Moisture problem in new shop!

[iublue]

what is the temp and outside rh, I have a 3 car garage workshop and remove that much a day when its humid outside, On the slab do you have a perimeter drain, it will help moisture migrating under the slab, in the house we grew up in we had a sump hole in the spring and a lot of the summer it ran a lot, we had a lot of ground water.

Here lately, the outside temp has been pretty moderate.  It is in the mid 30's now.
I have no idea what the RH is outside.  
We went through the same cold snap that most of the country endured.  It didn't get much above zero for almost a week.  Even running the dehumidifier constantly and at that temp with the outside being VERY dry, the RH did not drop much.
There was not a perimeter drain installed.  Mostly because I couldn't imagine water being a problem with the slope of the lot and the height off the ground of the slab.
That could have been a mistake on my part!
I guess if the moisture continues to be a problem, I will have to run the dehumidifier all the time.  Bummer


Thanks for the reply.

[Tony Z]

Thanks for the reply.

I think that ship has sailed if that would have been the solution.
I would have had to put pipe under the slab like a perimeter drain and of course, that would have to be done pre-pour.

Not necessarily, if whoever poured the floor put gravel underneath!

[iublue]

Not necessarily, if whoever poured the floor put gravel underneath!

There is a LOT of pea gravel underneath but there is also a concrete footing that on the south side (tall side) of the slab is close to 4 1/2 feet down from the top of the slab?
On the north side (short side) the footer probably extends 2 1/2 feet below the top of the slab.

Actually the footing makes this even more of a mystery to me since, although concrete is not a vapor barrier it is pretty effective in stopping liquid water without a crack in it.
I also used foam concrete form blocks to make the stem wall.  So you have the foam resisting water flow too.

At the end of the day, I guess the only real solution is to continue to use the dehumidifier during the time I am not using the AC.

[Roly]

There is a LOT of pea gravel underneath but there is also a concrete footing that on the south side (tall side) of the slab is close to 4 1/2 feet down from the top of the slab?
On the north side (short side) the footer probably extends 2 1/2 feet below the top of the slab.

Actually the footing makes this even more of a mystery to me since, although concrete is not a vapor barrier it is pretty effective in stopping liquid water without a crack in it.
I also used foam concrete form blocks to make the stem wall.  So you have the foam resisting water flow too.

At the end of the day, I guess the only real solution is to continue to use the dehumidifier during the time I am not using the AC.

Tape some clear plastic to the floor, about 2 foot square, sealing all around and see if you have condensation under it after a day or so.    That should tell you if it is coming through the floor.  Roly

[JosephP]

Wouldn't an epoxy coating on the concrete work as a vapor barrier? I understand the ideal is to have the barrier underneath the concrete, but that didn't happen. Can the problem be reduced now with epoxy? (Assuming it's moisture coming from the slab...which I'd bet on)

[jteneyck]

Here lately, the outside temp has been pretty moderate.  It is in the mid 30's now.
I have no idea what the RH is outside.  
We went through the same cold snap that most of the country endured.  It didn't get much above zero for almost a week.  Even running the dehumidifier constantly and at that temp with the outside being VERY dry, the RH did not drop much.
There was not a perimeter drain installed.  Mostly because I couldn't imagine water being a problem with the slope of the lot and the height off the ground of the slab.
That could have been a mistake on my part!
I guess if the moisture continues to be a problem, I will have to run the dehumidifier all the time.  Bummer


Thanks for the reply.

The outside RH was and probably still is screaming high.  RH increases as temperature decreases, for the same ambient vapor level.  People have a misconception that outside RH must be low since it gets low indoors in the winter.  What's happening is the indoor RH drops because our heating systems are raising the temp. from whatever the outside temp. is to the setpoint we set inside.  The RH outside is usually well over 70% in the winter where I live but inside it goes down into the low 30's unless I run a humidifier.

I'll bet the high RH in your shop is coming from the concrete slab.  It's water vapor, not liquid water, migrating through the slab because of ground moisture, regardless of any crushed stone under the slab.  You need a vapor barrier to stop that.  

Good luck finding a solution.  

John

[iublue]

Tape some clear plastic to the floor, about 2 foot square, sealing all around and see if you have condensation under it after a day or so.    That should tell you if it is coming through the floor.  Roly

Great idea!

Thanks

[iublue]

Wouldn't an epoxy coating on the concrete work as a vapor barrier?  I understand the ideal is to have the barrier underneath the concrete, but that didn't happen.  Can the problem be reduced now with epoxy?  (Assuming it's moisture coming from the slab...which I'd bet on)

That would be something to investigate after I use the plastic idea to determine if it is coming through the slab.

[iublue]

The outside RH was and probably still is screaming high.  RH increases as temperature decreases, for the same ambient vapor level.  People have a misconception that outside RH must be low since it gets low indoors in the winter.  What's happening is the indoor RH drops because our heating systems are raising the temp. from whatever the outside temp. is to the setpoint we set inside.  The RH outside is usually well over 70% in the winter where I live but inside it goes down into the low 30's unless I run a humidifier.

I'll bet the high RH in your shop is coming from the concrete slab.  It's water vapor, not liquid water, migrating through the slab because of ground moisture, regardless of any crushed stone under the slab.  You need a vapor barrier to stop that.  

Good luck finding a solution.  

John

Thanks for the reply.

I should have been more clear in my post.
The outside air is very dry when considering the absolute humidity, because at zero degrees the air just can't hold much water. In fact, according to an online calculator, the difference in the amount of water that air at 62F can hold compared to air at 1F is a factor of 10!   Any outside air that gets in will be very dry even in relative humidity once it is heated to 62F.

You are probably right and it is coming from the slab but I sure never thought about during construction!
The other issue I have with it coming through the slab is I would think there has to be some kind of pressure to force it through the concrete.  Like a high vapor pressure in all that fill and what would cause that condition?  
   I don't believe that the vapor would go through the slab just because it is there especially in the volume that it seems to be in.

I will do the plastic thing and go from there.

[firefighter]

Was the concrete ever sealed?