Moisture seeping up from garage floor?

[K8096]

I just built a 1500 square foot garage and was told by another hobbyist that I should keep plastic under all my cars for a year because moisture will seep up from a newly poured concrete floor. I did wait 10 days to let to floor cure before parking any cars on it. Have any of you ever heard of this? As time goes on I will post some info about my garage, including the threatened lawsuit by my neighbor over building it!

[Mark Huston]

Professional concrete contractors have told me that concrete takes about 90 years to fully cure, give or take a decade or two. While your garage floor is curing, the moisture in the concrete works it way up and evaporates into the air – which would include right under your car. If you have a lot of excess moisture work its way up through your garage floor you might see a chalky substance build up on your garage floor. This is a result of “wicking” when the moisture pulls minerals out of your concrete while working its way up and out of the concrete into the air. You might want to look into using a good quality concrete sealer while your concrete floor is still oil and grease free.

[jrbartlett]

I am not a builder, but just want to point out that a slab generally should have a plastic membrane underneath it. My 60-year-old house predates such technology, but the sand underneath was covered by a thick layer of tar before the foundation was poured, and this serves as the moisture barrier. The amount of water contained in the concrete is negligible compared to the amount of water in the ground that can work its way up through the concrete over the years. My father built a garage in the '50s with no moisture barrier, and that garage is noticably more humid than those built properly.

[K8096]

Oh yes, before they poured the concrete I made sure they put down plastic visqueen over the entire ground surface. The rebarb was laid down over the visqueen.

[mrpushbutton]

I think its a good idea to have a small, cheap fan under each car stored in a gound-level garage. Keep the air moving underneath.

[Ron Green]

My shop was built in 96 and is approximately 1000 SF and I have never experienced any moisture problems. They did spray concrete sealer on (typical practice) plus I also rolled another couple of coats on a week or so later.

[Robert's Cars]

My garage does not have a moisture barrier under the cement and I do get moisture on the floor on humid days. I like the idea of using fans at floor level. Does this really help? Is anyone using them now and seeing a difference? I seem to think that fans act like a dehumidifier which blows the moist air over a cold coil to form condensation. Using fans may make things worse? Thanks

[K8096]

A friend of mine has a large garage with ceiling fans that are humidity controlled. When the humidity reaches a certain point, they turn on. I may do something like that. I don't even have electricity in mine yet, that's next years project. Then I have to insulate & drywall the whole thing. I do have a ridge vent in the roof & soffit vents in the gables. I plan to have the entire ceiling drywalled & insullated as well.

[barry22]

You did good making sure a barrier was in place before the concrete was poured. I placed a layer of plastic on top of the slab in my garage years ago and don't regret it one bit. You can buy it in 20 foot wide rolls at Lowes or at other home improvement stores.

[Mike Cullen]

A moisture barrier under the slab and a couple coats of good sealer and then even an epoxy type paint on the floor will take care of it for a long time. Makes it easier to clean up spills too.