Moisture problem in new shop!

[JosephP]

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Obviously water and water vapor act differently but even if there is significant water vapor under the slab it would need a certain amount of vapor pressure to force it to migrate through the slab.  It is hard for me to see why there would be vapor pressure under the slab.
80-90% RH under the slab, 50% above the slab will draw the moisture up.

...I could seal some of the slab but I am not going to move all the benches and equipment just to seal the concrete.

I probably wouldn't bother if you aren't doing it all.  Kinda like patching some of the holes in a roof and being surprised when you still get wet inside.  Is there a way you could move everything to one half, and seal the open half, then switch?  Yeah, its a pain, but better in the long-term.

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I'm not a builder...but it is my understanding that a vapor barrier is pretty standard for a slab under any building that will be climate controlled.  Obviously, it is not beneficial if the building is not heated or cooled as you are going to acclimate to outdoor conditions.  Did the builder know you intended to insulate and heat/cool the space?  Did they offer a vapor barrier?

[KC]

I'm not a builder...but it is my understanding that a vapor barrier is pretty standard for a slab under any building that will be climate controlled.  Obviously, it is not beneficial if the building is not heated or cooled as you are going to acclimate to outdoor conditions.  Did the builder know you intended to insulate and heat/cool the space?  Did they offer a vapor barrier?

It's kinda hard for me to imagine that a reputable concrete contractor would pour a slab that was intended for a building without a barrier.  In the big picture of a total building, the material cost seems inconsequential.

[fredhargis]

Around here they put a barrier under building that will be heated/insulated but not under freestanding buildings that (they think) won't be. In my case I had to ask for it under my shop building, which to the builders was a detached garage; otherwise it would have been just slab on grade. Iublue stated it was an all metal building...if it was a post/frame structure they may have assumed iit wouldn't be insulated/heated. Speculation on my part, and none of it helps with his problem.

[JosephP]

Around here they put a barrier under building that will be heated/insulated but not under freestanding buildings that (they think) won't be. In my case I had to ask for it under my shop building, which to the builders was a detached garage; otherwise it would have been just slab on grade. Iublue stated it was an all metal building...if it was a post/frame structure they may have assumed it wouldn't be insulated/heated. Speculation on my part, and none of it helps with his problem.

I don't know...but IF the builder was negligent in following (or at least recommending) widely accepted industry standard practices, it may be worth a serious conversation where the builder is "offered them the opportunity" to address the problem by moving machinery and applying a sealer/epoxy to the top & providing and adequate de-humidification system in lieu of more drastic alternatives.  Obviously there are a LOT of variables in there such as communicated plan for the building, contracts, ability to establish/demonstrate industry standards, local building codes, etc. etc. etc...  Just saying, I wouldn't be real please if the builder knew it was intended to be insulated/climate controlled and didn't at least suggest vapor barrier.  If it was a franchised builder, my first call might to the mother ship to see what they do or do not recommend - not explaining the whole situation, just a simple "Do you recommend a vapor barrier under a slab for your buildings?".

[iublue]

I'm not a builder...but it is my understanding that a vapor barrier is pretty standard for a slab under any building that will be climate controlled.  Obviously, it is not beneficial if the building is not heated or cooled as you are going to acclimate to outdoor conditions.  Did the builder know you intended to insulate and heat/cool the space?  Did they offer a vapor barrier?

I have been out of the building game for a while now but when I was building homes it was not standard practice to have a vapor barrier under a slab.  Things change and maybe it is now.
If I had thought there would be a problem, I am sure the guy that poured the slab would have accommodated but as high as the building is off the original grade, it never crossed my mind that it would be a problem!

For me, it would not really be possible to move everything to seal the whole slab.

[MstrCarpenter]

Gutters may help; especially on the uphill side. The next thing I would consider is a perimeter drain on the uphill side, buried lower than the footing. I don't know how far the grade slopes uphill from your shop but there could be a layer of clay, fine silt, or other soil that slows the percolation enough that subsurface water flows over it to the point where the layer was broken. That would be the trench your footing is in. If the water goes under that footing, how easily can it get under the downhill footing? Could it be trapped within the foundation walls? You could drill some weep holes just above grade. If water dribbles out, the moisture level of the slab could drop enough. However; gutters and the perimeter drain would most likely get it (or at least more of it) before it's under your floor.

