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What’s the Best Basement Flooring System?

A homeowner remodeling a house with a walkout basement looks for a foolproof flooring option, but he might be worrying about the wrong thing

Posted on Apr 7 2014 by Scott Gibson

With a basement remodel underway, Jeff Dieterle weighs his options for a trouble-free floor. "We want to do the kitchen and bathroom in tile or stone and the rest of the area in wall-to-wall carpet," he writes in a Q&A post at Green Building Advisor.

So far, he's found two products that would seem to work: Ditra-Heat under the tile or stone sections of flooring, and a product called Delta FL in tandem with tongue-and-groove plywood under carpeted sections.

Dieterle has never had a bulk water problem in the basement in the 17 years he's lived in the house. Even so, he's not sure that the Delta membrane/plywood combination is a good idea.

"Not fond of the T&G plywood in the basement as everything else will be mold-proof," he writes. "Are there other options beside inorganic individual tiles? Or is the Delta FL system reliable?"

That question is the start of this Q&A Spotlight.

Two potential water problems, not just one

There are two types of potential water problems in a basement, GBA Senior Editor Martin Holladay says. One is the possibility of water leaking in from the outside; the second is that a cool concrete slab can become a condensing surface for moisture, particularly in the summer. The first is apparently not a problem here.

"In general," Holladay adds, "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."

Most older houses don't have a layer of foam beneath the slab. Assuming that's the case here, Dieterle could add a layer of foam on top of the concrete, followed by one or two layers of 3/4-inch tongue-and-groove plywood and then the finished flooring.

Holladay's suggestion of foam and plywood seems "almost identical" to the Delta FL system, Dieterle replies, so he'll just stick with that. And even if the best possible flooring in this situation might be concrete, or tile over concrete, his wife "is not going to budge from wall-wall carpet."

No, the two approaches are not the same

You're wrong about that, writes Dana Dorsett. The Delta FL systems bears little resemblance to rigid foam insulation and plywood.

"Foam and subfloor have a substantial and knowable R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. , whereas Delta FL has no rated R-value," Dorsett says. "While the material itself isn't mold-food, almost anything put put on top of it will be — especially the rugs."

The R-value of the Delta FL membrane could be less than R-1, he adds, and while that would be enough to protect bare feet from the cold concrete, it would still create mold conditions on the bottom of the rugs during the summer when the dew points of the room rise.

"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 outdoor air dew points is to invite a mold disaster," he says. "The R-value required to get there varies with location, since the outdoor dew points and subsoil temperatures are all local. But any place cold enough to warrant a wood stove in the basement [as Dieterle plans to do] would have subsoil cold enough to need at least R-3 under the rug to be safe anywhere in the ‘A’ zones of the U.S. (climate zones 4A though 7A).

"And heating with resistive mats without at least R-5 (R-10+ would be better) would have a disproportionately high operating cost, too," he adds.

The problem isn't moisture migrating through the concrete

Quoting literature from the manufacturer's website, Dieterle says that Delta FL is designed to stop water vapor from seeping up through the concrete.

But it's not that kind of moisture that Holladay and Dorsett are concerned with. Assuming the Delta membrane will prevent the migration of moisture upward through the slab, that still leaves the problem of condensation (or more properly, Holladay says, "adsorption").

"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," Dorsett says. "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 the room air to the cold underlayment."

Mold loves relative humidity levels of 65% and greater and temperatures of 60°F and above, he says, and that's exactly what you'll get when you put carpet with an R-value of 1 or 2 over Delta FL in much of the northeast U.S.

With room air at 72°F and 60% relative humidity, common in summer in the eastern U.S., the dew point would be about 57°F, Dorsett writes. With subsoil temperatures under 55°F, the relative humidity of the air on the underside of the rug could easily be greater than 75%, "and rife with mold growth due to the cooler temperature under the rug."

Headroom is an issue, but with as little as 1/2 inch of poly-faced EPS foam insulation plus plywood would sharply reduce the chance of mold growth.

Our expert's opinion

GBA Technical Director Peter Yost added this:

Great points all around; I will just add some food for thought.

