Helpful? 0

Still trying to understand why you don't care about trapping vapor under a vapor barrier

Still trying to understand why you don't care about trapping vapor under a vapor barrier if there is vapor constantly pushing up from the earth. Also interested in good sealant.

I am having trouble nailing down the answers to two questions: first I asked about creating a greenhouse effect by putting a layer of poly over the slab on grade basement floor. I have an imperfect vapor barrier below the slab but there are seams and strips with missing plastic. So in short, if vapor is constantly pushing up why arent you concerned about trapping it over the slab, creating a mold inducive environment?

Second, I would like to use a sealant on the concrete floor and walls. Ive learned of two: Enduro Seal which supposedly infiltrates the concrete and fills in the capillaries to block moisture pushing upward to the slab surface. They claim to have no offgassing. I wonder what your team thinks of such a product for an insulated living space?

Asked by Chris Campbell
Posted Thu, 02/20/2014 - 18:40
Edited Fri, 02/21/2014 - 09:38

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13 Answers

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1.
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When its raining outside you dont care because you have a barrier protecting you from the rain,

If done right the barrier will protect you from the moisture and mold on the slab. The moisture isnt going to hurt the slab.

Answered by Robert Hronek
Posted Thu, 02/20/2014 - 19:38

2.
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I appreciate your response, but the analogy doesnt seem apt because the moisture in your example can ventilate away. I am talking about a situation where moisture penetrates the structure of the house and has no way to get out. If vapor is constantly pushing upwards against a vapor barrier that sits on top of a slab on grade, shouldnt we expect that moisture trapped against the barrier will eventually bead up - over the top of the slab foundation. As moisture from continuous vapor pressure accumulates, with nowhere to go, stagnant in a dark place, why does this not create a breading ground for mold using dust on the slab as food? My reading says that once formed, mold - which is now on the inside of the structure - will find a way to the seams. What specifically is inaccurate about this concern?

Also, do you have a thought about sealants to use? Have you ever used Enduroseal?

Thank you.

Answered by Chris Campbell
Posted Thu, 02/20/2014 - 19:51

3.
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Hi Robert,
It seems like if you put a vapor on top and bottom of the slab you would be trapping the moisture in the slab and then it would try and wick up the walls. Maybe you should go with a sealant and use a dehumidifier to dry out the area. I am definitely not an expert in this area. Just thought I would add my 2 cents if you don't mind.

Answered by Tom Perkins - Midcoast Maine CZ 6A
Posted Thu, 02/20/2014 - 21:40

4.
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Chris,
The moisture isn't trapped. It's just there.

You evidently think that moisture that is in the ground needs to be "ventilated away." But you can't do that; the moisture is infinite. It would be like trying to dry out the Atlantic Ocean. Sometimes we just need to accept the fact that some areas of our planet are wet. It's not our job to ventilate them.

You also seem to think that water vapor is "constantly pushing up from the earth." That isn't really the case. The moisture is one one side of the barrier; you are on the other. There is no flow.

When water vapor is moving across a building assembly, we can explain the moisture flow direction by noting that the vapor pressure is determining the direction of flow. But if you have a good layer of polyethylene, there is no vapor flow. Nothing is being pushed. The moisture in the ground is relaxed; it's happy. You are in your dry basement. You are also relaxed and happy. (I hope.)

Finally, you referred to a "greenhouse effect" caused by installing polyethylene under your basement flooring. The "greenhouse effect" refers to the heating of air in a greenhouse when sunlight passes through glass. I don't think the same effect can happen under your basement flooring.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/21/2014 - 09:47

5.
Helpful? 0

Chris,
How imperfect is the poly under your slab?
If it is, say, 95% continuous then it is essentially 95% effective.
As an experiment get a damp towel and fold it up, then put a piece of poly over it and sit on it - the seat of your pants should remain dry.
Now poke a bunch of holes in the poly or make a few small tears, put it back on the towel again and have another seat - I'll bet you could sit there all day and the seat of your pants will not get much more than a few damp spots.

