Insulating heated slab where foundation is raised in parts

Discussion about insulating slabs and slab edges

Insulating heated slab where foundation is raised in parts

Postby jmarshallnz » Thu Jun 27, 2019 12:07 pm

Hi there,

First off, excellent set of tools and great to read through the forum - it's been most useful for learning how things work!

We're planning a build at the moment on a hilly site. It'll have garage/entrance underneath and concrete slab on top, partially suspended over garage. At the end that's not over the garage, the distance from top of slab to ground is about 1.8m at extreme where the hill starts falling away. Rest of it is mostly around 300mm above ground level. The slab will be heated, walls 140mm timber, located on Kapiti coast.

My question is how do we best insulate the slab? Underneath is easy enough - can go with probably 100mm EPS given it's heated (soil is sandy, so probably pretty dry). Perimeter insulation we were considering something like insulfound (66mm XPS), which goes to heights of 1.2m, replacing boxing. Presumably it'd be worth insulating the entire external face of foundations, which I guess we could do with extending the insulfound down to bottom of foundation using E/XPS? Would it also be worth insulating the interior face of foundation? e.g. using something like ICF for the foundation walls, with insulfound or similar perimeter insulation up to the wall framing?

Thanks for any and all input! :)

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Joined: Thu Jun 27, 2019 11:28 am

Re: Insulating heated slab where foundation is raised in par

Postby Albrecht Stoecklein » Mon Sep 09, 2019 2:09 pm

Hi Jonathan,

Firstly sorry about my late reply. I hope my answer is still relevant.

The situation sounds a bit complicated. I wonder whether you could upload some drawings. Alternatively please feel free to e-mail me directly with some drawings.

But if I understand your details correctly then your questions all relate to the upper suspended slab, not to the garage slab. If that's the case then I would say that the exterior vertical perimeter insulation which goes down to 1.2m from the slab would be sufficient. I don't think you need to extend that down the full height beyond 1.2m.

But the air space under the slab and 100mm EPS would still be cold because it is not completely enclosed and insulated (at least towards the garage end). So that cold air still has access to the inner side of the foundation and from there directly into the slab which is only 100mm away at the closest point. Therefore I think it would be useful to also add some insulation to the inside of the foundation.

Just a note: This situation is different where you have a slab on ground. There the assumption is that the ground under the house will warm up over time because of the immediate contact with the slab. In your case the ground is not in direct contact and the air in that space can move and won't worm up over time.

I hope that helps

Sunny regards

Albrecht Stoecklein
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Joined: Sat Jun 13, 2009 5:30 pm

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