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To install engineered hardwood flooring over a basement slab, can I use (2) 3/8″ CDX plywood over 1/2″ XPS?

kpeters4 | Posted in Energy Efficiency and Durability on

QUESTIONS: Can I use 2 layers 3/8” CDX Southern Pine plywood (joints staggered, glued & screwed together, but NOT mechanically attached to the slab) over 1/2″ XPS? Whether I install the plywood sheathing over a vapor-retarder or rigid foam insulation, is CCA treated plywood necessary in either case? Which installation method (e.g. rigid foam or vapor-retarder) do you recommend in view of my concerns below?
DETAILS: I live in a condominium built in 2002 in region 5A (near Detroit), with a 8′ poured concrete full basement. The basement has an existing finished bathroom with a threshold height of 1-1/4” above the concrete slab.
I will be finishing the remaining basement space, and would like to install pre-finished engineered wood floors. I have read FHB 248 “Build a Risk-Free Finished Basement, FHB 169 “The No-Mold Finished Basement,” FHB 206 “A Wood Floor That Can Survive Anywhere,” and Building Science Corp’s 0308 “Renovating Your Basement.” Each provides very informative, but slightly different approaches to finishing basement spaces. I do not want to cut corners by skimping on necessary materials for a mold-free sturdy floor installation, and am also concerned with minimizing VOC’s and the floor transition at the existing bathroom threshold. Therefore, I like the minimal thickness accomplished by 2 layers of plywood over a vapor-retarder as detailed in “A Wood Floor That Can Survive Anywhere,” but see the benefits of a ‘breathable’ installation over rigid foam insulation detailed in the other articles. Which should I do, and what are the minimum materials that I should use?

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Replies

  1. charlie_sullivan | | #1

    Key question: is there insulation under your slab already? Do you know?

  2. Richard Beyer | | #2

    Most engineered wood floors are floating systems. There's no need to install plywood below the engineered floor. Typically 6mil poly, engineered wood floor foam pad or floating wood floor felt pad is installed first. All seams must be taped and then you can proceed with the wood floor installation. Your questions sound to me like your over thinking this process.

    If your intent is to staple down the floor over floating plywood, I do not recommend this in a basement.

  3. mfredericks | | #3

    Kevin, it's certainly hard to be sure which basement retrofit advice to follow! However, you might want to add this recent article to your reading list:
    http://www.buildingscience.com/documents/insights/bsi082-walking-the-plank/

    In this article, Dr Joe advises against using 6mil poly over top of the slab because a vapor barrier over a slab in a retrofit needs to be continuous and totally air tight, and this is tricky to ensure with a sheet of poly. A liquid applied epoxy coating is mentioned as a better practice. The take away message that I got, is that you want to stop any moisture from the slab from entering the indoor space, so whatever you apply should be vapor closed to totally seal the slab. After that, your plan sounds sensible - a layer of XPS for insulation and 2 layers of plywood. That should be a warm and solid base for the new wood flooring. I don't think there would be any need to use pressure treated plywood if you've sealed the slab below.
    Good luck!

  4. kpeters4 | | #4

    Thanks for the responses, and the additional resource Mark.
    Charlie -- I do not know for certain, but it's unlikely that there is any insulation existing under the slab. I am thinking of 1/2" XPS with 2 layers of 3/8" plywood in order to minimize the transition in floor heights at the existing bathroom. However, providing sufficient insulation (to minimize the chance of condensation on the slab) and providing a secure base for both the engineered flooring & partition walls are the priority ... I'm hoping to manage the floor height differences at the bathroom door threshold.

  5. Richard Beyer | | #5

    Mark,

    Good advice however, "A liquid applied epoxy coating is mentioned as a better practice" will fail miserably if this gentleman has any form of hydrostatic water issues not recognized and will add insult to injury with his indoor IAQ/IEQ. These products require specialist to install. Poly has been used under floating floors for many years. If moisture and water are an issue this needs to be resolved before anything commences. Another option is to install Schluter Ditra and tape all seams and edges. This will make your floor water tight even though it is designed for ceramic and stone installations. It will not fail under the conditions I previously explained.

  6. Expert Member
    Dana Dorsett | | #6

    Condensation on the slab isn't a problem. Condensation the top side of the foam layer is, since that's where the moisture-susceptible material is.

    In a zone 5A climate having only R2.5 is a bit marginal unless you promise to never put a rug on top of that floor, which would lower the temperature at the foam/plywood interface.