Answered by Lucas Durand - 7A
Posted Fri, 02/21/2014 - 11:46
Edited Fri, 02/21/2014 - 11:51.

6.
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The vapor barrier under a slab keeps the moisture in the ground where want it, keeping it away from mold/rot susceptible materials (like flooring) in your house.

Vapor pressure doesn't "accumulate"- it's a function of temperature and saturation of the materials that are containing the water as adsorb (in this case, the temperature of your subsoil.) Except in very arid areas the moisture saturation of the subsoil is always at about 100%- as the soil changes temperature it either takes on or releases its moisture to/from the atmosphere above grade, since soils are generally extremely vapor permeable. With a mostly impermeable vapor barrier under the slab it forces that moisture migration path to go around your house rather than through your house, but it doesn't trap anything- the average moisture content of the soil just below the slab is still just a function of it's temperature (unless you're at the water table or have bulk water drainage making it liquid water under the slab rather than adsorb.)

With a vapor barrier under the slab the moisture content of the slab then tracks the moisture content of the conditioned space air, also relative to the temperature of the slab. With both a vapor barrier and insulation under the slab, in areas where the subsoil is colder than room temp the slab stays (generally) warmer == drier than it would be without the insulation. But with vapor-permeable insulation and NO vapor barrier the ground moisture will still be higher in the slab that it would be WITH the vapor barrier, since with the warmer slab & room above there's a vapor-pressure difference drawing moisture through the insulation. If the subfloor or finish floor above the slab is very vapor retardent it'll trap the moisture in the slab, which will then track the moisture content of the subsoil. That's potentially a problem, since a wooden subfloor is susceptible to moisture whereas the slab itself is not.

Answered by Dana Dorsett
Posted Fri, 02/21/2014 - 16:43

7.
Helpful? 0

Thanks for the answers. Some responses:

Martin, you say that with a polyethylene barrier on the slab, no vapor is trapped. It just is. There is no flow upward because vapor cant flow. I can buy this. And perhaps I misused the term "greenhouse effect," but with a layer of plastic over the floor, wouldn't vapor be likely to bead up under the polyethylene barrier and sit stagnant over the cement slab for long periods, which with dark stagnation and dust would create the circumstance for mold?

(Note that Dana above says, "If the subfloor or finish floor above the slab is very vapor retardent it'll trap the moisture in the slab, which will then track the moisture content of the subsoil. That's potentially a problem, since a wooden subfloor is susceptible to moisture whereas the slab itself is not.")

Mike said: "It seems like if you put a vapor on top and bottom of the slab you would be trapping the moisture in the slab and then it would try and wick up the walls. Maybe you should go with a sealant and use a dehumidifier to dry out the area." Doesnt he have a point?

Lucas - agreed. General coverage of vapor barrier under the slab is majorly helpful.

I do thank you for your attention.

Answered by Chris Campbell
Posted Wed, 02/26/2014 - 15:19

8.
Helpful? 0

Chris,
Of course you need a layer of polyethyene somewhere in your floor assembly, between the soil and your finish flooring. If you don't have any polyethylene, your flooring can get damp.

Your most recent answer appears to show that you are worried that your flooring will be damp. You quoted Dana Dorsett on that point. But Dana was talking about a floor assembly with no polyethylene. He wrote, "But with vapor-permeable insulation and NO vapor barrier the ground moisture will still be higher in the slab that it would be WITH the vapor barrier, since with the warmer slab & room above there's a vapor-pressure difference drawing moisture through the insulation. If the subfloor or finish floor above the slab is very vapor retardent it'll trap the moisture in the slab, which will then track the moisture content of the subsoil."

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/26/2014 - 15:55

9.
Helpful? 0

The moisture won't "bead up" under the vapor barrier, since it's at the same temperature as the concrete or soil that it's in contact with. The beading-up phenomenon you see when putting poly on the surface soil is due to the saturated soil not being allowed to track it's moisture with temperature when warmed by the sun, while the vapor barrier is at about the ambient air temperature, and below the dew point of the air in the sun-warmed soil beneath it. That's never going to happen under your slab, since the subsoil & slab isn't going to be much warmer than your basement room air.