Bulk water leaks: Most insurance companies will tell you their two single largest claims related to moisture are for ice dams and ruptured clothes washer hoses. If your basement does not leak, that’s great, but make sure that you have control of your clothes washer as a major source of interior water leaks. I recommend that any clothes washer, no matter what floor it is located on, should be controlled by a single-throw combo valve, conveniently located to treat water pressure to your clothes washer like a light switch: on when doing the laundry, otherwise off.

Moisture evaporating (unseen) from your basement slab: There are two tests — one qualitative and one quantitative — for determining whether moisture is coming up through any concrete slab.

Insulation under the floor: I could not agree more that elevating the basement floor temperature is essential in terms of managing mold, but also dust mites. Be careful when you go looking for EPS. There are many different types, as you can see in this Table of EPS Types and properties. There are other options for rigid insulation but availability and price for materials such as Foamglas and rigid mineral wool are issues.

I would lobby hard for rugs rather than carpets in the basement; they are designed for relatively easy cleaning if — or more likely, when — your basement floor gets wet, or even just damp.


Tags: , , , ,
1.
Mon, 04/07/2014 - 11:57

What would you recommend for flooring in suburbs of Atlanta?
by Nat Hilton

Helpful? 0

This forum is right on time. I'm in the process of finishing my basement. I will have 2br, a family room and bathroom. I was thinking about doing carpet in those areas, (mostly comfort for kids) but with an underlying system like rigid foam, and then ??? I don't mind doing tiles, which is water proof right? I don't ever have to change if I get a leak? What sought of tile outs be best?
What is the process for this? I'm putting up rigid foam ( 3/4 rigid foam sheathing , bc studs already up, can't find 1" and don't have enough space for the 2" on basement wall.

Thanks..
I'm enjoying all the education from this website.


2.
Mon, 04/07/2014 - 12:24

Response to Nat Hilton
by Martin Holladay, GBA Advisor

Helpful? 0

Nat,
If your basement slab lacks sub-slab insulation, you’ll need to install some rigid foam above your slab -- especially if you decide to install carpeting. Otherwise, the slab or the flooring will be damp during the summer (due to condensation on the cool slab).

The usual technique is to install 1 or 2 inches of XPS or EPS foam insulation on top of the existing concrete, followed by a layer of plywood that is fastened through the foam to the concrete with TapCon fasteners. (If you are still worried that your slab may sometimes be damp, you might want to install a layer of dimple mat under the foam.) When installing this layer of foam, it's important to make the installation as airtight as possible, to make it impossible for any humid interior air to contact the concrete. Seal the edges of each piece of foam insulation with a high-quality European tape, with caulk, or with canned spray foam.

If you don’t want to lose the height required for rigid foam, you could try installing a dimpled subfloor product like Delta-FL. (Note that some similar products, notably DRIcore, have mixed reviews from some builders. For more information on this topic, see comment #6 from Mike Guertin, below.)

For more information on insulating existing basement slabs, see:


3.
Wed, 04/09/2014 - 16:38

Any chance someone could post
by dennis brown

Helpful? 0

Any chance someone could post a link or two for the washer switch stated above? Methinks this is something I need to do.


4.
Wed, 04/09/2014 - 18:01

Edited Thu, 04/10/2014 - 05:40.

Response to Dennis Brown
by Martin Holladay, GBA Advisor

Helpful? 0

Dennis,
It's a valve, not a switch. The classic valve has been manufactured by Watts Valves for decades -- the good old Watts #2.

Here is a link to a page from a retailer:
http://www.amazon.com/Watts-Brass-Tubular-2-M2-Valve/dp/B00128816E

Here is a link to the item on the Watts website:
http://www.watts.com/pages/_products_details.asp?pid=736


5.
Thu, 04/10/2014 - 09:30

Existing plumbing fixtures and cabinets
by Darcy Grant

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I have an existing finished basement that I want to renovate in order to add insulation to walls and floor (currently no floor insulation and carpeted, and walls have batt and poly). My question is in the bathroom, do I lift out the shower, cabinets, toilet, etc. to install the XPS and plywood, then reinstall the fixtures on top (while extending the drains, of course). Are fixtures normally installed over top of this subfloor system?
Thanks
Darcy


6.
Thu, 04/10/2014 - 09:51

Response to Darcy Grant
by Martin Holladay, GBA Advisor

Helpful? 0

Darcy,
Q. "In the bathroom, do I lift out the shower, cabinets, toilet, etc. to install the XPS and plywood, then reinstall the fixtures on top?"

