We have a 1960 two story colonial with a finished basement in New England. The basement floor is wall-to-wall rug, on a rug pad, on old vinyl tiles, and then a concrete slab; I assume there's no explicit water control anywhere. We're on top of a well-drained hill, so standing water hasn't been a problem, although the basement can get a bit humid in the summer.

Two weeks ago I put down a 4' x 5' clear plastic chair mat in front of my basement desk. Today I noticed that moisture is condensing under parts of the mat; if I lift the mat up there are drops of water condensed on its bottom surface, and the rug feels damp.

BTW, I believe the affected areas are where the water pipes from the boiler to the radiators run through the slab. I can track the pipes with an IR thermometer; the rug over the pipes is about 5°F warmer than the rest of the rug. A second mat, which has no pipes under it, isn't showing condensation.

I assume that if I do nothing I'll get an unlicensed ecosystem (i.e. mold) under the mat. Without pulling up the rug is there anything I can do to reduce the moisture?

  • Halfway through writing this question I figured out the answer, which I posted in case it helps anyone else. Comments and alternate answers welcome. Dec 6, 2020 at 20:34

1 Answer 1


Unfortunately, unsealed concrete slabs are fairly water-permeable, so the moisture level in the concrete will be pretty high. Until now the moisture has wicked through the rug and evaporated into the air, but it's now building up under the mat. Having the heating pipes in the concrete makes it worse, evaporating the water up through the rug; since the mat is cooled by the (cooler) basement air the water condenses on it.

Having a water-saturated rug, especially one that's warmed by the heating system, is asking for trouble.

Pulling up the rug and adding a waterproof layer would likely help, but that may not be in the cards; for now remove the mat and accept that the chair may damage the rug.

  • I agree the standard test for a wet slab is to place plastic sheeting down after 24 hours if there is moisture like a wet spot on the slab or droplets like you have a moisture barrier is the only way to go. To help with the humidity get a dehumidifier but don’t put anything over the carpet that won’t breathe or you will end up with mold problems.
    – Ed Beal
    Dec 7, 2020 at 14:34

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