poly under or over floor insulation

[gogolf0401]

For concrete radiant heat applications I've heard of putting poly under the 2" foam insulation and also heard of it going over top (under the pex). Curious what you guys think is the "correct" way to install a vapor barrier.

Initially the thought of putting poly over the insulation, directly under the concrete, seemed silly since it's supposed to be a vapor barrier and here I am going to put staple holes through it when installing pex. But someone somewhere made the point that the staple holes don't add up to much area.

Also, if doing this without a roof and the possibility of rain water accumulating on top of the poly started me thinking that maybe over top is better, since I imagine water would be trapped between the poly and foam and take ages to dry out causing the water to remain when pouring concrete. Unsure what issues this would cause if any.

Thoughts guys?

 

[The Cobbler]

many folks say you don't need poly under eps as it acts as it's own barrier
I did poly under the eps

 

[gogolf0401]

many folks say you don't need poly under eps as it acts as it's own barrier
I did poly under the eps

I think if you would tape the foam seams this would likely be true since EPS is closed cell and doesn't allow water through it. Taping the seams would **** though..

 

[The Cobbler]

I taped mine. no biggie

 

[850xpeps]

I’d put it under. If there’s moisture between foam and poly it’s no big deal. It’ll drain or evap eventually. You shouldn’t have that much if grading is proper.

 

[gogolf0401]

Any other comments?
Anyone think running poly over the foam / under pex is a better idea?

 

[850xpeps]

I think it can get annoying by bunching up while your trying to lay pex and rebar.

 

[like2wheel]

These people say it needs to go over the insulation. However, after reading the explanation, I think it doesn't apply to my situation, & I'll be putting it under the insulation for practical reasons.
I'd be interested in other people's thoughts of the article.

Edit:
Oops I forgot to include the link:

https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs






.

 

[yeldogt]

I always put a nice heavy VB on top of the compacted base -- foam on top -- I do tape the joints of the foam .... just to hold it all together.

Tubing attached to the wire mesh -- I like the wire because I can place the tubing anyplace I want.

 

[kj_mustang]

I don't agree with that article. You can be careful and place the plastic and not allow rain to get on top of it, pretty easy to do if the slab is inside a pole building. A better argument is that over time XPS insulation has been known to adsorb moisture and degrade its R value. Putting the plastic barrier below the insulation will help prevent that.

 

[yeldogt]

Greenbuilding is interesting -- but, keep in mind ... they have not been consistent with all the recommendations over time.

Also - the link is to a slab w/o radiant.

Remember: every group ... concrete ... radiant heat ... wood flooring association. Wants first and foremost to sell a product. They are trying for various methods - to sell a product ..have it work and be as cheap to install as possible.

The sand -- no sand thing seems to be an ongoing situation. Concrete guys love it -- but is it the best. Some say only stones ... others want fines?


I have found that a nice crushed stone base is quite smooth. I have tested more than once using a heavy poly under insulation and stepping all over it and checking the polly .. the polly is fine.

Some places say you can staple though the polly if you are attaching the tube directly to the foam -- the small holes will not matter ? Why bother with the polly VB? And who wants to fight with polly on top of the foam while trying to install the tubing?

I tape the foam to keep it in place -- the tape will hold for long enough. Will it last 4 months outside. Unlikely. I'm talking about inside radiant slabs.

For me the placement of the tubing is relevant in the slab -- I'm typically doing 4" or a little thicker. I want the tubing at the bottom ... it's not because the wire is going to raise it up a bit and if it 1/2 tubing -- it's going to be 1' above in many places anyway. How high do you want it? Insulation is more important.

I get a little higher mix -- seems to always work.

In a perfect world -- the foam is tight -- or the tubing panels fit exactly. With a nice VB under everything you know it tight and you can bring the VB up above all around the permitter.

I'm also in a radon area -- I want the polly.