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Community and Q&A

Choosing a Basement Flooring System

dietejrwso | Posted in General Questions on

Remodeling a 1500sq.ft. walkout that previously had carpet on the concrete. We want to do the kitchen and bathroom in tile or stone and the rest of the area in wall-wall carpet. The only ptroduct I’ve found thats meets my requirements are Delta Fl with t&g plywood for the carpeted area and Schluter¯-DITRA-HEAT for the tile/stone area. Not fond of the t&g plywood in the basement as everything else will be mold proof. Are there other options beside inorganic individual tiles or is the Delta Fl system reliable? In the 17 years we lived here, never had water in the basement and the foundation has a perimieter drain that gravity flows downhill from the house.

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  1. GBA Editor
    Martin Holladay | | #1

    There are two issues to consider when choosing flooring for a basement slab: one is the possibility of water entry; and the other concerns the temperature of the slab. (Cool slabs can be a condensing surface for moisture, especially during the summer when humidity levels usually rise.)

    If you are sure that your basement has no water entry problems, then the first issue is one you don't have to worry about.

    In general, you don't ever want to install carpeting on a concrete slab unless you are sure that there is a layer of horizontal rigid foam under the slab. (The foam insulation keeps the slab near the interior air temperature, greatly reducing the chance of condensation.) Without the foam insulation layer, you can get moisture build-up and mold under the carpet.

    Homeowners who live in older houses usually assume that their basement slab is uninsulated. If you want to install finish flooring on such a slab, you can install a layer of rigid foam on top of the slab, followed by one or two layers of 3/4-inch-thick tongue-and-groove plywood, secured to the concrete with long concrete screws through the foam.

    To answer your original question, "What's the best basement flooring system?," I would have to answer: The best flooring is concrete. The second-best flooring is ceramic tile over concrete.

  2. dietejrwso | | #2

    The rigid foam & plywood screwed to the concrete plan is almost identical to the Delta FL system. So I think I'll stay with that as my wife is not going to budge from wall-wall carpet. On another matter we have an efficient wood stove in the basement and once we start using it in the early winter the fire never goes out till spring. The basement remodel along with Delta Fl flooring system will have metal stud walls insualted with spray foam against the concrete and covered with mold resistant wallboard and a suspended ceiling. I wasn't planing to insulate the ceiling thinking that helps the heat transfer from the wood stove to the upper floor. Is that reasonable or should I insulate the ceiling?

  3. GBA Editor
    Martin Holladay | | #3

    If you keep a wood stove burning in your basement all winter long, I see no reason to insulate your basement ceiling.

  4. Expert Member
    Dana Dorsett | | #4

    Rigid foam & plywood is NOWHERE NEAR identical to the Delta FL system.!

    Foam + subfloor have a substantial and knowable R value, whereas Delta FL has no rated R-value. While the material itself isn't mold-food, almost anything put put on top of it will be- especially the rugs, and it would be surprising it were more than R 1.5, and could be less than R1. While that is sufficient to provide barefoot comfort against cold concrete, it's a serious problem for creating mold conditions on the bottom side of the rugs during the summer when the dew points of the room air rises with outdoor air/ventilation air dew points.

    To use rugs in a basement without sufficient R-value below it to keep the bottom of the rug well above the dew point of the peak summertime out door air dew points is to invite a mold-disaster. The R-value required to get there varies with location, since the outdoor dew points and sub-soil temperatures are all local. But any place cold enough to warrant a wood stove in the basement would have subsoil cold enough to need at LEAST R3 under the rug to be safe anywhere in the "A" zones of the US (4A though 7A).

    And heating with resistive mats without at least R5 (R10+ would be better) would have a disproportionately high operating cost too.

  5. dietejrwso | | #5

    Dana you don't agree with Delta FL regarding their flooring systems ability to stop moisture movement when installed correctly. Here is a quote from their webpage. " n a conventional basement, dry air in the living space acts as a magnet for water vapor seeping up through the concrete. But with the DELTA®-FL system, the space between the dimples provides a sufficient air-gap to equalize the water pressure above and below the concrete slab " Another reason I'm looking at their system is the minimal headroom I have to forfeit, anything more the an inch under the hvac trunk soffits would be very low headroom.

  6. GBA Editor
    Martin Holladay | | #6

    Water can evaporate from a damp slab to the basement air; that's the type of moisture movement that the Delta FL literature is addressing. However, Dana and I are talking about a different phenomenon: condensation (or, more accurately, adsorption) of moisture from the humid indoor air onto (or into) the cold concrete slab during the summer. Ideally, there will be a layer of horizontal rigid foam between the warm, humid indoor air and the cold concrete slab -- or a layer of horizontal rigid foam between the concrete slab and the cold soil. It's unclear whether Delta FL is airtight enough to prevent the type of condensation or adsorption that Dana and I are discussing.

  7. Expert Member
    Dana Dorsett | | #7

    I don't care at ALL about adsorption of room moisture into the concrete- the concrete isn't affected by moisture. I'm concerned about the relative humidity of the entrained air in the rug underlayment or bottom of rug, which can easily hit levels high enough to support mold growth if you don't allow the temperature of the underlayment to track the room temperature. I'm sure Delta FL stops the moisture going from slab to rug, but it doesn't have any means of stopping the moisture from going from room-air to cold underlayment.

    Mold growth starts to take off at air humidity levels of ~ 65% relative humidity with temperatures above 60F, and that's exactly what I'd expect to see with an R1-R2 rug over Delta FL in much of the northeastern US.

