Slab perimeter insulation for radiant floor heat

[challenger]

Getting ready to prep for my concrete pour in a month. 40x64, West Michigan. Need to run some electrical conduit, then put down the foam. I am planning to do 2" 25PSI foam under the slab.

My question is, what PSI foam should I do around the perimeter? I am planning to dig down 2' inside the skirt boards. Is 15PSI good or should I pay for the added integrity of 25PSI there, too?

 

[chinboys]

Foam board insulation with a higher PSI value is more resistant to compression thus provides better insulation performance as you are buying R-value per thickness and don't want any compression.

 

[ConCretin]

There is no reason to install 25 psi insulation in a vertical application around the perimeter of what I assume is a pole structure. The loads will be far less than that A bigger concern to me is how you are going to achieve adequate soil compaction under the perimeter of your slab. Are you digging a trench wide enough to get compaction equipment into? Having a good plan for compaction plan is more important than the insulation psi.

 

[challenger]

There is no reason to install 25 psi insulation in a vertical application around the perimeter of what I assume is a pole structure. The loads will be far less than that A bigger concern to me is how you are going to achieve adequate soil compaction under the perimeter of your slab. Are you digging a trench wide enough to get compaction equipment into? Having a good plan for compaction plan is more important than the insulation psi.

Thanks for the feedback. Honestly, for compaction, I was just going to dig down with a narrow shovel and pop the foam in so that it was at floor height. Not worry about the compaction down there under the foam. Then use sprinklers and a gas powered packer to pack the sand for the floor at finish grade.

 

[ConCretin]

Thanks for the feedback. Honestly, for compaction, I was just going to dig down with a narrow shovel and pop the foam in so that it was at floor height. Not worry about the compaction down there under the foam. Then use sprinklers and a gas powered packer to pack the sand for the floor at finish grade.

That's probably a decent plan. You're not going to get full compaction down 2' by running a compactor over the surface but if the trench is kept fairly narrow, it probably won't bother. You could consider thickening the perimeter of the slab a couple inches, which is something I usually like to see especially at building entrances where you drive onto the slab.

 

[chinboys]

There is a concrete expert who works in upstate NY who uses perimeter closed cell XPS from board to prevent frost heaving on his slab pours.
I think it is revolutionary as many of us still dig to the or below the frost line for slab-grade concrete flooring.
His Youtube channel is Bondo.