Here's my own examples. The walk-out basement of my 24 x 48 shop was always relatively dry thanks to a perimeter drain beside the footing. Then I noticed condensation in one "uphill" corner. I dug up the downhill corner (where I drive in) and found the drain was crushed, torn, and blocked. I dug a new pit and connected the pipe to a 55 gal. barrel with an open bottom. Problem solved.

My house also has a walk-out basement. In about 20 years it flooded 3 times and we would need to control humidity. Following the third time, I cut 12" trenches in the slab and put perf pipe next to the footing from end to end and front to back in the middle, then under the footing to another perf pipe running the length of the house (outside) to a 55 gal. pit. That pit drains to daylight about 150' further downhill. My basement is powder dry. No pumps. No dehumidifier.

[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

So you think a day or so would be enough time?

I taped two pieces of plastic down to the floor in two spots.
Both pieces are about 2' x 3'.  I used packing tape and some masking tape to hopefully seal them to the floor.
I put one piece over the middle cut in the slab that is for controlled cracking.  I also sealed the crack with caulk under the tape.
I put one piece on the end with a lot of gravel (~3 feet) and the other on the end with shallow gravel (~1 1/2 feet).

[Roly]

So you think a day or so would be enough time?

I taped two pieces of plastic down to the floor in two spots.
Both pieces are about 2' x 3'.  I used packing tape and some masking tape to hopefully seal them to the floor.
I put one piece over the middle cut in the slab that is for controlled cracking.  I also sealed the crack with caulk under the tape.
I put one piece on the end with a lot of gravel (~3 feet) and the other on the end with shallow gravel (~1 1/2 feet).

I would think a day or two would be enough.  Roly It would be best after it has rained

[iublue]

So you think a day or so would be enough time?

I taped two pieces of plastic down to the floor in two spots.
Both pieces are about 2' x 3'.  I used packing tape and some masking tape to hopefully seal them to the floor.
I put one piece over the middle cut in the slab that is for controlled cracking.  I also sealed the crack with caulk under the tape.
I put one piece on the end with a lot of gravel (~3 feet) and the other on the end with shallow gravel (~1 1/2 feet).

The mystery deepens.

Using Roly's suggestion, I did what is above about taping plastic to the floor.

I removed the plastic today.  The tape adhered so good I had to use a scraper to get it up.
The plastic had been down for 6 days.
Both pieces were as dry as the proverbial bone.  
No water detected, either by touch, by sight or by paper towel.  NOTHING!
I hadn't even emptied the dehumidifier for almost two days since I had been busy, so the RH had risen in the building.

Not only did Roly suggest that test but I also read the same thing on the internet.
Assuming the test is accurate, then moisture is not coming through the slab because even in very small amounts there should be some on the plastic after 6 days.

Definitely scratching my head now!

[Cabinet Monkey]

I have been out of the building game for a while now but when I was building homes it was not standard practice to have a vapor barrier under a slab.  Things change and maybe it is now.

Don't know when that was or if it's peculiar to Indiana, but in the southeast:  AL, GA, SC, and N.FL vapor barriers for a slab in enclosed buildings have been on every project I've ever been associated with since the early 70's



Suggest you forget what you know about slabs (or think you know) and have a practical and simple think about your issue.

You have a very tight (presumably) building that is all but impervious to water from the outside.  The roof doesn't leak, the walls are impervious to water and they are sealed on both sides via the CLOSED CELL FOAM.   You have a hard side cooler, in effect.  Almost.

Yet, you have very high levels of moisture in the air.   FORGETTING your building knowledge bias, where is the most LIKELY place the moisture is coming from if it's not from your roof or through the walls ?  Remember liquid water ain't coming in because of the metal skin and water vapor is prohibited (for all practical purposes) by the insulation.

For me, it would not really be possible to move everything to seal the whole slab.

Then, your only choice is to mechanically deal with the moisture that will continue to enter the building. You'll almost certainly need another / larger mini split -or- a much larger dehumidifier.