    When the room is running 72F/60% RH (or higher) as it might during much of the summer anywhere in the eastern US, the room air has a dew point of about 57F. If your subsoil temperature is under 55F, the RH of the air at the underside of the rug could quite easily be well above 75% and rife with mold growth due to the cooler temp under the rug. Since the rug is insulating, lowering the temperature at the top-side of the Delta-FL to a few degrees above the deep subsoil temp, and even R1-R2 of rug will take it there. But when you put even as little as R3 between the rug and the subsoil the temperature at the bottom of the rug splits the difference between the room temp and subsoil temp, keeping it at a humidity level that isn't strongly supportive of mold growth.

    Half inch vinyl or poly-faced EPS runs about R2.1 (actually a bit higher if it's on a cold slab) and has a vapor permeance well under 1- it's a vapor barrier, with very low capillary draw. Add half inch plywood over that and you're at 1", and looking at ~R2.5 between the slab and the rug. If you can use 3/4" EPS you'll be over R3.5 between slab & rug, an give you some temperature margin on mold growth under the subfloor as well. The subfloor doesn't have to be anywhere near as stout as subfloors going between joists, since it's fully supported by the foam & slab, not spanning between joists- it only has to have sufficient fastener retention for anything that's nailed/tacked to the top of it. In most case half-inch will be plenty. When the foam is fully supported by the slab, with 15psi rated foam between the half-inch ply and the slab the weight distribution of the plywood to the foam results in less deflection than 3/4" plywood subflooring between 16" o.c. joists.

  8. dietejrwso | | #8

    You-all have made me a believer, as I run a dehumidifier most of the summer months. That was one of my goals in the remodel, to reduce that electrical expense. After an hour of Googling I haven't found any links to vinyl or poly-faced EPS, do you have retail sources? I'm assuming the EPS should be sealed at all joints to maintain it's vapor permeance and this subfloor should be under any areas I plan to tile as well as the carpeted areas. Are those assumptions correct ? How will this system handle the weight of a pool table, about 500lbs resting on four 6" dia. feet ? I'm very glad I stumbled on to your sight, you both have been a tremendous help.

  9. Expert Member
    Dana Dorsett | | #9

    Some EPS products come with polymer facers that are sufficiently low-permeance, but foil facers are fine as long as it has foil on the top side. But it's just as easy to put down 6-mil poly between the EPS and subflooring (or between the EPS and concrete) for vapor control.

    A 6" diameter foot is 28.27 square inches, so with four of them you're looking at 113 square inches. With say 800lbs (your 300lb brother in law sitting on the 500lb table), you're looking at a static load of 800/113= 7psi, less than half the 15psi typical rating of Type-II EPS (even without the subfloor to further distribute the load.) Methinks you'll have more than sufficient margin to handle the dynamic loading unless you expect have four rowdy 200lbers having a polka-party on top of the table. ;-)

    You don't need to seam-seal the foam or sheet-poly vapor barrier to achieve good water vapor control. Vapor diffusion is a vapor pressure x surface area divided by permeance kind of deal- the surface area of any seam openings are pretty tiny in the grander scheme. But sealing the seams is important for air-tightness, and if the slab has any cracks in it (most do) a surprising amount of air can be pulled through slabs in a depressurized house. Go ahead and seal up the foam, and the subfloor too, but unlike above grade walls and upper floor ceilings you don't have to be super-obsessive about it. Taping the seams of the foam with housewrap tape is "good enough", even though it doesn't really stick super-well. The seam tape will be held in place by the subflooring anyway.

  10. mackstann | | #10

    Dana, what's your preference for securing the plywood subfloor and preventing edge curling?

  11. dietejrwso | | #11

    Nick, glad somebody else is following this thread and I'm interested in Dana's reply also. I haven't been able to source 1/2" T&G plywood or OSB, the thinnest I've found is 3/4". I suppose you
    could use biscuits on1/2" that is butt jointed or route the edges for shiplap joints. In my situation unless I find 1/2" T&G, I'll go with 3/4" and use 3 tapcons to each long side and 2 centered in the middle of the sheet.

  12. dietejrwso | | #12

    Is there a reason to use EPS foam board instead of XPS beside cost? XPS has a slightly better R value and higher perm. rating. Owens Corning;s Formular product description states it has a laminated film on both sides. If this product is suitable, would taping the seams with a low perm. tape eliminate the need for a sheet-poly vapor barrier?

  13. mackstann | | #13

    XPS is blown with an extremely potent greenhouse gas that mostly negates its beneficial insulative qualities (as far as the atmosphere is concerned).

  14. SusanBlue | | #14

    I think wooden flooring would be perfect for winters. We too have recently installed plywood flooring in our basement and it is working well.

  15. Sunduck | | #15

    Revisiting this convo with an uninsulated slab on grade home — would it be best to put faced foam with no moisture barrier but properly sealed, and then subfloor on top? I'm looking for wood or engineered wood floors with a thin floor assembly bc of 8' ceilings... I live in northern California, zone 3, 1960s slab on grade.

    I'm considering 2 options

    1) 1/2" XPS/EPS foam (it's hard to find GPS foam suppliers here, and at under 1"), 2 sheets of 19/32 Advantech, floated wood floors

    2) Delta-FL, 2 sheets 19/32 Advantech, floated wood floors

    I worry that with no insulation below finished floors, it could trap moisture, as Dana mentions.

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