Slab wicks moisture

[Jaybob]

The concrete slab in my shop/garage literally pulls the moisture out of the air.On a humid day if you leave the roll up door open the floor would be soaking wet within minutes. Unfortunatly my slab quy elected to not use a vapor barrier. The substrate is rocky and well drained but you'd think I was in the swamp. I tried a sealer which didn't help.I stripped that and applied two part epxoy floor paint which helped a little but now looks bad and any moisture brought in just sits there.
Anyone have any suggestions that don't involve a jackhammer?I might add that the shop is insulated but not often heated.The exterior walls also are often wet, but my house and attached garage 30' away are dry as a bone.
I've been putting up w/this for years now and have'nt been able to find a answer.

[ThingOfBeauty]

The moisture is probably coming from the outside air, not under the slab.
Just like a glass of ice water is cold enough that moisture in the air condenses on the outside, your slab is often cold enough to condense water out of the warmer, more humid outdoor air, after you open the door and let the warm air in.
My own garage does this, though only at certain points in the year. At my garage, the weather has to have been cool enough for the slab to be pretty cold, and then the weather has to change to be warmer and more humid (BTW, warmer is pretty much always more humid).
If your shop is nestled in a hillside with no sun or windows, like my brother-in-law's shop, its slab might be pretty cold a lot of the year, and any time it's reasonably warm you'll get condensation on top of the slab.
You don't "need" to fix this problem; it's a simple matter of physics and doesn't do much if any harm to the slab itself. However, if you could keep the average temperature in the shop higher, it would happen less.
You could also verify if this diagnosis is correct by checking the temperature of the slab, and the 'dew point' of the air...whenever the temperature is below the dew point, this can happen.

[Ablesupe]

I agree with Doug. The other day I opened up my above grade shed/shop with crawl space below. It had been chilly in the night so the i/s temp. was probably 15 degrees colder that the o/s by the time the sun came out . In just a few minutes everything that was metal, i.e. tablesaw, propane bottles, jointer - everything was dripping with condensaation. No different than a glass of ice tea on a hot humid day in the summer. It's not the tea coming through the glass, but rather the moisture in the air hitting and condensing on the cold surface.

[Mark Parlee]

Doug
Good informational post
Are we being spammed with new members that do not know how to fill in their profile; what's up with that?

[DaveProfitt]

Good explanation, Doug. I use the "ice tea on a glass top patio table" to to explain to customers here in the Charlotte NC area why their crawl space has water on top of the vapor barrier, the ductwork is dripping and their hardwood floors are cupping in the summertime.

And my shop floor does the same thing several times each spring when the night was cool followed by a warm humid morning. Physics works the same every time.