\"Roofing Insulation\" beneath slab

Greg_40

I have a customer planning to use polyisocyanurate insulation 2-1/2" thick beneath his slab for a radiant floor in a commercial shop building. The R-value is high, but will this type of insulation product perform long-term under concrete? I haven't heard of anything good or bad on this product regarding soil contact.

Any problems? Thanks!

ALH_4

Polyisocyanurate

Polyisocyanurate absorbs moisture more readily than EPS or XPS. I would not use it in a moist environment like beneath a slab. The foil backing is useless beneath a slab anyway.

kevin coppinger_4

as long as..

you put a good vapor barrier under it, it should be fine.kpc

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George Peteya_4

Polyisocyanurate

Greg, unless your supplier tells you that "iso" is definitely suitable for ground contact, don't use it.

BTW, ants love to nest in it.

Tim P._2

What is the compressive strength of the polyiso he proposes to use?

Will headroom improve with time?

Unknown

Had a guy;

that used this stuff on me not long ago under the slab, it was 2". Said the "reputable" insulation guy commented it needed no VB,, I kinda wondered,, but I thought is was better than double-bubble. So you tell me,,,was this no good?

Dave

Glenn Sossin_2

Try this experiment

The foil is used to reflect radiant heat energy. Under a slab there is no air space, hence no ability to reflect anything. Look at the foil faced insulaton in you attic on the AC duct work. It works here, reflecting heat energy to minimze heat in your attic from being absobed by the duct work.

Here a quick easy way to demonstrate the charcteristics of foil. Take a sheet of foil and tape it to a piece of cardboard. Now hold it an inch or so from your face. Feel the heat ? Thats heat from your body (cause your warmer than the surrounding air) being reflected back at you by the foil. Now take the foil and press it against your face..... uh oh..... not warm anymore. Foil is a great conductor. My $.02, it's a useless component for insulation under slab.

Brad White_9

strength

Strength is the issue here. Most roofing grade foam boards have a strength of 15 psi, underslab 45.

Stick with the properly listed products if you want your project to last and maintain it's integrity. Read the label, don't take someone's word for it or go by color alone. The compressive strength is printed on the label. The wrong stuff might save $3-4 a sheet right now, but will cost later in a very big way.