Moisture problem on concrete floor

[Mike D]

i converted a 12x24 semi-attached garage into living space 6 years ago. On the 'crete floor I had painted, then a layer of 4mil plastic, then a layer of bluefoam insulation, then 3/4 particle board (used the blue screws to secure the boards), then commercial carpet.

The carpet is moist and the room smells like a stagnant locker-room. I ridge vented, but the trouble is (now) obviously that the concrete is leeching ground-moisture upwards. So.. Ill take up the particle board and do what?

I'd float the floor on 1x's but they'l rot.. Any Ideas?

Mike D

[manhattan42]

The main problem is the 4 mil plastic.

4 mil plastic will do little to stop moisture flow.

You generally need at least 2 layers of 6 mil plastic laid with al seams taped or sealed even at the walls or foundation

-- manhattan4

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[dirt farmer]

Make sure the entire slab is above grade. Water could be getting in through the walls, sill plate, or migrating from the roof down the outside of the walls and wicking through the mud sill. Put in a french drain.

[Joseph Meehan]

Dirt is defiantly correct. Common concrete is not water proof. You may need to lower the water table around the pad.

[Goedjn]

The concrete isn't waterproof, but the 4-mil plastic ought to be, and the foam, while not waterPROOF should be water resistant. I'd look around for another source of water-intrusion before blaming ground-water.

My instinct says that you ought to have put the plastic on TOP of the blue foam, on the theory that the top edge is warm and not likely to condense.

[manhattan42]

4 mil plastic is "waterproof" but it is not "vaporproof"....4 mi plastic has a relatively high perm rating which means that it readil allows gaseous water to flow thru it. 6 mil plastic sheeting has a much lower perm rating and doubling th sheets and taping the seams will eliminate most gaseous water.

The slab itself should be painted with a vapor barrier coating t further eliminate any gaseous water entering the floor thru the porou concrete slab.

The proper place for plastic sheeting is below the concrete in a livin space.

It is this gaseous water that will condense and cause mold and milde growth deapite liquid water entering from anywhere else

-- manhattan4

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[nicksanspam]

Then again,

says 4 mil polyethylene film passes 0.17 gr/h-ft^2 per "Hg of water vapor pressure, vs 0.11 for 6 mil, vs about 20 (200X more) for 1/2" drywall. A 1" hole in a sheet of drywall can pass a lot more water vapor.

A house with a 1000 ft^2 50 F wet basement floor with 70 F air at 50% RH would have Pw = e^(17.863-9621/(460+50)) = 0.367 "Hg at the floor and Pa = 0.5e^(17.863-9621/(460+70)) = 0.374 "Hg in the air, so moisture would flow FROM the air TO the floor through a 4 mil poly film layer at a rate of 1000(Pa-Pw)0.17 = 1.2 grains per hour, ie 1.2/7000 = 0.00017 pounds per hour, ie about 1 drop of water every 2 weeks. Changing to 6 mil poly would reduce this to about 1 drop every 3 weeks :-)

Nick

[manhattan42]

The problem with all the scientific rhetoric is that is does not tak into account the reality that gaseous water (humidty) passing u through the concrete can be well more than 100% and that th tempartures can drop well below freezing.

This adds up to temperatures and conditions well below the dew poin for that portion of air even at 70F or above.

The calculations above also fail to take into account that gases flo from areas of high concentration to areas of low concentratio regardless of the air pressure.

This means that the natural progession of moisture will be up from an through the concrete basement floor and into the living space abov where it will condense under most circumstances.

One thing you got correct, however, is that 4 mil poly is 3 times les able to retard water vapor than 6 mil poly, and why it is not permitte as a true vapor barrier by building codes

-- manhattan4

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[nicksanspam]

Humidty more than 100% and basement tempartures below freezing? :-) It seems you can't even spell, much less appreciate science...

Bullshit. What HAVE you been smoking? :-)

No. Water vapor likes to flow from greater to lesser vapor pressures.

Wrong wrong wrong again.

Nope. It's only 0.17/0.11 = 1.5X less permeable.

Nick

[manhattan42]

This isn't even worth a reply since what you have stated is NO science.

Diffusion is the method by which gases flow from areas of highe concentration to areas of lower concentration regardless of pressure This is why gaseous water routinely enters basement spaces from th ground below where it is more concentrated to the basement where it i less so and often where the basement has a higher temperature an pressure.

Dew points can be reached at temperatures in the 70s or highe depending upon relative humdity of the air mass. For example, you ca get condensation forming when surface temperatures are at 73 degrees and the air temperature is 75 F with a 95% humidty.

There isn't much else to say except that perhaps you need to fin another major at Villanova if 'science' is yours.......

-- manhattan4

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[nicksanspam]

Incorrect.

We might compare this to Andersen's estimate that an average family of 4 evaporates 2 gallons per day.

It takes all that into account. And RH can never exceed 100%, and basement floor temps below freezing are extremely rare south of Alaska, and the last two facts are irrelevant here. Just pointing out how utterly wrong you are, again.

How vague. Just what do you mean by this, exactly?

They flow because of vapor pressure vs air pressure differences. "Areas" (vs volumes) with differing concentrations have different vapor pressures.

If the vapor pressure under the vapor barrier over the basement floor is less than the vapor barrier in the room air (as above) water vapor will flow FROM the room air TO the basement floor, through the resistance of the barrier, in a miniscule fashion.

True, but again, vapor pressure and air pressure are different.

We might talk again after you learn to spell "humidity" :-)

Nick