Thermal break between foundation & slab
davidmeiland
| Posted in General Discussion on
Two story house, 8″ stemwall, 2×6 exterior walls, slab-on-grade first floor with the slab exposed and stained/sealed. Apparently a thermal break is required between the stemwall and the slab, which I guess means insulation of some thickness although I have not investigated the exact requirement yet.
If this is a two-pour job, with the footing and stemwall first, then the slab within the walls, two problems. One, how to create the needed thermal break, and two, how to avoid seeing a bit of the top of the wall around the edge of the room?
If this is a monopour (which I doubt, it’s not a very flat site), then how do we accomplish the thermal break?
Replies
I have built some houses like that... Attached is a drawing. It was something that went in with the building permit apps, but doesn't show the grade beams and rod chairs that were used. I think it will still give you an answer to a few of your Qs - or at least some ideas.
The insulation I used was 1" borate treated EPS rigid foam. I imagine the thickness would vary depending on your climate. I think you would look at the foam as more of a thermal break rather than actual insulation.
Edited 8/8/2006 8:04 am ET by Matt
You got away with murder on that job.You really need foam in both places.And the edge foam needs to be much deeper. IIRC 18-24" below grade.
Maybe not Bill. The perimeter insulation need only go to the frost line depth. Same holds for the 2' under the slab. It may be enough for the area in which he is building.
Here 30" for perimeter and 4' under slab are nominal. Go north to those 40-48" frost lines and 2' isn't enough. The purpose of the perimeter insulation is to insulate the soil under the slab.
Dave
Where does the bottom plate go in that drawing? I assume it's inboard of the brick so that it covers the insulation and sits on the slab....?
The wall plate goes on top of the brick with the sheathing flush with the outside of the brick. For the slab edge perimeter insulation, if you are using 2x6 walls it will cover the insulation. 2x4 walls and the insulation will just peak out from under the base but as long as you are using floor covering it doesn't matter and especially in areas that get shoe mould. I've tried both ways of installing the insulation. Each has it advantages/disadvantages but I perfer the insulation layed flat method. There is no frost line here.
search http://www.buildingscience.com for "slab insulation". you'll get several different building types and option for this.
The break is usually 2" of extruded polystyrene (blue board).
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David, I have seen people in our area pour slabs in the living area flush with top of the concrete stem wall. This however only works with traditional floor coverings, and not with stained concrete. Also, you really should have a thermal break between the stem wall and slab, but alot of inspectors in certain areas of western washington don't call you on it. (moderate climate)
Consider bucking out the top inside of the stem wall so that you have enough room for the thickness of your slab and so that it can be poured flush with the inside of your finished wall. In theory you only need to take 2 inches out of the inside. Put a piece of half inch rigid between the stem wall and your slab. Also, 2 inch rigid is what I've seen people use as the buckout material in their forms for this situation.