Under-Slab Insulation Question

[Shoester]

So browsing the net I've come across two different depictions of proper under-slab insulating:

1)

2)

In the first, the insulation goes directly beneath the slab. In the second, the insulation extends down along the footing/stem wall and does not run directly beneath the slab.

Which method did you use, or which method do you think is best?

 

[sands35]

#2 is dependent upon using the correct stone to minimize heat loss into the ground. Not sure, but this may not be allowed for in-slab heat.

#1 has better thermal performance for in-slab heating. Direct isolation of the ground from the slab is going to be better than relying on compacted gravel to do that.

Some locations drive to #1 by code interpretations (at least that is what my local AHJ had me do, but I didn't read that part of the construction code).

There are other variations with insulation on the outside of the foundation wall as well.

 

[Randy in Maine]

I sort of used the top drawing, except my stem wall 2" insulation starts at the bottom of the concrete floor (before it was poured) and stops at the top of the stem wall ~ 8" above the heated floor. That insulation is covered in pine now as are the walls.

 

[Chris705]

If your project has exposed foundation walls to the exterior a combination of 1 & 2 would be ideal to afford freeze protection to the foundation wall from expanding soils inside the perimeter. I have 6' of exposed foundation walls in one corner of my build and opted to do both.

 

[Shoester]

#2 is dependent upon using the correct stone to minimize heat loss into the ground. Not sure, but this may not be allowed for in-slab heat.

#1 has better thermal performance for in-slab heating. Direct isolation of the ground from the slab is going to be better than relying on compacted gravel to do that.

Some locations drive to #1 by code interpretations (at least that is what my local AHJ had me do, but I didn't read that part of the construction code).

There are other variations with insulation on the outside of the foundation wall as well.

I would be interested in hearing more about this. When you have a thickened edge slab that is, for example, 10" above grade, how do you properly insulate this exposed concrete edge? I could imagine running a 2' deep piece of xps into the ground that extends up to cover the exposed edge, but then you have 10" of insulation showing above ground and susceptible to the elements. Is there a better way to protect a thickened-edge type slab?

 

[sands35]

Google "Foundation Insulation" and look at the images. Lots of variants.

Above ground insulation needs to be protected with something. There are paint on stucco like products, cement board or flashing. The exterior insulation can run in a continuous sheet from the footer to the eave.

I used AL flashing capped with AL window cap.

It really depends on what you want to do and what your local AHJ will approve.

Where do you live?

 

[Shoester]

Good info.

After a little searching, I think I may use some pre-painted Fiber Cement Lap Siding from menards to cover the insulation. The 8 1/4" stuff seems like it would do the trick to mostly cover up the visible insulation.

Do you think screwing/nailing into the XPS along with a healthy dose of construction adhesive would keep it in place?

I'm just outside Kansas City.

 

[sands35]

You will need to through fasten it to the concrete behind the foam. I wouldn't rely on just construction adhesive for above ground work. Either use concrete screws of the appropriate length or expanding nylon bolts.

The slab should be insulated under it, and ideally a thermal break to the foundation walls. For a monolithic slab, the thermal break to the foundation wall is not possible, so insulation on the outside of the wall is the next best option.

 

[cleveman]

I think they are both wrong. You may want to insulate your footing as well.

 

[Steve in Mi]

Quote:
Originally Posted by sands35 View Post
#2 is dependent upon using the correct stone to minimize heat loss into the ground. Not sure, but this may not be allowed for in-slab heat.

#1 has better thermal performance for in-slab heating. Direct isolation of the ground from the slab is going to be better than relying on compacted gravel to do that.

Some locations drive to #1 by code interpretations (at least that is what my local AHJ had me do, but I didn't read that part of the construction code).

There are other variations with insulation on the outside of the foundation wall as well.

I would be interested in hearing more about this. When you have a thickened edge slab that is, for example, 10" above grade, how do you properly insulate this exposed concrete edge? I could imagine running a 2' deep piece of xps into the ground that extends up to cover the exposed edge, but then you have 10" of insulation showing above ground and susceptible to the elements. Is there a better way to protect a thickened-edge type slab?

I used aluminum flashing to cover the exposed rigid foam between the siding and ground level.

 

[theoldwizard1]

#1 but the edge insulation MUST go as deep as the frost line in your area.

 

[Radix2]

Good info.

After a little searching, I think I may use some pre-painted Fiber Cement Lap Siding from menards to cover the insulation. The 8 1/4" stuff seems like it would do the trick to mostly cover up the visible insulation.

Do you think screwing/nailing into the XPS along with a healthy dose of construction adhesive would keep it in place?

I'm just outside Kansas City.

Sounds like you are building a shallow foundation under a stick built wall? ( one that does not go deeper than the frost line in your location)

If so, you need to search for specifications for "frost protected shallow foundations". And follow them closely - there are different requirements for heated and unheated ( maintained above 65f) buildings.

For these shallow foundations in freezing climates, the issue is not just making the floor heat work, the bigger issue is to prevent frost heaving and cracking of your foundation.