I've read conflicting recommendations for the placement of polyethylene in an insulated slab. Seems like the most common reference is to place the plastic vapor barrier (polyethylene) underneath the insulation and pour the concrete on top of that. In my case 2" XPS. But, the following Building Science Corporation article says the plastic goes on TOP of the XPS, NOT underneath! Here's the link: http://www.buildingscience.com/documents/insights/bsi-059-slab-happy So, what do you guys say?

These guys say the poly goes under the XPS. http://www.radiantcompany.com/

I say put it where ever you want- for unfinished slabs it can work fine both ways.

But if you ever plan to put a finish floor above the slab (or even paint it) you'll be better of putting the poly between the foam & slab, not under the foam. Reason:

When you pour the slab if you have poly under the foam the excess water will end up puddling in the pockets between the foam & poly, which can take literally years to dry through 2" of foam (especially XPS, which has a vapor permeance of onluy ~0.6 perms @ 2"). Concrete is just fine being saturated with ground moisture- in fact it helps it cure to a higher strength faster.

I also say no matter which way you stack it relative to the poly, use EPS at 1.5lbs density or higher (Type-II or Type-IX), not XPS.

Reasons:

1: EPS is cheaper, at about 10cents/R per square foot to XPS at 12-13 cents.

2: XPS loses performance as it's HFC blowing agents leak over time, and in a handful of decades is about the same as EPS of the same thickness & density, whereas the hydrocarbon blowing agent for EPS is pretty much gone before it hit's the distributor's yard, and it's performance at age 50 is pretty much the same as on day 1.

3: The primary blowing agent for XPS is HFC134a, which has a greenhouse potential about 1400x that of CO2, whereas the blowing agent for EPS is pentane, at 7X CO2. The greenhouse hit of the blowing agent in some sub-slab installations may take centuries or or even never full "pay back" that greenhouse hit by lower atmospheric carbon from the saved energy use. (It depends on what the energy sources and subsoil temps are over then next 50-100 years.)

4: In submersion situations such as floods EPS quickly takes on ~7% moisture ever, where it temporarily loses a bit less than 10% of it's R-value, but dries just as rapidly when the tide goes out, whereas saturated XPS can take months or years to dry.

5: Under slabs space is not a premium so it doesn't really matter that you have to use 1/4-1/2" more foam thickness to hit the same ~R10 nominal R.

Dana, good information. Thank you!

3: The primary blowing agent for XPS is HFC134a, which has a greenhouse potential about 1400x that of CO2, whereas the blowing agent for EPS is pentane, at 7X CO2.

I poisoned the atmosphere needlessly.... :-(

I would follow Building Science over TRC every time. However, concrete doesn't dry so much as it cures. It is an exothermic process that presumably never ends. OK the exothermic part does, but the concrete shrinks and gets harder with time. We have installed miles of tubing in various slabs with and without insulation or vapor barrier. Much depends on the weather, the substrate quality and condition, not to mention the mix and installer.

I have seen no difference in the quality of finished concrete in any case. We use XPS because of its ready reliability and consistent cost but have no objection to EPS as it the common insulating component in all of our raised knob systems as well as the Roth panel, at about 60 psi.

I have taken to leaving out the vapor "barrier" all together (may own shop last fall) and still alive. As Dana points out, the 0.06 perms, it's enough.

If you can get distributors around the country to stock 2" EPS i will specify it more often.

Yes, BSC over TRC every time...good science matters... The VB may be optional for XPS, but that’s not the case for EPS. We use EPS and we put the VB on the top as BSC suggests. This is also one the reasons why we raise our HR PEX to the middle of the slab in lieu of just stapling it to the EPS and creating many holes in the VB.

How do install PEX in the middle of the slab? Is there a device made to hold the tubing up while preventing puncture of the VB? Also, do you guys use metal mesh, rebar, or an additive to the concrete? My site is in Northern Michigan (LP) with good loamy soil, on top of a hill. FWIW, I'm not doing the install, I just want to know it's getting done right! Thanks for the help.

Type-II EPS is less than 1.5 perms at 2.5" /R10-ish, a strong class-III vapor retarder. Type IX EPS is less than 1 perm @ R10, a minimal class-II vapor retarder. R10 XPS is about 0.6 perms, a middle-of-the road class-II vapor retarder.

Poly sheeting is a cheap way to drop the vapor permenace another order of magnitude, and make it (nearly) air-tight to soil gases- it's worth it.

It would take a gadzillionstaple holes to appreciably change the vapor permeance of the slab stackup. Vapor-permeable foil radiant barriers are made with a tight grid of tiny holes (holes that aren't filled with staple-shaft either), at somewhere between 1/4" & 1/2" spacing comes in between 10-20 perms (eg: http://www.atticfoil.com/technical-information/product-specs.html ). If you're using THAT many staples for the PEX you're clearly doing something wrong! :-)

And for that 0.001% of the surface area of the poly that is vapor open due to the tiny tears & holes, it's backed up by the still significant vapor retardency of the foam.

Bottom line, there's very little point to raising the PEX, particularly from a vapor permeance point of view. Stapling through the poly into EPS is just fine, but you get better staple retention if you bump up to Type IX (2lb nominal density) EPS from Type II (1.5lbs nominal density).