A. Yes, that would be the best way to proceed.

Q. "Are fixtures normally installed over top of this subfloor system?"

A. I'm not sure what you mean by "normally," but if you have a bathroom in your basement, and if you want to install rigid foam above the slab and on the interior of your concrete walls, the way you describe is the best way to proceed. It probably wouldn't hurt to support the toilet and bathtub with a few lengths of pressure-treated lumber (installed as sleepers, replacing some of the rigid foam).


7.
Thu, 04/10/2014 - 22:47

dmx 1 step
by Nat Hilton

Helpful? 0

Has anyone heard about DMX one step? It seems to be similar to Delta-fl, but dmx has an R value and also some antibacterial foam, which they say helps with the clicking noise delta makes. 10mm or more laminate can be installed over this , offer at HD, but needs to be special order. It's also a little pricier..


8.
Mon, 04/14/2014 - 09:56

EPS / XPS foam density
by Mark Fredericks

Helpful? 0

I'm working on a project where we're needing to match an existing floor height. We're dealing with an existing basement in Climate Zone 6. A portion of the basement has already been finished using 1.5" EPS foam and 3/4" plywood. I'm considering using XPS or EPS foam at the same thickness so all of the floor heights match up. I'm comparing prices but I'm unsure of the appropriate density of either material. Does it matter much if we're covering it with 3/4"plywood + flooring anyway?
The EPS seems cheaper, but it won't accept tape as well as XPS for sealing all the seams. Any suggestions?


9.
Mon, 04/14/2014 - 10:30

Response to Mark Fredericks
by Martin Holladay, GBA Advisor

Helpful? 1

Mark,
The short answer is that ordinary EPS has sufficient density and compressive strength to work in this application.

To read about a successful use of EPS for this type of job, see Green Basement Renovation.

The same job is also profiled in this article: The Stay-Dry, No-Mold Finished Basement.


10.
Tue, 04/15/2014 - 14:24

best alternative without using rigid foam on the floor
by Fred Zheng

Helpful? 0

I have a 7yr old house at West Chester, PA, with concrete slab in my unfinished walk-out basement. I don't know whether the builder had put rigid foam under the slab. I am in the planning stage of having the basement finished. What is the best alternative to provide a subfloor for either carpet or laminates without using rigid foam? Any thick subfloor may require redo the stairs for meeting the code. Thanks.


11.
Tue, 04/15/2014 - 14:48

Response to Fred Zheng
by Martin Holladay, GBA Advisor

Helpful? 0

Fred,
The correct way to insulate a concrete slab is with rigid foam. If you don't want to use rigid foam, that means that you want to do it the wrong way.

If you intend to finish your basement, and your basement slab lacks sub-slab insulation, you’ll need to install some rigid foam above your slab. Otherwise, the slab will be damp during the summer. (If you aren't sure whether your slab has insulation underneath, you can drill a hole through the concrete to inspect what's under there.)

The usual technique is to install 1 or 2 inches of XPS or EPS foam insulation on top of the existing concrete, followed by a layer of plywood that is fastened through the foam to the concrete with TapCon fasteners. (If you are still worried that your slab may sometimes be damp, you might want to install a layer of dimple mat under the foam.) When installing this layer of foam, it's important to make the installation as airtight as possible, to make it impossible for any humid interior air to contact the concrete. Seal the edges of each piece of foam insulation with a high-quality European tape, with caulk, or with canned spray foam.

If you don’t want to lose the height required for rigid foam, you could try installing a dimpled subfloor product like Delta-FL. (Note that some similar products, notably DRIcore, have mixed reviews from some builders.)