    Mark Fredricks writes: "In this article, Dr Joe advises against using 6mil poly over top of the slab because a vapor barrier over a slab in a retrofit needs to be continuous and totally air tight, and this is tricky to ensure with a sheet of poly."

    That would be true if there WASN'T XPS in the stackup. The cricism was related to wood in direct contact with the poly sheeting, which is not the case here.

    What Dr. Joe actually wrote was:

    -----------
    "Why is using a plastic sheet a dumb idea? Folks on the Internet love it. It has got to be the right way to go if it is on the Internet. If the vapor barrier layer is not completely fully adhered condensation will occur in air pockets. If these air pockets communicate with the interior you will grow mold and algae. And the floor will smell. What are the odds that a thin sheet of plastic will be installed in an absolutely airtight manner on the top of a concrete slab where the perimeter of the plastic layer is sealed in a continuous airtight manner to the concrete and where the thin sheet of plastic will survive the construction process and not have any holes in it such as holes necessary to fasten the flooring?

    That the polyethylene plastic sheet recommendation still comes from many in the wood flooring industry including wood floor trade associations is flabbergasting and disappointing. You would think they would know better. The old guys knew better with the bitumen layer. But apparently they all died off and didn’t tell the new guys. Come on guys, put the plastic sheet under the concrete not on the top of the concrete.

    What if I want to insulate the floor? Easy, place the insulation under the plastic vapor barrier and over the top of a granular capillary break (Figure 8). Do not, and I repeat, do not install the insulation over the top of the plastic vapor barrier and under the slab. The insulation will get wet and stay wet forever. Under the plastic it can dry into the granular layer. Memo to everyone, with insulation you absolutely need a granular layer with no fines to act as a capillary break and further note that the only insulation that will work is an extruded polystyrene."
    -----

    And the caption to Figure 8 (which shows sub-slab insulation) reads:

    -----

    "Figure 8: Insulating the Slab—Install the insulation under the plastic vapor barrier and over the top of a granular capillary break. Do not, and I repeat, do not install the insulation over the top of the plastic vapor barrier and under the slab. The insulation will get wet and stay wet forever. Under the plastic it can dry into the granular layer. Memo to everyone, with insulation you absolutely need a granular layer with no fines to act as a capillary break and further note that the only insulation that will work is an extruded polystyrene."

    -----

    With the poly sheeting on top of the slab but under the foam, any moisture pockets that form are isolated from the wood by the foam. The stackup is fine as long as you add poly sheeting under the 1/2" XPS, but 3/4" or even 1" XPS would be advisable given the low subsoil temperatures in MI.

    I would take issue with Joe over XPS being the sole solution here, and would very much like to see comparative data on the capillary draw of EPS.(both unfaced & vinyl or poly faced EPS) compared to XPS at any given thickness or R-value. My strong belief is that EPS is suitable for both sub slab insulation or above the slab, but could be swayed by actual data. The EPS industry has a long & successful track record of use in sub-slab and foundation wall insulation.

  7. Richard Beyer | | #7

    Great explanation Dana, as always. Another solution is to install Schluter Kerdi-Board and tape all seams and change in plains... This will also eliminate the moisture concern and provide the thermal value.

    http://www.schluter.com/7757.aspx

  8. kpeters | | #8

    Thanks for all of your responses. Although, admittedly, I'm not fully certain that I followed correctly. Based on your recommendations and my research in Fine Homebuilding and buildingscience.com, I am currently planning on the following construction sequence. Do you recommend that I make any alterations?:
    (2) layers tongue & groove 1" XPS against the poured conc foundation wall (seams staggered, sealed w/ expanding foam, and taped);
    followed by 10-15 mil polyethyene vapor retarder over the floor slab (lapped 12", seams sealed w/ waterproof tape, and wrapping up the wall XPS by 6"), 1" tongue & groove unfaced XPS (again, seams sealed w/ expanding foam & taped), 2 layers 3/8" 'floating' plywood subfloor (1/8" expansion gap between sheets; glue & screw layers together 90 degrees to eachother);
    then 2x4 wood studs @ 24" o.c. directly against the wall XPS and mechanically anchored to the floor sheathing (NO additional cavity insulation), 1/2" paperless drywall;
    followed by engineered hardwood flooring (stapled to the plywood sheathing), and topped by area rugs.
    Do you recommend that I adjust any of these materials or the construction sequence?

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