Under slab foam board placement

[StefanNH]

For those of you with thickened edge slabs single pour slabs (Alaskan slab), did you put foam board down in the footer or haunch?

From my research I've seen it done both ways. I'd prefer to put foam board down over the whole thing as I want to minimize any "thermal bridging" where my radiant heat pours heat into the subgrade. My concrete finisher is advising against it as it is very difficult to get the sloped section flat enough, which would create voids under the foamboard and could lead to slab failure.

Thanks,

 

[jasondavidmann]

I have a thickened edge and did not put any under the footer. I couldn't find much information regarding it and didn't want to risk anything down the road. Admittedly I'm a mechanic and not a concrete guy and don't know what I am doing. Having said that my inspector didn't blink and my concrete friend didn't say a thing. It has been a few years and I have no regrets.

 

[AEAdam]

The equation for heat transfer is delta T X area. The delta T under a slab shouldn’t be that high. Air temps are typically colder than ground temps in cold climates.

So thermal bridging is real, but if the area is small, don’t worry about it. In the end, there are always losses. You size your heat source for them.

When you ride a bicycle, the reason you constantly pedal is to overcome the losses, caused by rolling friction, aerodynamic drag, etc. If you limit the losses, you have to pedal less. Make sense? You can’t get rid of the losses.

 

[StefanNH]

Admittedly I'm a mechanic and not a concrete guy and don't know what I am doing.

Same!!! Lot of people smarter and more knowledgeable than me on this forum. They way I see it the more information I can get the better.

 

[StefanNH]

You can’t get rid of the losses.

Agreed but they can be limited, especially with heated floors where the slab is the warmest part of the building. I wanted to figure out how high the risk is of putting down possibly poorly unsupported foam and if the benefits would outweigh that.

 

[finn]

You want a vertical foam board at the form, at the very least.

It takes a little more effort to insulate under the thickened edge, including the sloped portion that meets the slab, and I’ve seen it done both ways.

Considering you’re using rebar anyway, a contractor with good attention to detail should be able to handle insulating the whole thing without leaving large voids, given a decent base.

A dump and trowel guy, not so much..

 

[StefanNH]

Considering you’re using rebar anyway, a contractor with good attention to detail should be able to handle insulating the whole thing without leaving large voids, given a decent base.

I ended up spending a lot of time smoothing out the bottom of the thickened edge which I did insulate. I got the first few inches of the slope flattened out too and may run a piece up it but not going quite all the way to the top. I will include some pictures once it’s finished so the next guy with this question will have a reference. I also plan to use tons of rebar incase it does end up spanning a small gap.

 

[StefanNH]

For anyone who looks this up later for refence gets how I ended up doing mine. All parts were later taped together with pex stapled to them. I put foam where I could get it flat enough to not rock or create voids. Any place that ended up curved I left alone.

 

[sjvicker]

following this closely. I'm going to do a mono-pour insulated slab with radiant for my house and will need to tell the engineer/designer what I want for insulation so its on the plans.

I'd like it fully encapsulated but am considering just insulating the flat surface. Frost depth here is 1' and I'd rarely heat more than 35deg above outside temps with the radiant.

 

[StefanNH]

I'd like it fully encapsulated but am considering just insulating the flat surface.

If you spend some time getting at least the footer flat it’s not too bad to insulate. Where I live the frost is 4+ feet so I wanted to protect the outside edge from frost heave. I would have done a conventional foundation but already had hammered out feet of hard granite to get it where it is now and didn’t want to go any lower haha. One side is right on ledge with 6 inches of gravel and the other side is over 4 feet of drainable fill material. Figured the less water under there the better.

 

[Tom Sestito]

I put it under mine. Poured the slab three years ago and put the house on it 2 years ago. So far so good