Yes, concrete cures by reacting with water, but it also dries- it's porous. It's poured with excess water intentionally, and best-practices are to keep it flooded and saturated for at least a day after the pour to keep it curing faster/better. It's during that phase that liquid water leaking between the seams of the foam forms puddles in a bottom-side-poly stackup.

We just tie the PEX to the rebar or WWF (what we use depends on what the actual structural requirements call for) and we just use a lifting hook during the concrete pour to raise the PEX/rebar/WWF assembly into the middle of the slab. Rebar chairs can also be used to accomplish the same thing. We sleeve the PEX and keep it low where there are concrete control breaks.

Structural engineers typically call for the rebar/WWF to be located within the center 1/3 of the slab. There is also some heating efficiency gains that can be realized by getting the PEX closer to the concrete surface. The significance of this heating efficiency improvement diminishes proportionally with increasing under-slab insulation R-value. For a 4” slab using 2” EPS, we estimate you gain about 2.5% by having the PEX in the middle versus at the bottom of the slab. We also strongly believe that PEX is less likely to be damaged by freezing conditions when fully encased in concrete, but we don’t have enough data yet to fully substantiate this belief.

We also don’t have any data to quantify VB performance degradation resulting from staple holes. Since we don’t introduce any staple holes, this has frankly not been any concern to us to cause us to give it much thought. However, the water vapor diffusion mechanism can get pretty complicated and involves both the effective material permeance and the vapor pressures that are developed across the material. So it isn’t entirely clear to us if a small area of missing VB has a very small or a very large negative impact on VB performance. A small leak in the building envelope can have a very large negative impact on building infiltration rate as this pressurized air flow mechanism takes the path of least resistance much like electrical current. However, we suspect the pressure gradient is likely pretty small for under-slab VB and you are likely right Dana. We place the VB on the top of the EPS because we often finish the floor and we are schedule driven.

Some folks just staple the PEX to the insulation and call it good enough. A little research will show the pros/cons to each approach and this is often a very hotly debated subject on this forum. What gets done is pretty much what the contractor feels is appropriate in meeting their objectives and what they are comfortable/confident in successfully accomplishing.

Certainly the vapor pressure gradient between the sub-slab air and the concrete is never going to turn into a large moisture movement number through 1.5 perm EPS, given the extremely small collective cross sectional areas of the staple holes even if you were banging in a staple for every foot of PEX.

Even if it added up to an square inch for every 10 square feet of slab (which it clearly wouldn't) a 1000' slab would have 100 square inches (a 10 inch square, not even a square foot) of "exposed" 1.5 perm EPS.

If that's a problem skating by with only 2" (0.6 perm) XPS and NO poly would be a problem of disasterous proportion, since you would have 1,440 x more non-poly surface area with only the foam for a vapor retarder, but at 0.4x the vapor permeance. That means at a first- order estimation there would be (0.4 x 1440=) 576x more moisture coming through the slab with the no-poly XPS than with the extremely hole-ridden XPS + poly.

Yet experience would indicate that even at 576x the moisture migration it isn't a problem. Don't sweat the staple holes from a moisture diffusion point of view, it's a NON-issue.

Yes, we would agree with your math Dana and experience seems to indicate that this isn't a significant issue.

Posted By Dana1 on 17 Mar 2014 06:03 PM
I say put it where ever you want- for unfinished slabs it can work fine both ways.

But if you ever plan to put a finish floor above the slab (or even paint it) you'll be better of putting the poly between the foam & slab, not under the foam. Reason:

When you pour the slab if you have poly under the foam the excess water will end up puddling in the pockets between the foam & poly, which can take literally years to dry through 2" of foam (especially XPS, which has a vapor permeance of onluy ~0.6 perms @ 2"). Concrete is just fine being saturated with ground moisture- in fact it helps it cure to a higher strength faster.

I also say no matter which way you stack it relative to the poly, use EPS at 1.5lbs density or higher (Type-II or Type-IX), not XPS.

Reasons:

1: EPS is cheaper, at about 10cents/R per square foot to XPS at 12-13 cents.

2: XPS loses performance as it's HFC blowing agents leak over time, and in a handful of decades is about the same as EPS of the same thickness & density, whereas the hydrocarbon blowing agent for EPS is pretty much gone before it hit's the distributor's yard, and it's performance at age 50 is pretty much the same as on day 1.

3: The primary blowing agent for XPS is HFC134a, which has a greenhouse potential about 1400x that of CO2, whereas the blowing agent for EPS is pentane, at 7X CO2. The greenhouse hit of the blowing agent in some sub-slab installations may take centuries or or even never full "pay back" that greenhouse hit by lower atmospheric carbon from the saved energy use. (It depends on what the energy sources and subsoil temps are over then next 50-100 years.)

4: In submersion situations such as floods EPS quickly takes on ~7% moisture ever, where it temporarily loses a bit less than 10% of it's R-value, but dries just as rapidly when the tide goes out, whereas saturated XPS can take months or years to dry.

5: Under slabs space is not a premium so it doesn't really matter that you have to use 1/4-1/2" more foam thickness to hit the same ~R10 nominal R.

Dann is absolutely correct.