Replies

1. 
   Marson | Jul 19, 2008 11:19pm | #1

   You should be using foam that is impervious to water (high density extruded polystyrene, such as foamular 250.) We put the VB under the foam mainly because it would get torn to heck during the pour, installation of rebar, radiant heat tubes, etc.

1. 
   stagehand8 | Jul 20, 2008 12:07am | #2

   Thanks for responding. If XPS is so good, why do I need the vapor barrier in the first place?

1. 
   Marson | Jul 20, 2008 12:13am | #3

   Excellent question. I think it wouldn't be needed if your foam fit together perfectly, was stable enough to tape the joints and have it stay, was cut close enough to the plumbing penetrations so that you could caulk them in. But then there are field conditions. The $40 roll of poly is cheap insurance.

1. 
   stagehand8 | Jul 20, 2008 12:58am | #4

   My Energy Star rater recommends installing the barrier above the insulation to keep water and water vapor from getting into the insulation. I take it you don't think that's a problem. Do you have number for the permanence of the foam.

1. 
   Marson | Jul 20, 2008 01:19am | #5

   Don't have a number off the top of my head. I am talking about underslab insulation. If I was concerned about the moisture reducing the effectiveness of the foam, I would be more concerned about moisture from the ground.This is not something I just dreamed up. Where I live, I have always seen the plastic under the foam.

2. 
   frammer52 | Jul 20, 2008 02:26am | #6

   Where I'm from we don't bother with vapor barrior at all!

3. 
   Marson | Jul 20, 2008 04:15am | #7

   Yeah, I asked the BI about that. No go.

4. 
   Matt | Jul 27, 2008 07:40pm | #16

   >> Where I'm from we don't bother with vapor barrior at all!. <<

   Every interior slab needs a vapor barrier under it to keep ground moisture from comming up through the concrete.  Also, it slows down the curing process and helps make the concrete stronger as otherwise the substraight can suck the moisture out of the concrete.  Here, it is also code required.

5. 
   frammer52 | Jul 27, 2008 08:49pm | #18

   We always used to use it, but it fell out of favor about 10 years ago.  It takes the slabs too long to cure.  Masons would rather keep a slab wet.

6. 
   brownbagg | Jul 27, 2008 09:24pm | #19

   those that refuse to use a vapor barrier are hacks, they dont care about PSI strength or moisture in the finish floor material. If you dont use a vapor barrier the slab will crack, that is a gaurentee. The finisher just want their money and they are gone. All slabs crack, you been listen to hacks too long

7. 
   JeffinPA | Jul 27, 2008 04:51am | #11

   Vapor barrier above insulation to keep insulation from getting moisture???

   Round our parts, the moisture comes up from below.  That is what the poly is for.

   If the poly is above the insulation, the insulation will be next to the stone in the moisture.

   From a functionality standpoint, I dont see any problem with either way and like the idea of under purely for ease of working foam then rebar etc.

   I would not use 2 layers.  No reason and no idea what happens when you trap moisture like that under a slab.  (I know what happens when you do that in a wall and it aint good)

    

   So long as the 6 mil is installed well, I dont think it matters.

8. 
   stagehand8 | Jul 27, 2008 05:13am | #12

   I talking about New York near Albany. My source is "Builder's Guide to Cold Climates" published by the Energy & Environmental Building Association. I don't think I care if vapor comes up from below because it won't have much of an effect. However, if moisture in the house permeates the slab and then the insulation, it can condense on a vapor barrier below the insulation.

9. 
   JeffinPA | Jul 27, 2008 07:16pm | #15

   "I don't think I care if vapor comes up from below because it won't have much of an effect. However, if moisture in the house permeates the slab and then the insulation, it can condense on a vapor barrier below the insulation."

    

   i think if you knew how much moisture comes thru from below, you might care. 

   Moisture comes up from below and wreak havoc.

   From a logic standpoint, I would agree with you that the vapor barrier should be on the warm side.  (that is the rule after all and i did not think that thru on the  last post)

   With that said, I retract my last post and say go on top of the insulation.

   I just saw Joe L's article that someone sent you.  They are out of Boston Area and he spent most of his life in Canada so knows cold well.  The article is about California and Florida but I have worked with Building Science in the phiadelphia PA area and they are the rocket scientists of the building industry.

10. 
    stagehand8 | Jul 27, 2008 08:36pm | #17

    Thanks

2. 
   RedfordHenry | Jul 20, 2008 04:24am | #8

   On my last insulated slab, I put the poly on top of the foam.  Seems to work ok.  By the way, I learned that slab guys do not like to see the poly (and in many cases, foam) under the slab so unless they are used to it, you might want to let them know in advance that you will be using it.  When I asked the slab guy what he didn't like, he had a couple of issues.  First, the foam, especially with poly over it, is slippery to work on when they are slogging around during the pour.  Second, the foam/poly slows down the cure so afternoon pours can be a problem since they have to wait so long before they can float it out. 

1. 
   Marson | Jul 20, 2008 04:51am | #9

   I think if I was going to do it that way, I might go with one of those super tough membranes, like Rhino. The poly under the foam here is pretty entrenched. It's on every construction detail I have ever seen.Masons in this neck of the woods are pretty used to pouring on foam or poly. I'd say the slab poured straight on sand is the exception.

1. 
   stagehand8 | Jul 26, 2008 10:21pm | #10

   If you put the poly under the insulation, you risk water accumulating between the poly and the insulation. (Less than an inch of unfaced XPS is considered vapor semi-permeable; more than an inch is considered semi-impermeable). Therefore, if you are going to use a vapor barrier it should above the insulation. Although the insulation maybe semi-impermeable it absorbs little water and it's insulation properties should remain relatively unaffected.

3. 
   shelternerd | Jul 27, 2008 08:28am | #13

   Joe Lstiburek has a pretty good explanation of why you want to put the vapor barrier on top of the plastic in contact with the bottom of the slab at this site http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

   He also strongly recommends a wt cure for the first week or so, burlap, not poly, We've used wet straw with good effect and it makes good mulch for cutting down on the mud once the building is dried in and the slab is protected from the sun.

   Be well

   M

   ------------------

   "You cannot work hard enough to make up for a sloppy estimate."

1. 
   Marson | Jul 27, 2008 02:21pm | #14

   Different. He is arguing against putting a layer of sand on top of the VB. We are talking about putting extruded polystyrene between the vb and the concrete. Extruded polystyrene does not soak up water.