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.