Warm Day - Moisture on Slab

[ddurrett896]

yesterday was the first warm day of the year with little ran. Opened my garage and turned on my ceiling fans and the garage slab almost instantly got darker from moisture.

Confirmed it didn't come from below when I moved a bucket and it was normal, plus have a 15mil vapor barrier under the slab.

After reading, it would like the warm air hit the cold slab and condensed. Is that the case?

Is there anything bad that can come from this? If so, is there a way to mitigate it?

For reference, I have blown ceiling insulation and rockwool in the walls. Thanks

 

[Buckgnarly]

Warm moist air hits cold stuff and condensation occurs. There are always a few warm days in Spring I avoid opening my garage door to prevent condensation on tools and everything in my shop.
I always wait a few days until Temps equalize before opening the door

 

[ddurrett896]

Warm moist air hits cold stuff and condensation occurs. There are always a few warm days in Spring I avoid opening my garage door to prevent condensation on tools and everything in my shop.
I always wait a few days until Temps equalize before opening the door

Would a portable dehumidifier fix it? Something like this....

https://www.amazon.com/dp/B073V9MG3Y/?tag=atomicindus08-20

 

[Snip]

I had the same here yesterday in Tennessee. Even before I opened the doors the floor (40x66) was damp. We had had a couple days of rain and temps warmed into the 60's during the day, only down to the upper 50 overnight. Slab was still cool. No ceiling or insulation as of yet and vented eves didn't help i'm sure. I do have double bubble under the metal roof and house wrap on the vertical and a VB under the slab. I opened the doors and ran the fan but with the pending rain in the air it never did dry out. Can only guess it is due to the slab temp still being cool? I left the fan on over night. Our first year here in a humid climate, lived in a dry one for the last 40 years. Lots of learning and adjusting to go,lol

I just went out to the shop and checked the floor, about 3/4 of the floor is now dry, even though we had over an inch of rain overnight. I guess the fan thing and air movement makes a big difference........Note to self.

 

[Crawlin]

Same happened to my new shop in NC. It is finished but haven't had the floor heat hooked up yet. I left some OH doors open for a few hours since it was warm, then noticed slab wet the next day. Its dry now but smells musty. Guess I should run some fans.

 

[Buckgnarly]

Would a portable dehumidifier fix it? Something like this....

https://www.amazon.com/dp/B073V9MG3Y/?tag=atomicindus08-20

It would take the moisture back out, but could not do it fast enough to stop it from happening I would think.

 

[tsubahog]

I had the same problem years ago in my shop in NJ. Spring humidity on the cool slab resulted in a wet floor and possibility of mold. I do have ceiling fans and that helped but to solve the problem, I built two of the "contraptions" shown in the picture.

Inside the box is a radon fan, mounted horizontally, vented to the outside. They have been running non-stop for 15 years. On top of the box, is an Aprilaire de-humidifier, with the drain running through the wall to outside. I only run these in the spring as the electric meter spins furiously when they are on.

Never had the problem again.

 

[yeldogt]

yesterday was the first warm day of the year with little ran. Opened my garage and turned on my ceiling fans and the garage slab almost instantly got darker from moisture.

Confirmed it didn't come from below when I moved a bucket and it was normal, plus have a 15mil vapor barrier under the slab.

After reading, it would like the warm air hit the cold slab and condensed. Is that the case?

Is there anything bad that can come from this? If so, is there a way to mitigate it?

For reference, I have blown ceiling insulation and rockwool in the walls. Thanks

The air above will be warmer .... it's going to have more moisture in it. The fan will push the warmer air down ... same with any air outside thats above the humidity inside.

 

[nadogail]

Until the slab gets warmer, the moisture in the air will condense on the cold slab.

 

[finn]

Happens every spring. Worst case is the garage with what looks like an 8” slab and is unheated all winter.

The other garage receives some heat via a wood stove and overhead propane heated for at least part of the winter, so the slab isn’t as cold in the spring and there is little condensation. If I fire up the wood stove, it clears up rapidly.

The shop has radiant in floor heat, and there’s not a sign of condensation in the spring when the doors are open.

The lower the thermal mass of the slab, or the lower the temperature differential between the slab and outside air, the less condensation you will see.

High school physics.

 

[glentre]

This is a lesson many of have learned the hard way. With warmer/humid Spring days and a cold slab, leaving the big doors open will result in condensation on the floor every time. And, if you're not careful, the next time you'll get condensation over all your cars, tools, walls and especially leather and fabric items.

This problem can be resolved by using dehumidifiers but they are expensive to buy and energy hogs when running. Best thing to do is keep air circulating if you can, get some heat in the garage or don't open the big doors under these conditions.

There are many older threads on this subject with a variety of ideas on how to control condensation.

Glen

 

[PWC Repair]

It was the quick temp change PLUS the humidity. It had been cooler weather, then rained some, then cool overnight......Then a quick unusual warmup that felt GREAT! Throw open the doors and enjoy it....BUT...the warmer humid air condenses quickly on the colder items. Next time it's noticeably warmer and humid outside than inside, turn on a circulating fan and crack the overhead door open 6" for an hour, then another ft the next hour, then half open for an hour, then all open. This method seems to really help keep the condensation at bay for me.

 

[Sweetcorn]

It's the same thing as setting a cold pop can on a picnic table on a warm day. Your slab is cold and the air is warm, like the pop can is cold and the air is warm. It sweats due to the differential in temp between items.

If your shop was heated and the floor was warm, it wouldn't be sweating. Like a warm can of pop won't sweat on a hot day.

 

[toolmiser]

My parents long time ago had the same issue. We found if we opened the attic "hatch" and left it open, the moisture would disappear.

Won't cost much to try.

 

[firebirdparts]

yesterday was the first warm day of the year with little ran. Opened my garage and turned on my ceiling fans and the garage slab almost instantly got darker from moisture.

Confirmed it didn't come from below when I moved a bucket and it was normal, plus have a 15mil vapor barrier under the slab.

After reading, it would like the warm air hit the cold slab and condensed. Is that the case?

Is there anything bad that can come from this? If so, is there a way to mitigate it?

For reference, I have blown ceiling insulation and rockwool in the walls. Thanks

It is as you say. Just have to wait until tomorrow or sometime when the weather changes.

You could heat the slab, and lots of guys do, and love that style heating. That would solve it. You could also keep the shop closed up and dehumidify it so that the dew point is the below the temperature of the slab.