Vapor Barrier

What kind of vapor barrier do you use under a concrete floor? I always saw the plain old 6 mil or 10 mil plastic, but looking around online I see they sell actual vapor barrier. It is quite a bit more money than the plastic. Is is really worth it or is the plastic alright? Another question, I"m installing radiant floor heat with 2" EPS insulation on a 1" sand bed, where do you recommend the vapor barrier to be? Under the sand, between the sand and insulation, or above the insulation? I"ve seen recommendations for all three, but am leaning towards between the sand and insulation.

just my thoughts but in all the buildings Ive done floor heat in with 2" foam in the floor no vapor barrier is needed

The vapor barrier is really important you don't want that floor to sweat. I would not go cheap get the stuff made for concrete.

I would put double bubble with foil on top of the sand, then the styrofoam. The double bubble will provide vapor barrier and the foil radiates the heat back.

I agree. if you tape the seams on the foam, it is a non permeable vapor barrier. Now if you are preventing radon intrusion that is another story for a different day.

From what I read EPS is "moisture resistant' but is nor a vapor barrier.

I've heard this, too, but also heard that you don't want to allow the foam to stay wet. Supposedly the foam will degrade. It seems that everyone has a different opinion and there's not a lot of standardized practices for this stuff. I'm not concerned with preventing radon, I just don't want a floor that sweats.

I don't think you would use the white bead style foam under the floor. You would want the pink or the blue as it is denser and would act as a vapor barrier. That white stuff comes in a couple different densities but is not as good as the pink or blue non bead style.

Sweating is caused by the concrete being cooler than the dew point of the air above it. Do not use white foam under concrete, it will absorb moisture. You need to consult a knowledgeable builder on this subject.

As a contractor I have built several comercial and farm shops with radiant floor heat.
The key to your sucess is a properly constructed base for your floor. First off you need a good substraith. On top of a very solid base you need a fine agrigate that will compact smoothly and lock together like concrete. I sugest stone dust, sand will not compact. This layer has to be graded dead on with no low spots so the styro, if that is what you chose to use doesn't have to bridge low spots. I prefer insulated blankets, that incorperate a vapor barrier and a radon barrier. The blankets when covered with crete follow any variations in sub grade and can be formed under floor drainage systems to maximize strength and continuity of the vapor barrier in this area. The rebar schedual you use is also critical. We layed down a grid of #4 bar 5' OC on 2" (chairs) and then tied 5'x10'x6" mesh to it. We then tied 7/8" radient tubing 16" on center with plastic tielocks to the mesh.
You are also going to need to install a vertical insulation barrier 2' deep around the perimiter of the building, and between all the exterior aprons and the main floor. We pour a full 6" floor with a min. of 4000# crete, which is power screeded to elininate air bubbles, and settles it in tightly around all the rebar and tubing. (this is very important to strength and transfer of heat from the radiant tubing. On large floors like in the pics. I posted we cut the floor into 20' sections on the column lines to the center drain system. I don't perticularly like center floor drainage systems but the two buildings I picured had to incorperate oil/water seporator systems. The Teed section on the rear of the 12,460sq ft red construction shop is an enclosed and heated wash bay, and the seporator tanks are buried just to the left of it. The pics of the radiant tubing are in the office area of the building and we did use 2" styro there with 6" ICF frost walls, where there would be no heavy floor loading. The shop floor was designed to hold the abuse of big construction equipment and point loads from floor jacks and blocking suporting heavy equipment, and was a 4500# mix.
To sumerize
1 compaction, compaction, compaction.
2 smooth and precisely graded
3 put vapor barrier under styro., if you use it. Rebar/mesh and pouring it will compromise it.
4 vibrate it in with a power screed so it fills all the gaps around rebar and tubing, for strength and heat
transfer.
Insulation coments.
These are Steel freespan buildings, but this applies to all steel clad buildings.
1spray on "open cell insulation, not closed. (closed will not expand and contract with the steel, and cracks or looses it's bond to the steel clading.
2leave an air gap, and then install 2" thick DUPONT Thermax which has a one hr. fire rating required by code, and be sure to install it as directed to to form a continuous vapor barier to the inside of the building.The Thermax creates the condensation barier so that moisture will not condense above it in the roof and wall air pockets.
3 Put a protective wall of plywood or steel on the inside of the lower wall portion of the Thermax as it is prone to damage if not protected in your work zone.
Loren, the Acg.

Keep in mind, any floor that has a temperature below the dew point of the surrounding air will sweat. All the vapor barriers in the world will not change that fact. The sweat comes from the air, not from the capillary action of water through the concrete. The foam would brake the capillary action of ground water but a vapor barrier will not stop a cold floor from sweating on a humid day. For that, you have to turn the heat on and keep the concrete warm.

Well said!