Moisture problem in new shop!

[iublue]

I have a moisture issue that I can not figure out.

Some background first - 

The slab was poured in July of 2022.
The all metal building in September of 2022.
The foam was sprayed in October of 2022.

Once the overhead door was up, I used propane heaters to heat the space.
I stripped the building out with 2x4's and hung drywall in December of 2022 and January of 2023.

In February of 2023 once the electrical was installed, I switch to a 220 electric space heat with the propane as supplemental heat.
I started running a dehumidifier and was emptying it twice a day.  I know the propane put a lot of moisture into the air but with emptying the bucket twice a day, I was taking out about 1 3/4 gallons a day!  I bought a hygrometer when I started to use the dehumidifier and the relative humidity was almost 85%!
  It never did rain!  Whew.

Using the dehumidifier brought that number down to around 70%.  I wasn't really worried because I assumed that going through the summer and using the air conditioner would bring that number down further and it did to a little under 60%.  Not bad.

I quit using the dehumidifier and now into this heating cycle and the number went back to 70%!
So I started it back up and empty it about 1 1/2 times a day.  The number is back down to 60% but no lower.
One day I was gone and the RH went back up 2%!

WHERE IS THE MOISTURE COMING FROM???

The concrete has been poured almost 16 months so I would think that it is fully cured and no moisture coming from it.
The building is on a slope lot so the slab has approximately 2 1/2' of pea gravel under one end and 1' under the other end.
The gravel and the fact that the concrete is not wet tells me that no water is coming through the concrete.
For the framing lumber, I used kiln dried pine and once again, I would think any moisture in that component is gone.
I assume that the foam might have a water component but after well over a year and using the AC and dehumidifier, wouldn't that moisture have off gassed?
I do not store any quantity of lumber in the shop - green or otherwise.
I do use water based finish but I haven't sprayed anything in quite a while and even then I exhaust the over spray outside.

I found an absolute humidity calculator online and it had 9 pounds of water in the shop volume with and RH of 60% and a temp of 62F.  That surely can't be right since I am removing close to that much every day.  I must have made a mistake.

I have racked my brain and tried to think of every contingency and I have got nothing.
I have asked friends family and once again nothing.

What does the Woodnet brain trust say???

[Stwood_]

It's coming in somewhere, plus the LP doesn't help matters.
Run your humidifier 24/7. Most have an option to hook a drain hose to the drain pan, and then run it into a drain or out under a door, etc.
Is insulation on the drywall in the ceiling, or just the foam on the bottom of the roof?
Is the attic vented?

[fredhargis]

I do agree the LP should be discontinued, but I'm wondering about ground moisture coming through the concrete...was there a vapor barrier put down?

[R Clark]

I think most of the moisture is coming up through the concrete, and to some extent likely always will.  Concrete is not an impermeable barrier.

Do you have a basement in your house?  If so, what's the humidity situation there?

It's best if the LP heat is a sealed combustion unit, using outside air for the burn and then exhausting all the gas outside.  How big is the shop?

[Tony Z]

Might want to consider a sump pump, with the sump made to drain moisture from under the slab. You can either form and pour from concrete, with weep holes to under the slab, or buy a pre-made sump to hold the pump, that typically have weep holes molded in.

[wing nut]

I have a moisture issue that I can not figure out.

Some background first - 

The slab was poured in July of 2022.
The all metal building in September of 2022.
The foam was sprayed in October of 2022.

Once the overhead door was up, I used propane heaters to heat the space.
I stripped the building out with 2x4's and hung drywall in December of 2022 and January of 2023.

In February of 2023 once the electrical was installed, I switch to a 220 electric space heat with the propane as supplemental heat.
I started running a dehumidifier and was emptying it twice a day.  I know the propane put a lot of moisture into the air but with emptying the bucket twice a day, I was taking out about 1 3/4 gallons a day!  I bought a hygrometer when I started to use the dehumidifier and the relative humidity was almost 85%!
  It never did rain!  Whew.

Using the dehumidifier brought that number down to around 70%.  I wasn't really worried because I assumed that going through the summer and using the air conditioner would bring that number down further and it did to a little under 60%.  Not bad.

I quit using the dehumidifier and now into this heating cycle and the number went back to 70%!
So I started it back up and empty it about 1 1/2 times a day.  The number is back down to 60% but no lower.
One day I was gone and the RH went back up 2%!

WHERE IS THE MOISTURE COMING FROM???

The concrete has been poured almost 16 months so I would think that it is fully cured and no moisture coming from it.
The building is on a slope lot so the slab has approximately 2 1/2' of pea gravel under one end and 1' under the other end.
The gravel and the fact that the concrete is not wet tells me that no water is coming through the concrete.
For the framing lumber, I used kiln dried pine and once again, I would think any moisture in that component is gone.
I assume that the foam might have a water component but after well over a year and using the AC and dehumidifier, wouldn't that moisture have off gassed?
I do not store any quantity of lumber in the shop - green or otherwise.
I do use water based finish but I haven't sprayed anything in quite a while and even then I exhaust the over spray outside.

I found an absolute humidity calculator online and it had 9 pounds of water in the shop volume with and RH of 60% and a temp of 62F.  That surely can't be right since I am removing close to that much every day.  I must have made a mistake.

I have racked my brain and tried to think of every contingency and I have got nothing.
I have asked friends family and once again nothing.

What does the Woodnet brain trust say???

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.

[iublue]

It's coming in somewhere, plus the LP doesn't help matters.
Run your humidifier 24/7. Most have an option to hook a drain hose to the drain pan, and then run it into a drain or out under a door, etc.
Is insulation on the drywall in the ceiling, or just the foam on the bottom of the roof?
Is the attic vented?

I have not used LP since May of 2023.  Since then the shop has went through an entire AC season.
There is really not an easy way to run a hose.
The entire building was sprayed with closed cell foam.  2" on the underside of roof and 1 1/2" on the walls.  The walls probably ended up with close to 2" because I had a problem with the first installer and another company had to touch up areas.
No attic.

Thanks for the reply.

[iublue]

I do agree the LP should be discontinued, but I'm wondering about ground moisture coming through the concrete...was there a vapor barrier put down?

No vapor barrier was used.  And I understand that concrete is not a vapor barrier but since the slab is so far off the ground and the ground has a pretty large slope, it is hard to imagine that much vapor coming through the slab.  Also, the slab shows NO signs of any condensation even when the RH was 85%.

Thanks for the reply.

[iublue]

I think most of the moisture is coming up through the concrete, and to some extent likely always will.  Concrete is not an impermeable barrier.

Do you have a basement in your house?  If so, what's the humidity situation there?

It's best if the LP heat is a sealed combustion unit, using outside air for the burn and then exhausting all the gas outside.  How big is the shop?

You could very well be right about the moisture coming through the concrete.  Hard for me to imagine with the height of the slab but....
The shop is a stand alone building that is not even on the lot where my house is.
LP is no longer used.
The shop is 30'x50' with approximately 10' ceilings.

Thanks for the reply.

[iublue]

Might want to consider a sump pump, with the sump made to drain moisture from under the slab.  You can either form and pour from concrete, with weep holes to under the slab, or buy a pre-made sump to hold the pump, that typically have weep holes molded in.

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.