Moisture will wick up an untreated foundation wall whether there is a vapor barrier over (or under) the slab or not, and the presence or absence of the slab vapor-barrier will not affect the rate.

In new construction it's best practice to put a capillary break between the footing and foundation wall, and a thermal & capillary break between the slab edge and foundation wall.

As long as the vapor barrier is between the 100% saturated ground moisture and the susceptible material like subfloors or surface-paints, there isn't a ground moisture problem, whether it's under the slab or on top of it.

Masonry sealers will slow down the capillary draw at both the slab and foundation walls, and as long as you have reasonable drainage that would be "good enough" for unfinished walls & slabs. If you want to paint them, a vapor-permeable paint will last longer than a thin impermeable paint, since there will still be a vapor diffusion issue, even if the capillary draw has been slowed. by 90% or better, often enough to cause blistering of the paint. Thick impermeable epoxy will usually hang in there for quite awhile though, if it's thick enough to be sufficiently rigid to not blister.

Mechanical dehumidification is more likely to be needed in summer, when your ventilation/infiltration air is likely to have a dew point close to or above the temperature of an un-insulated slab/foundation. Unless you have serious drainage issues or zero air exchange in your basement , ground moisture isn't going to raise the basement humidity to mold-inducing levels in most climate zone 4 & higher locations.

Answered by Dana Dorsett
Posted Wed, 02/26/2014 - 15:57

10.
Helpful? 0

Chris,
Concerning your worry that a damp slab with polyethylene above and below the slab could wick moisture at the slab perimeter to the concrete walls:

1. If there is poly under the slab, there won't be much moisture in the slab, especially if the slab finds a (slow) drying route at the slab perimeter.

2. The amount of moisture that could be transmitted by this route (and the rate of moisture transfer) would be quite small.

3. Best practices call for the installation of vertical rigid foam at the slab perimeter; this foam acts as a capillary break and a vapor retarder.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/26/2014 - 16:00

11.
Helpful? 0

I think this medium is making the communication difficult. Martin, I am having trouble getting a direct answer from you to this question: Just say you have no vapor barrier below the slab. If you put polyethylene over the cement slab, wont vapor from below the slab bead up under it over time?

thank you.

Answered by Chris Campbell
Posted Wed, 02/26/2014 - 19:32

12.
Helpful? 0

Chris,
Dana Dorsett has provided an answer. He wrote, "The moisture won't 'bead up' under the vapor barrier, since it's at the same temperature as the concrete or soil that it's in contact with." That answer sounds correct to me; I assume that any theoretical "beads" of water would be absorbed quickly by the concrete, which is hygroscopic. There wouldn't be any reason for water to form visible beads.

The larger question, of course, is, "Who cares what happens under the polyethylene?" It's invisible; it's dark; it's damp. You can't see it. You can't touch it. It can't hurt you.
Don't worry about it.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/26/2014 - 20:38

13.
Helpful? 0

" If you put polyethylene over the cement slab, wont vapor from below the slab bead up under it over time? "

No, it won't EVER bead up because the vapor barrier will be at a temperature above the dew point of the air entrained in the sub-soil.

The concrete will have a higher moisture content, but it won't be in the form of liquid water, only adsorb + water vapor.

When you put a sheet of plastic on the above-grade dirt and shine some sun on it the surface soil heats up to above the ambient air temperature, releasing it's adsorbed moisture, but the plastic remains at or near the ambient air temperature. When the dew point of the damp air under the plastic reaches the temperature of the plastic, it condenses on the plastic forming beads of liquid.

You'll never see those conditions at a basement slab, which doesn't get a lot of sun even when it's not covered. If the vapor barrier is covered be a finish floor there will be NO solar heating of the slab- the room will be getting most of the solar heating, and the room temperature will rise first, reliably keeping it above the temperature of the slab.

Answered by Dana Dorsett
Posted Thu, 02/27/2014 - 14:15
Edited Thu, 02/27/2014 - 14:16.

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