Thinner plastic can't stand up to the abuse of construction. I agree, but I think a much simpler, more forgiving approach is to break the slab up with control joints. With EPS being greener than This plastic sheet is in direct contact with the concrete—as it should be in a rational world. poly placement and radiant heating When I remodeled, and added 3/4" xps, sleepers, and a plywood subfloor, and then wanted to install a finished oak wood floor I was worried about the floor cupping as I have heard stories. Wood Flooring on slabs Shouldnt the EPS/XPS be elevated above the bulk water layer by the stone. Taping the seams in the polyethylene layer, and sealing penetrations, causes no harm, and probably makes air sealing easier once the slab has been placed. Alain and Karl, More information on PEX for Alain Hamel Alain, 2. In the old days, not much — just dirt. Comes in different widths and lengths so it's easy to economize and minimize seams. Response to James Huang (Comment #11) All vapor barriers should conform to ASTM E-1745; but consider going with a barrier that has water vapor permeance less than 0.3 perms. Here is information to help you understand how moisture moves in a slab and how using a vapor barrier can help control the problem. However, looking into the issue the "science" I found was mostly from manufacturers and struck me as similar to debates over which rigid foam is superior below grade; mostly it is competitors sniping at one another. Primarily, plastic is installed in crawl spaces to residential buildings to create a barrier against moisture or vapor. The ground is always damp. You wrote, "It's now part of the 2010 NBC (and valid in most of the [Canadian] provinces except Quebec) to put a 6 mm poly or even thicker under the slab and to seal each penetration correctly and perimeter to the air barrier -- they called it radon protection -- so nobody has a choice, it's mandatory.". There is no reason that the poly cannot be an effective air,vapour and radon barrier if it is well sealed and protected. Don't worry if the concrete contractors put a few holes in the poly. Anyone that has had to face a moisture problem with a concrete slab understands the damage that excess moisture can cause. Should we worry about the RH below grade. This thicker material is much less susceptible to tears or punctures and has a lower permeance. Other cold zone areas require deep footings. A. What goes under the concrete in a slab-on-grade home? 6-mil plastic often gets damaged during placement of reinforcement and concrete, creating holes that can let a considerable amount of water vapor into the slab. Crawlspaces are area based applications. Residential PV Installations in Florida Remain Strong. What is a vapour barrier under a concrete slab and why do you need one? One inch of extruded polystyrene (XPS) has a vapor permeance that ranges from 0.4 perm (that would be a vapor barrier) to 1.6 perm (a vapor retarder, not a vapor barrier), depending on the brand. Although it may seem water-tight, this grade of material allows a lot of water vapor to pass through. Well-sealed 10 mil poly under or over your foam, no. My personal home built in the 50's is a sandy base with a 4" slab cast atop it, and no problems exist. This feature has been temporarily disabled during the beta site preview. It's called Perminator a product from WR Meadows. It depends on the type of insulation and its thickness. I personally think there were four reasons for this easy, non-cracked floor: fibermesh, pouring in shade (SIP walls and roof were up) and most significantly the physical embedded reverse muffin tin shape of the Creatherm product: 4" oc, 2" diameter, 1" tall knurled protrusions into the bottom of the concrete and finally NO uneven water wetting the subbase just prior to pour thereby NO uneven drying or curing. Signs that you might have a leak under the concrete slab of your home include: A sudden decrease in water pressure; Water collecting in areas it shouldn’t Perfect timing This leads to less cracking when it gets trampled by trades, and it largely negates the risk of the floating icebergs that Steve Baczek mentioned. The best recommendations I've read, based on ACI, suggest 0.5% reinforcing ratio. Although it may seem water-tight, this grade of material allows a lot of water vapor to pass through. Termites, spiders, scorpions, black widows, rattlesnakes, and the list goes on. 6-mil plastic often gets damaged during placement of reinforcement and concrete, creating holes that can let a considerable amount of water vapor into the slab. Anything less is a very bad idea. Choose a method that works for you. Most are poorly insulated, leaky and moisture ridden. Q. This approach helps minimize the risk of mold or mildew intrusion into the home. Whenever I have seen holes drilled through concrete slabs, I have never noticed any degradation in the polyethylene layer. Steve, 7. The other thing always forgot by some experts is that concrete always crack even with control joints, it's really difficult to predict accurately how concrete is going to react, it depend on a lot of factors. The crushed stone provides a capillary break that reduces the amount of moisture flowing upward from the damp soil to the permeable concrete. On new interior slabs, moisture can be easily controlled and mostly eliminated. An application of 2 inches of closed-cell spray polyurethane foam would have a vapor permeance of about 1 perm, which meets the definition of a vapor barrier. Plastic sheeting in your crawl space can serve a number of useful purposes. > permeance of 0.7. Although, even when the concrete is topped with a vapor closed flooring such as VCT we wouldn't omit the poly. "Would spraying 2 inch of closed cell work?". A supplier pointed out "research" that said polyethylene deteriorates below concrete slabs due or the conditions of the subslab environment and lime in the concrete; they recommended polyolefin as a superior specification for subslab vapor barrier. The poly vapor barrier is 10 mil, not 40. But if PROPERLY placed and cured, this slab will need no control joints and will have an array of very fine, almost invisible cracks. The order upwards for the slab is: undisturbed soil, crushed stone, 4-inches of Type I EPS, 6 mil polyethelene sheet, another 4-inches of Type I EPS and then the 4-inch concrete slab for a total insulation value of R30. I had NO cracking at all. According to most building scientists, here’s how the layers should go, from the bottom up: crushed stone; rigid foam; polyethylene; concrete. Because concrete is an extremely effective vapour barrier, we aren't concerned about vapour from the house condensing on the "cold side" of our insulation against the poly and building up there below the foam (which is why one would normally place poly on the warm side of their insulation a-la walls and roof). This doesn't strike me as one of those building science issues with warning signs written all over it. ACI 302.1, Guide for Concrete Floor and Slab Construction, states that the minimum thickness of an effective vapor barrier is 10 mils. If you didn't do it right and they found radon in the basement, you could easily be sued by the owner. One of the reasons those old basement floors were like that was because they had no vapor barrier beneath them, leaving an easy path for water vapor from the soil to migrate into the slab, assuring that the cold clammy damp feeling never went away.
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