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What happens to the moisture?

Posted by microxl (My Page) on
Sun, Oct 28, 07 at 17:51

A question and a little info sharing

I continue to debate with myself (and my wife) the pros and cons of the various basement subfloor applications like DriCore/Subfloor and Delta-FL and Platon products.
I think some GW posters have made it pretty clear that Delta-FL or Platon type materials are the most economical way to go. I like them too because there is no OSB involved, at least if I use a floating laminate floor on top.

But, I have this nagging question as to what happens to all the moisture and air that circulates under the material? Can anyone explain this? Does it work its way to the sides of the room and escape up along the walls? I would think it has to go somewhere.

According to the Platon website it says you can even use the material on the interior cement basement walls securing and sealing it at the top with a caulk (PL procuct?). (It doesn't say if you seal it to the floor Platon as well or not.) It goes on to say that if using fiber glass insulation to be sure to use the standard vapor barrier before installing sheetrock. This would in effect create a double vapor barrier, one on each side of the insulation I would think.

By coincidence I came across a Univ. of Minnesota study that tested and compared different basement moisture barrier applications:

In short,they came to find that in cold climates moisture can condense on either side of a single moisture barrier depending on the time of year because moisture will go both ways. They concluded that the double barrier was the best way to keep the insulation dry. When installing framing for the basement perimeter they installed and sealed poly sheeting on the back side before hoisting the framing into place.

Follow-Up Postings:

RE: What happens to the moisture?

I would think moisture trapped would do what moisture does, it grows mold. It's a simplistic solution on how not to get your carpet wet but doesn't address the issues generated by the application.

RE: What happens to the moisture?

Install ceramic/porcelain tile. Carpet has NO place in a basement. Tile can be "cold" in winter, but then you can lay out area rugs. In the summer roll them up and put away. Rugs always get musty in the basement. Try pulling one up...yuk!!!

RE: What happens to the moisture?

Regarding the Univ. of Minnesota recommendation to use double vapor barriers around the insulation, I've always thought the approach to plan and construct such that the moisture dries to the interior is the only safe (perhaps sane?) way to handle the moisture issue.

RE: What happens to the moisture?

Here's what the Building Science Consortium says about double vapour barriers:

"The main problem with a double
vapor barrier wall is that it cannot dry to
either the inside or the outside should it
ever get wet. In addition, it requires a
perfect air barrier on the interior to prevent
warm interior air from contacting and
condensing on the cold foundation wall
where it may be trapped. This type of
construction should be avoided."

Even the University of Minnesota study notes that in a basement without an internal vapour barrier, a dehumidifier would prevent excessive water vapour problems. Yet time after time, I come across musty basements that would have been fine if the owners had only bothered to invest $200-$250 in a portable dehumidifer and $50 a year in electrical charges. (I was just at a customer's home today. Sure enough, the dehumidifier I bought and hooked up to run continuously was stuffed in a corner of the basement.)

I've used glue-down carpet for years in basements with absolutely no problem. But only low-pile with a jute backing that doesn't trap water vapour. Only in new homes and only with mechanical dehumidification.

Here is a link that might be useful: Basement Insulation Systems

RE: What happens to the moisture under the Delta FL

Delta FL equalizes the pressure above and below the slab, thus reducing the drive of water vapour through the slab. Water vapour still moves into the conditioned space but in much smaller quantities.

Here is a link that might be useful: Delta FL--How it Works

RE: What happens to the moisture?


i have a large basement (>2000 sq feet) but plan on putting carpet in one small section (less than 300 sq feet). this is a work out room and we would probably put in a low pile carpet and padding designed for basement usage.

do you think we should put a dehumidifer into this room? if so - can you recommend a brand?

For the rest of the basement - I am starting to look at stained concrete and a new product called polycrete. The subfloor products like dricore are so expensive - I can get the entire basement stained with a nice finish for about the cost of just the dricore subfloor. Do you have any experience with these stained concrete products? any pitfalls?


RE: What happens to the moisture?

Every basement where the humidity goes above 45%-50% should have a dehumidifier. I wouldn't bother with a separate one for the workout room. Just put it in the main area and leave the door open when you're not using the room. You might check here for dehumidifier reviews. I've been using Woods' models for years and they are holding up.

Avoid any foam or rubber type underpad for basement carpeting. It will only trap water vapour moving up through the floor. Luxury new homes in my area usually have plush carpeting over thick underpads, even laminate flooring(!) Looks great for a couple of years. Till the mould overwhelms even the most blocked sinuses.

I've not used stained concrete, so I can't comment.

RE: What happens to the moisture?

I believe Dricore used to provide the same installation instructions as Delta-FL, seal in the moisture between the concrete and the poly barrier. These days Dricore calls for a 1/4" gap at the walls, to allow moisture to ventilate to the sides and up while keeping the finished floor dry. You will need a dehumidifier.

Any plastic barrier in a basement makes me nervous. A different approach is 3/4" XPS as a vapor retarder (not a barrier), then 1x4 strips and floating plywood floor above that. The XPS insulates the floor but vapor can still escape up through the floor, just at a slower pace so the subfloor is not soaked. See for a picture (Read This Before you Build/Renovate....).

RE: What happens to the moisture?

Building Science lists the dimpled plastic membranes like Platon and Delta-FL as an approved way to provide vapor control and drainage. But it does appear that assumes an air tight seal at the perimeter which I don't think Dricore would provide.

"Vapor pressure on top of slab and under slab equalizes, thereby stopping capillary transfer of water and soluable mineral salts (moisture content in air space and under slab remains the same; ie., wet)"

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