For more information on insulating existing basement slabs, see:


12.
Wed, 05/21/2014 - 14:06

Flooring options
by Michael Ashooh

Helpful? 0

I am finishing a basement renovation using the method that you have outlined here and in several other articles. I have 1inch of XPS covered by 2 layers of 1/2 inch exterior rated plywood. I wanted to use bamboo flooring in the main areas of the basement, but I wondered whether underlayment options for bamboo would interfere with the drying principles of this system. Can you suggest how to go about installing bamboo or other engineered wood flooring options that is consistent with the principles of this method? Additionally, I have a small bathroom and an entry area. I am assuming that tile cannot be laid over this flooring, and I don't want to use vinyl, but do you have other suggestions? Thanks


13.
Wed, 05/21/2014 - 14:40

Response to Michael Ashooh
by Martin Holladay, GBA Advisor

Helpful? 0

Michael,
If you install rigid foam on top of a slab, you don't want to encourage any moisture flow upward. The intent of the flooring assembly is not to allow the slab to dry upward.

Q. "Can you suggest how to go about installing bamboo or other engineered wood flooring?"

A. Any engineered flooring product must be installed according to the manufacturer's recommendations. Consult the manufacturer if the instructions that came with the product are unclear.

Q. "I am assuming that tile cannot be laid over this flooring."

A. If your subfloor is adequately screwed to the underlying concrete, I see no reason why tile flooring couldn't be installed over the plywood subfloor. But most builders would advise you that it is easier to install the tile directly to the concrete.


14.
Thu, 05/22/2014 - 08:31

Flooring options follow up
by Michael Ashooh

Helpful? 0

Thank you, Mr Holladay for your quick response. I have been a big fan of your work for some time in FH and here. It's a pleasure to be able to ask you some questions about these issues, if you don't mind.

Just a clarification. When you say: "The intent of the flooring assembly is not to allow the slab to dry upward." Do you mean that the intent is to try to prevent the diffusion of vapor from the slab toward the interior? Or that the system is not designed to encourage or facilitate upward drying? I've always understood it to be the later. Or am I missing a significant distinction between drying and vapor transmission? I've understood the system to be designed to allow for the fact that vapor or water molecules will diffuse toward the interior. So, what I hear you saying is that we are not trying to facilitate that process, only account for it. Am I wrong? For instance, I did not include a vapor barrier between the XPS and the concrete, as many suggested, because as I understood this, the XPS allows for diffusion. But if the goal was to prevent diffusion or vapor transmission, then it seems that I should have included such a barrier. Can you help me to understand that better?

I was concerned with both hardwood underlayments and tile would have very low permeability, lower than the XPS anyway, and so trap vapor in the plywood. Now I am starting to think I have misunderstood something.

BTW, a few tiling contractors thought that the XPS would not be rigid enough over time to prevent flexing, which was another concern, but I am very happy to hear that you think I could lay tile over this. Any thoughts on that concern?

Thanks again for your help. I have been wanting to ask you these questions for a long time.


15.
Thu, 05/22/2014 - 08:40

Response to Michael Ashooh
by Martin Holladay, GBA Advisor

Helpful? 0

Michael,
The soil under your basement slab will always be damp. So will your slab, unless your builder installed a polyethylene vapor barrier between the dirt and your slab.

The interior of your house will be warm and dry, while the dirt under your slab will always be cool and damp. We don't want to encourage any moisture flow from the damp soil to your dry interior. We don't want any water vapor flow; nor do we want any liquid water flow. What we want is a barrier to separate these zones.

The usual barrier is polyethylene. If polyethylene was installed under your slab, you are all set. If there are reasons to believe that the builder didn't include any polyethylene under your slab -- a very possible situation for older homes -- then you need to install a layer of polyethylene above your slab. Rigid foam is almost as good as polyethylene, because it stops most of the water vapor transmission -- but not quite as much as would polyethylene.

Again, you are looking for a barrier to separate these zones. It makes no sense to encourage the flow of water vapor from the damp soil under your slab to the interior of your house.


16.
Wed, 06/11/2014 - 22:24

Textured XPS in lieu of a dimple mat?
by Brent Mellen

Helpful? 0

For slabs that might sometimes get damp, could a textured XPS such as Owens Corning Foamular Insul-Drain be used in lieu of a dimple mat? Maybe the drainage would not be as good as a dimple mat, but better than standard smooth XPS? Thanks.


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