View Full Version : Wall Framing and Flooring over Subfloor

12-27-2012, 10:12 AM
As I mentioned on another thread, I am preparing to finish my basement. I am considering the following subfloor stackup:

Existing Concrete Floor
Delta FL Vapor Barrier
1" XPS
5/8" Plywood (Tongue and Groove)

The finish flooring will, most likely, be vinyl plank in the living areas and ditra/tile in the bathroom.

I have several questions:

Does this subfloor stackup look reasonable?
Can the Plywood be left floating in the living areas?
Should the Plywood be Tapcon'd through the XPS to the existing concrete in the bathroom?
Should the partition walls be built on top or next to the built up subfloor? (see sketch below). In either case the base plate would be fastened to the existing concrete floor.

In the sketch below, I've shown "on top" as A and "next to" as B. If A is used, to the left would be an unfinished utility room with a dehumidifier to pull any moisture from below the Delta FL. If B is used, any moisture below the Delta FL have to absorb back into the concrete or into the base plates (which would be treated, of course). The exterior stud walls are in, so B will be used around the perimeter of the basement regardless.


Thanks in advance for your replies!

12-27-2012, 04:00 PM
Don't use DELTA FL and XPS together.

Frame after you finish the floor, it will be much better that way.

You will need to tapcon the ply, it wont stay there floating perfectly without.

12-30-2012, 10:08 AM
6 mil vapor barrier, then?

12-31-2012, 08:39 AM
A 6 mil vapor barrier would be fine.

The foundation wall also needs foam between the concrete & studs, as well as between the concrete & sub-floor edge.

You have serious moisture trap risk to the plywood with vinyl flooring on top, which is also a strong vapor barrier. Using something more vapor open than vinyl for the finish floor is STRONGLY recommended.

12-31-2012, 10:25 AM
Thanks for the replies. Are there any "non-carpet" flooring options that would be suitable below grade other than carpet? My thought with vinyl was that it would be moisture tolerable, but I've kind of eliminated that line of thought with the plywood in the subfloor stackup.

12-31-2012, 11:24 AM
Dana, regarding the top vapor barrier, is the tile in the bathroom going to be a problem? Would I be better to use cement board rather than plywood in there?

Thanks again!

12-31-2012, 03:52 PM
Tile is usually quite vapor permeable, despite being waterproof to liquid water. Cement board provides better rigidity and a better bonding surface for tiles than plywood, which is also important to consider.

Most laminate flooring is at least somewhat vapor permeable, but get specs on it. Standard hardwood & bamboo flooring also works, and with the foam & poly underneath would only have moisture issues if the basement has flooding problems, which would hopefully be addressed BEFORE you finish it out nicely.

With an inch of foam you'd have sufficient R under it to avoid summertime condensation/mold issues under the carpet, if you went with a carpeting solution. Without the foam carpet would be all but guaranteed to have rot/mold issues. (Not that I'm a fan of carpeted floors, in basements or anywhere else.)

06-05-2013, 04:28 PM

Would a thinner insulation (say 3/4") still work under the plywood subfloor?

06-06-2013, 12:33 PM
Depends on what you mean by "work".

At 3/4" XPS is only about R3.5, which is on the thin-side for the ~45F western WI deep subsoil temps (http://mb-soft.com/solar/soilmap.gif), and you may need to use dehumidifiers in summer to keep the subfloor from taking on moisture from the ventilation air. The more R you can get between the subfloor and concrete the better, up to a point. Average summertime outdoor dew points in western WI are in the low 60s, and if the bottom surface of the subflooring stagnates at 60F or lower it'll take on moisture. With a 70F room, 65F floor and 45F slab, you have a 20F difference. If the finish floor + subfloor add up to R1 you have about R4.5 , total or 20/4.5= ~4.5F per R. That means the bottom of the subfloor will be running about 65F-4.5F, or 61.5F or about the same temp as the average http://weatherspark.com/#!dashboard;a=USA/WI/Eau_Claire. That kinda makes it on paper, but not with a lot of margin. With R5 (or a warmer subsoil temp- can you move the house to Peoria? :-) ) you'd have a bit of margin.

If you put a dehumidifer down there and keep the most humid rooms under 60% RH all summer it'll have quite a bit of margin.

06-06-2013, 06:23 PM
Unless you seal the edges, Ditra has channels to manage water vapor underneath it, so is certainly easier to install and, personally, a better tile substrate than cbu. It is the only one specifically allowed to be installed on a slab as soon as you can walk on it...most require you to wait at least 28-days because of that vapor management and your old slab should be a piece of cake for it to handle. FWIW, Schluter is coming out with a heating mat with Ditra integrated. No idea when it will be released, but it may be nice for the bathroom.

06-06-2013, 06:56 PM
Thanks for the quick replies!

Dana, I will be using a dehumidifier but I'd rather not depend solely upon it. Therefore, 1" certainly seems like the way to go. I think I'm moving back toward carpet due to WAF (Wife Acceptance Factor).

Jim, that is very interesting that Schluter is is looking into that, that would be perfect for my install. Any idea when they might release?

Should I be using Cementboard for the subfloor? If so, stick with the 5/8" thickness? Is the 1" XPS underlayment going to be a problem (it seems to me like there would be a little flex here, maybe no more than floor joists?)?

06-10-2013, 12:33 PM
Channels do not have any effect on water vapor- channels are about reducing the capillary wicking of liquid water.

But the materials Ditra is made of make it a class-I vapor retarder, a true vapor barrier, so be careful what you trap underneath it, since whatever that is will end up being nearly 100% saturated at least some of the time.

Cement board isn't necessary, but would probably be fine for use as a subfloor under carpet. To meet code it has to be thick enough to meet the ignition barrier requirements for the foam. But wood is fine, as long as you have the poly under the foam, and sufficient foam-R to avoid the summertime moisture issues. There's actually LESS flex than with joisted systems, since the foam & slab are supporting it over the entire surface, the slab is what ultimately determines the flex. If the slab is pretty flat, the flexing of the subfloor will be near-zero.

06-10-2013, 07:24 PM
Dana, you need to look at the construction of Ditra! There are unobstructed channels at 90-degree angles that run entirely across and along the sheet membrane about 1/8" tall and about 1/4" wide, not counting the fleece that should be embedded in the thinset. It does not trap moisture vapor from underneath and allows it to vent. No idea when Schluter will come out with their Ditra Heat product...a couple of weeks ago at a class, they said the prototypes had passed all of the tests, but now they have to get around to making it in volume, designing marketing, distribution, training, etc. From what they said, it will be awhile, but they may have been hedging their bets in case of any glitches. They're pretty conservative, and I expect when it is released, it will be a nicely engineered product.

06-10-2013, 09:21 PM
Jim, if Ditra is butted up against the walls and flooring goes over the top of it, where will the water vapor vent? I don't understand how it is supposed to work.

06-11-2013, 10:39 AM
Jim- Just because it has air channels doesn't stop it from being a vapor barrier. A sheet of poly with big slashes is still a vapor barrier that will trap moisture, even though it's far from air tight. Without any convective, mechanical, or other drives to move that air displacing it with drier air, it'll just sit there.