I've attached a drawing for my foundation. It calls for 10" Nudura ICFs on the perimeter, which have 2" foam on the internal and external surfaces. The top of the internal foam will be cut away to allow for the cement slab to be continuous with the perimeter foundation. See this in the attached file... My question... is the slab properly insulated this way? Heat could theoretically move into the perimeter foundation and go down into the bedrock on which it sits, but I'm not sure if this would be a significant heat loss.

Your design looks good, although NUDURA has 2 5/8" EPS each side. If your frost-walls go below the frost, your fill under the floor will maintain a more consistent temperature.

But if you want a better design and much more energy efficient, use 4" or 6" NUDURA on your above grade walls too. It simplifies the process too.

Integrate a shallow frost protected foundation design. Basically run an insulated skirt around the whole thing. A 4' skirt of foam board insulation will change the frost line around the whole building and decrease the heat loss. Not stop it but reduce it.

ICFconstructions suggestion also sounds good.

Jimmery
you asked "is the slab properly insulated this way?" No. You should have a thermal break between the floor and any cold concrete - at least 2" of foam. There is a mathematical way to detwemine the amount of energy you will lose through that thermal break but it is too complicated to show here.

This thermal break requirement applies to any concrete slab in any insulated living space.
Bob Irving
rhirvinghomebuilders.com

Bob I,

Are you saying that the foam that is under the concrete slab should have extended across the top of the ICF blocks or are you saving that the foam should be under the footer also.  That is the only two places I do not see foam insulating the slab.

I don't understand this either. Insulation already surrounds almost everything.

I think Bob means that the underslab insulation should extend over the top of the ICF concrete and contact the outer ICF foam, therby isolating the slab concrete from the foundation concrete. I can see this as a good idea in colder climates. Alton, I doubt it would be necessary in Alabama, and maybe not even here in KY.

Bob I is correct, but it is not an energy code requirement everywhere. You can get a fair amount of heat loss from the slab through the foundation and into the bedrock. But it will be only to the extent of the temperature of the bedrock, and as Wes pointed out, it's probably not a problem where you are. But in areas where it is a concern, or if you are concerned, a floating slab on well compacted soil separated from the foundation wall would probably be a better thermal answer.

So the dowels from the ICF will pass through the insulation and up into the slab? The exterior walls will be sitting on a 4" slab with 2" foam underneath ? Is this structurally sound? I thought the walls should sit on the ICF foundation, without foam separating the two. I've attached a new image of what I understand is being proposed. The builder is happy to do as per the original plan, but if there is a better option, I would rather propose the change. Bye the way, this if for Ontario (by the Minnesota border), so a nicely insulated slab is priority.

I tried to get that one past my building inspector and he would not allow it. I wanted to cover every possible cold bridge area. He did not like that. I would say because it is not what is traditionally done. Not a good reason but that doesn't matter to the person that has complete control.

I wanted to insulate either under the footing or between the footing and the slab. No way. Not going to happen. Did not matter if I had a good argument.

Just a cautionary tale. Check with your inspector before going too far.

Geofoam is used under highways.  I see no reason why it can not be used under a footer.  It may take an engineering stamp on the plans but depending on the weather it may be worth it.

Check out Highload 60/100 or Foamular 600/1000 XPS foams. They are used in commercial slabs with heavy loading often. They will handle 60psi and 100psi respectively.

You will very likely need an engineer to sign off on it, find someone who does commercial building. They recemond a safety factor of 3 for static loads and 5 for dynamic loading. This means a 60psi foam will support 20psi (2880psf) static load.


Hope that helps, we are doing this but in a raft style foundation, so it's a bit differant application.

No, I wouldn't do it that way. You're right, it's not structurally sound. More like the first detail, but with the foam going all of the way up. It's a trade-off between insulation and detailing difficulties.
Your anchor bolts seem a little clost to the outside if your scale is right. One of the inherent problems with having stick-frame on top of ICFs. Especially if it's 2x4. Might want to check into that.
Foam is fine under most any slab or roadway. But I wouldn't put it under a footing of any sort.

I have a strange idea. I may post it later to see how it flies.

How deep is the bedrock you're sitting on?

I'm not sure how deep the bedrock is, but this is "Canadian Shield," (rocky terrain) several hundred feet away from the shore of Lake Superior. I'm guessing the bedrock should go down quite far. The well isn't drilled yet (coming soon), which would have helped. Why do you ask?

Since you wondered about the walls, they are 2X6, with OSB and 1" SM foam (R-5) for sheathing, R-20 batts between studs, and fiber cement siding.

If the bedrock is below frost level, then this is similar (but far less area and more distance) to insulating basement floors - doesn't make much difference if you aren't using radiant floor heat.

Maybe you could put the foam on top of the slab?

jonr,
The bedrock is not below frost level--it is exposed at ground level, and the house will sit on top of this. Heating will be with a hydronic radiant floor, and also by lots of South-facing windows for passive solar. The insulation will need to stay below the slab.

I'm no expert here, but it seems that you have a non-structural slab that is connected to bedrock on the edges and only sand in the middle. Seems like this is asking for major cracks. If the slab didn't extend over the beams, then the slab would be more consistently supported (and you could fully insulate it).

Jimmery,

I have a situation that is close. In my case a flat concrete roof deck on ICF walls. I plan to use a cource of Y-Tong blocks at the top. This material is AAC has compressive levels of approx 800lb/in. The R-value is 1.2 per in. With a 8"x8"-24" block, R-9.6. This will be a decent thermal break. In my case, having a -20F roof deck attached to my 70 deg walls would have been a major heat loss. Your 40-50 deg footers won't pull much heat, delta T
In your case, you could do the same thing with AAC.

While searching for a solution to this issue, I did find two other options. One was a footer form made of high density foam. Saw it on a European green building site. The second was a foam glass brick. It was shown being used as a first cource on top of the footer. Only avalible in the UK

http://www.foamglas.co.uk/building/foamglas_perinsul/

Y-Tong, Safecrete and Arcon are all AAC options

Patrick T

Thanks, those ideas above have got me thinking. PatrickT--those blocks are nice but it would be too difficult to scribe them to the contours of the bedrock. The ICF foundation is starting this week, but we (builder and myself) haven't come up with anything better. I wish I knew how much heat loss this is addressing...

Anyway, I put jonr's idea to use with the ICF foundation. To summarize, the 2X6 exterior wall is kept in place, but the ICF foundation is moved outwards a few inches. Then there is room for 2" foam to surround the bottom and side of the slab floor. Hopefully the builder will say this could work. I guess it boils down to how much contact the 2X6 wall needs to the foundation below it. The rebar coloured in red might not be necessary anymore, since this is a floating slab now.

Jimmery,

I am very interested to know how you made out and what detail you ended up going with. I have a site in Kenora ON with exposed bedrock and will be building somthing similar next spring. (slab with radiant heat, footing scribed to bedrock with compacted fill and slab)

Did you end up just going with compact fill under the slab and no supports. My deepest spot might be 3 or 4 ft from Bedrock to the underside of the slab. Did you have anything comparable, of was the bedrock you built on free of major impressions?

Let me know what you did and how everything went, I have lots of questions you may be able to help me with.

Did you go with a floating slab or was it tied to the icf footing? any concerns with settling of the slab?

Thanks