Is closed cell foam a vapor barrier?

Paul_11

I'm having 1800 square feet of  2 1/2" of closed cell foam installed over 6" of gravel on a slab on grade radiant installation.

Airtight from PA is doing the foam work in Bechtelsille, PA.

They say that the closed cell foam acts as a vapor barrier and I don't need to install the 6 mil vapor barrier that I intended to install over the gravel and under their foam.

Do you guys agree or disagree?

Paul

eluv8

well,

Closed Cell (extruded) insulation will absorb water to an extent and moisture can definitely pass through each seam unless sealed. So, in short it kind of acts as a vapor barrier but not  as good as a true vapor barrier. Note: Expanded insulation while less dense (lower psi compression strength) does offer a higher resistance to water retention and will maintain a higher R-Value long term.



I have heard of people using it as a vapor barrier, but have not seen an insulation manufacture state that as a use of the insulation.



With that if you are trying to protect hardwoods against known moisture underslab then I would not rely on the insulation to protect it.

Techman

vapor barrier

Closed cell insulation is a "vapor barrier" only in the sence of the temp difference/dew point,point .It does get wet .Your vapor will protect the entire area .

Paul_11

6 mil or not to 6 mil

There will be no seams in the 2 1/2" closed cell foam insulation which will be sprayed in. 

It is 2000 square feet wall to wall with ICF walls.

The floor finish will be tiles.

so in short you would still install the 6 mil poly sheet on top of the gravel?

OK , I got you.

I had always planned on installing the 6 mil, but when I heard this from my foam installer, I began to think about how tight the bond would be to the gravel.

All the foam would ooze between the small little spaces rather just sit on top of the 6 mil.

I began to rethink my plan.

Paul

Paul_11

to only foam or not to only foam??

What do you mean, that my vapor will protect the whole area?

You say the closed cell foam will get wet?

I'm sure it will get wet on the bottom or underside, but are you implying that the water will get through the 2 1/2" of foam.

eluv8

Sprayed?

Spraying will eliminate the seams, and then it comes to to water permissibility. I hope I spelled that right. Anyhow, if you talk to the insulation contractor he should be able to provide the ratings on the foam being sprayed. If the insulator is telling you that it qualifies as a vapor barrier then he should have a rating to back it up. I personally would prefer to install the foam directly on top of the rock to fill in all those voids that will be between the foam and the rock. I never liked the idea of installing foam board over uneven base for that very same reason.



 I had an insulation contractor approach me once about spraying underslab however he could not provide me any details with compression strength so I had to pass. Other than that I really liked the idea and the price was right.