Foam under slab?

[78C-10]

Hello everyone, I have a question regarding the 2" foam under a slab that is used with PEX. I am not going with in-floor heat, I know, shame on me. There are a few reasons why I am not. My question is should I still put 2" of foam under the slab or just a vapor barrier. I do not want the concrete to sweat with temperature/season changes. Will the plastic sheeting be enough or do I still need the foam to prevent condensation? I do plan on perimeter insulation though for sure. I live in northern Illinois, building is 30x40 pole barn(insulated), and I am planning on using a gas fired radiant tube heater. Thank you in advance for any opinions and suggestions.

 

[randydupree]

i would put insulation under any slab.

 

[jomobco]

wouldn't the weight of the concrete crush the foam making it lose it's R value?

 

[jlckmj]

I would put insulation and vapor barrier under the slab if you are in a cold climate.

I would also run pex tubing if you are going to insulate, for the $300-$4oo it would cost you. Even if you are not contemplating in floor heat at this time.

Jim

 

[canadiancobra]

what he said!!

 

[BigGMC]

wouldn't the weight of the concrete crush the foam making it lose it's R value?

It hasn't on millions of installations thus far........

The vapor barrier wont do anything to combat surface condensation - its to reduce/eliminate vapor transmission from the ground up through the slab.

I think its a great idea to insulate even without a radient system. The floor will stay noticabley warmer...... your feet will the be the first to notice.

Concrete slab will always condensate providing a big enough temperature differential and high humidity.

 

[oilslick]

I put vertical perimeter 2" on my 32x54 and it does seem to keep the floor warmer, I only heat when occupied about 2 days a week. To do again I would figure a way to insulate in front of doors underground, seems that's the coldest spot but maybe that's my all glass doors lettin in the cold. I am in central Illinois.

 

[86turbodsl]

Foam under a slab is done all the time.

The typical foam used is 15 psi foam. You can even get 25 psi easily.

It will support 25 pounds on each square inch. If you take a one inch square piece of concrete 4 inches tall (typical residential thickness) it weighs about a pound. So you could pour a foot of concrete over it and any bulldozer you wanted and you wouldn't harm the foam a bit.

I once calculated that the foam under my shop floor could support millions of pounds.

 

[joe_padavano]

As noted, the crush strength of the foam is much higher than the weight of the concrete and vehicles, ASSUMING that you have the foam uniformly supported underneath. You need to be sure the gravel base under the foam is level and well compacted. I used a dirty crusher run and ran a tamper over it for quite a while to get it compacted and flat before laying the foam panels.