Moisture On Concrete Slab

[Dave]

Guys, I hope some of you know the answer to this. I built a cabin up north on a concrete slab. The cabin half of the slab I sealed with some waterproofer. The garage half I haven't sealed yet and I think that's the problem. I'm looking for a absolute answer, however.

With warm weather, the cabin and garage stay quite cool when unopened for some time. The garage floor has a lot of moisture on the floor; I suspect from the coolness of the slab and warmer air temperature. Is the concrete NOT being sealed what is causing this moisture to be on top of the slab?

The cabin part is doing fine and this is the only difference I can see, is that the garage floor is not sealed. Is that a correct assumption?

And, what's the best sealer for garage floors, as far as, wearing off?

[sparcebag]

I dont believe moisture is comming up through the concrete.Its condensation just as you stated,cool itside warm & moist out, open the doors and cool concrete extracts moisture from warm humid air,Most likly it will happen inside also,if it was opened as much as garage.You need a dehumidifier to slow it down,or if the concrete warms up enough it will stop,but to warm enough you'd have to have it open most the time.Also under those conditions you should watch for molds growing in the lower parts of interior walls,DEHUMIDIFIER!

[Dave]

Actually, the garage is opened up more when we're there than the cabin part is. At first, I thought moisture was coming up from concrete. The same happened in the cabin, until I sealed the cabin concrete. The moisture is there before I open the garage door. This must also be the reason I'm seeing "dirty spots" (moisture) on the garage sheetrock

[sparcebag]

I would bet your (dirty spots)in the garage are molds

starting.

Which theres high chanch the inside of walls,Behind the rock,Is moldy,could create a problem down the road!I take it your cabin is new? Should have used mold resistant rock!

[Dave]

It's about 3 years old, I believe this is the 2nd summer since I sheetrocked the place. So, yes, it's relatively new. NOW you tell me about mold resistant rock;)

The only main place sheetrocked inside the cabin partition is the upstairs, which stays nice and dry. Also, I didn't seal the concrete in the unfinished bath either and that stays dry, come to think of it.

Looks like I should dehumidify, clean and seal the concrete in the garage to see if that helps. It can't hurt.

[CFRay]

The condensation or water transmitting through the slab question could be answered by taping a clear poly (3'x3') piece down on the unsealed garage floor next time you leave. Tape the edges completely to create an airtight seal. When you return if there is water trapped beneath the poly it is coming through the floor. If it is drier than the surrounding area it is condensation.

[Dave]

CFRay,

Ah, thanks for the idea. That sounds like something I should do. That's why I finally asked the question here.

[sparcebag]

When ya tape poly down,Let me know what tape will stick to damp crete.I need some!!

[Sifty]

Dave it has been my experice that a concrete slab will"wick" away moisture for many years.

We have gone to job site's that are twenty years old and have moisture problems.

When you seal a floor the moisture that is in the slab still needs a place to go therefore it will find the easist place to get out.... the unsealed area.

The idea with the poly is a good start. You can use duct tape and a hair dryer to get it to stay.

There are a number of reason's for the moisture to be coming through but the slab is a good place to start.

Sifty

[GotOne]

Try googling "moisture vapor transmission". I could get into a lot of details about "mve", but the pfd on "principles of water vapor transmission" by Moxie will give you an education on the subject.

[Dave]

Thanks for the info. I printed it out and will read it. I can dry out the slab pretty good by placing a fan inside the garage. So, the duct tape will stick, I believe.

[hanson]

Is there a vapor barrier underneath the concrete slabs?

If there is moisture coming up through the slab, that would have stopped it. Its too late to install one now however.

I think you are not seeing moisture on your sealed slab because it is sealed, like you mentioned.

[Dave]

I went up yesterday and duct taped a piece of poly down on the slab. We'll see what happens next time it gets hot outside. Moisture on the slab yesterday was very minimal, only about a 3X3 section.

[Dave]

Slab was still free of moisture last weekend. I checked under the duct taped plastic and saw no moisture there either. There was some moisture under a gas can I moved however. I suspect that may be moisture that was there when the gas can was placed there though. Looks like I need to continue monitoring this.

[GotOne]

Dave, moisture can be a seasonal issue (or if we have heavy rains). This time of year...plus, how dry it has been, most likely very little moisture vapor emissions (MVE). The moisture in the soil is lower this time of the year-less likely to have issues. My guess is that in the spring or after a heavy rain, the mat test could have different results.

[sparcebag]

If ya could get there in this warm HUMID weather,you could open it up and see if its condensation,Perfect time!

[Dave]

Well, it was pretty humid last Saturday (had to turn the A?C on) so, I thought that'd be humid enough to form condensation on the cool concrete. No go so far.

[Shack]

Dave, what type of soil do you have around cabin? What is water table like?

During wetter times, if soil is holding moisture for long periods of time, this would have an affect on the moisture showing up on your garage floor.

I think someone else touched on this, but concrete basically is a wick to what ever soil is under it. Like Hanson said, a barrier of sorts might have helped (foam board or sheet plastic) but again it is too late. I have used some commercial sealant in the past that has proven good to hold back moisture. I still have a half full 5 gal bucket of it at home and I will try to post name of stuff. It seems to last a very long time, even under heavy traffic. The ice rink look wears off in a couple of months, but the seal barrier has held up for about 4 years know.

[Dave]

Hard rocky clay soil. True, it's very dry up there right now. I don't know how deep the water table is. No matter what, I will be putting down a sealer. I'd be interested in the type you used.

[Shack]

If I was betting in Vegas, I would of bet "clay"...

They worst stuff for dissipating/draining moisture.

The stuff I have used is made by "Sonneborn". Its called Kure-N-Seal 25. It is high solid transparent curing and is sealing dust proof.

It is in an orange and black 5 gallon bucket!

[Dave]

I will check that stuff out. THANKS!

[Sifty]

Dave just remember that you will need to seal your whole floor.

If you just seal the "wet" area it will find it's way to the next area.

Sifty

[Dave]

Thanks, Sifty. The WHOLE floor is the wet area when the problem appears.