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David Meiland
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Checked out a manufactured home yesterday that had very high interior humidity. Underneath there's a concrete slab with poly sheeting over it. There is a lot of whitish crystalline material deposited on the top of the poly. It looked very similar to lichen but wasn't. There are also stains on the bottom of every pier block, most noticeable on the bottom block but also on the second and (and sometimes third) block in each stack. The home has an electric downflow furnace sitting on the floor of a closet with return air entering about 3 feet off the floor.

I take this to be condensation on the top of the poly, enough over the years to leave mineral deposits on the poly and cause some efflorescence in the blocks. I find it really interesting that the upper blocks also have some of that showing. It's surprising to me that the slab can get cold enough to condense, but with high enough humidity it's certainly possible. I've been under a couple of other places in this same 'hood and not seen this. It probably does not help the the FAU is pushing the humid air down into ducts where some of it doubtless leaks into the crawl.

Any comments on this?

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Hi David,

That's way beyond anything caused by condensation that I've every seen.

Are you absolutely sure there was never a layer of water about an inch deep no top of that barrier? The efflorescence around those piers is uniform in height at just about an inch; as if the base of the piers had been sitting in water.

Is there mineral content in the soil up there that might cause groundwater to leave mineral deposits like that when it evaporates?

Any possibility that the washing machine one overflowed with soapy water and it all drained into the crawlspace?

ONE TEAM - ONE FIGHT!!!

Mike

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Mike, it certainly could have flooded under there, and with skirting tight to the ground I suppose it could have gotten an inch deep and stood there awhile. Surface water, downspout problem, pipe burst, washing machine hose, not sure. The owner hasn't been there long and didn't have any history on it. I was impressed by the uniform nature of the deposits... evenly spread almost everywhere, nothing to suggest areas where water flowed and other areas left dry. The interior humidity was significant, 72F and 58%RH, with lots of sweat on the windows and a skylight, even a small area of mildew on drywall in the upper corner of a bedroom. I read my meter in grains per pounds, outdoors yesterday was about 30, inside my little slab-on-grade office was 42 with me sitting here, inside her house was 68.

I like Jim's comment as well. I assume they pour the slab, lay the plastic, roll a tractor trailer on with the house on it, then scurry around with blocks to get it on the ground. All that traffic on the plastic is going to leave a lot of tiny pinholes.

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Around here slabs like that are placed on 10" to 14" of #2 stone in a 16" to 18" deep excavation, then after the perimeter block stem walls are placed and the unit(s) set, the edges of the slab are buried. The net result is that the whole assembly floats on its own pond. The slabs are always wet. Placing poly on the top would draw water through the slab as if it were a sponge. There's enough loss at the plenum to turn the entire crawl into the crusty innards of an Aprilaire humidifier, it just hasn't run long enough yet.

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This one has numerous piers like those in the photo, but the perimeter is 1/2" plywood skirting that's tight to grade, either sitting on the slab or right next to it. Numerous vents cut in.

I will probably need to revisit this one after winter has really set in. It's about 1/4 mile away on the road to town.

Scoop a sample of that white stuff into a baggy. I'll bet it's what Mike L. said, Lime or borax or some other to kill the smell of cat urine on rat urine? Human sewage is also possible in that environment.

PS, if you every find the infamous dripping water bag hanging down from the frame, you must resist the urge to poke it with a screwdriver. Nothing good can come from doing that. [:)]

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Around here slabs like that are placed on 10" to 14" of #2 stone in a 16" to 18" deep excavation, then after the perimeter block stem walls are placed and the unit(s) set, the edges of the slab are buried. The net result is that the whole assembly floats on its own pond. The slabs are always wet. Placing poly on the top would draw water through the slab as if it were a sponge. There's enough loss at the plenum to turn the entire crawl into the crusty innards of an Aprilaire humidifier, it just hasn't run long enough yet.

I grew up in New York so I understand that. If they don't do them like that they heave like crazy when the frost sets in come winter. Fortunately, we don't get that here; hell, we're lucky (or perhaps unlucky) if frost goes 10 inches.

Around here in crawlspace country, the underside of these things acts pretty much the same way crawlspaces under regular homes act. We cap the soil, the soil beneath the plastic can't evaporate moisture, it becomes waterlogged, reaches equilibrium and conditions stabilize. I can remember a few times when I've been in previously flooded crawlspaces where some kind of white alkyline substance was left behind after the water trapped on top of the plastic evaporated. I think that might be what's happened here.

I don't understand the humidity though. That's pretty unusual and with an intact vapor barrier it doesn't make a lot of sense. David, did you check the marriage plenum underneath to see if it's full of water? If they had a pipe burst. It could have drained into both the crawl and the heating system and you might have some ducts half full of water that are functioning like a huge humidifier.

ONE TEAM - ONE FIGHT!!!

Mike

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I worked several years for an AHJ in a rural area where manufactured houses were numerous. Probably the biggest problem with installation was getting them to provide positive slope around the building. The installer may or may not have to do it, depending on the terms of his deal with the owner. Many people just give you dumb looks when you try to explain to them what it means.

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