Moisture problem in new shop!

[iublue]

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.



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.

I get it, the slab is the likely culprit but right now there is no empirical evidence to back it up.
If the test I perform is accurate, then the moisture is not coming through the slab.
Using the dehumidifier, the shop is at a stasis point right now at a RH level of 60%.  Since I am dumping about 9-10 pints of water a day, logically there is 9-10 pints of water a day infiltrating since the RH is stable.  But the test showed no sign of water on the plastic after 6 days.

A new mini-split is not an option besides that would not help during the heating cycle.
I could seal the slab and do the best I can to cover a vast majority of the floor but as you said, there would still be holes in the protection.
It appears to me that the only reasonable option at this point would be to get another dehumidifier or a larger single dehumidifier.

Thanks for the reply.  I will update if I get any new information.

[fredhargis]

 I will update if I get any new information.

Please do, I'm curious about what you find (if anything).

[Snipe Hunter]

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?

This. If the contractor didn't put down an impermeable barrier before the pour, you will always have a moisture problem. Concrete is porous.

I'd paint the floor with a durable, impermeable paint or epoxy. And probably install some sort of fresh air system but that will increase your heating costs. Heat won't dry the inside unless there is a way to remove the water from the air. 

Walls need to breath to release moisture when the outside humidity is less than the inside. Or when the outside is cooler than the inside. Steel buildings don't breath well and neither does spray foam insulation. So you have 3 strikes against you. No water barrier under the slab, steel building and spray foam. Houses are built to breath. Everything in a a traditional stick build house is permeable, the drywall, the insulation, the sheathing, the house wrap and the siding. It breaths just enough to move moisture but not much air. Basements on the other hand are usually concrete or block, both permeable. The walls are cool so most of the moisture in the house condenses in the basement so only the basemen needs a dehumidifier or powered air transfer, not the rest of the house. A properly built crawlspace needs vents and a vapor barrier. A slab foundation in a house had an impermeable vapor barrier.

I'm surprised they used pea gravel as opposed to a larger stone. A fairly inexpensive attempt to help keep the air dry under the slab would be to cut a 4" diameter hole through the slab and install a 4" PVC pipe into the gravel and install a high CFM radon fan and vent the fan outside. Ideally the hole would be near the center of the slab. Leave the fan running. It will pull the moist air out from under the slab and help dry the slab. Radon fans last about 20 years. You still might need the dehumidifier but have it only kick on when the humidity gets high. But the fan should pull out most of the wet air.

FanTech is a very reliable fan. Fairly quiet. You could even install the fan near the ceiling or even outside if you don't want to hear it running. If you put it outside, paint it or the UV rays will degrade the plastic housing. A 6" pipe will accommodate a 500+ CFM+ fan. 4" pipe will accommodate up to about a 260 CFM fan.

[Snipe Hunter]

I get it, the slab is the likely culprit but right now there is no empirical evidence to back it up.
If the test I perform is accurate, then the moisture is not coming through the slab.
Using the dehumidifier, the shop is at a stasis point right now at a RH level of 60%.  Since I am dumping about 9-10 pints of water a day, logically there is 9-10 pints of water a day infiltrating since the RH is stable.  But the test showed no sign of water on the plastic after 6 days.

A new mini-split is not an option besides that would not help during the heating cycle.
I could seal the slab and do the best I can to cover a vast majority of the floor but as you said, there would still be holes in the protection.
It appears to me that the only reasonable option at this point would be to get another dehumidifier or a larger single dehumidifier.

Thanks for the reply.  I will update if I get any new information.

Yes there is empirical evidence. It has always been a problem in home construction. Most jurisdictions require a 6 mil, impermeable barrier under the slab and still it isn't 100% effective. Any moisture evaporating from the floor, even small amounts will get stuck in the air and accumulate without any external ventilation. You've built a greenhouse with a constant moisture supply.

Have you put a moisture meter on the slab? Anything approaching or above 19% MC (moisture content) is very high. Ideally, you want to bring it down to about 5-7%. But even 5-7% will evaporate into the air in the building. I get readings on basement walls around 5-7% and higher on the floor routinely, even if the home is brand spanking new and I know the slab has a moisture barrier under it and the exterior of the foundation is sealed with a moisture barrier. But the subfloor above the basement is very permeable so much of the moisture escapes and you'll see it condensing on the windows on cold days. We need some moisture in the living areas or live is miserable. Even potted plants put moisture in the air, breathing puts moisture in the air. But most of it comes from the basement walls and floor. If it's built right, we might have to add moisture with a humidifier.

You know the moisture isn't coming through the walls or the roof and there's been no rain. It's winter so humidity levels outside are low. That leaves only 1 possibility for moisture intrusion... the floor.

[ajkoontz]

I just wanna chime in as a lifelong IN resident, for whatever reason it is not common practice to put vapor barriers under slabs of detached outbuildings around here and the chance of getting any builder to correct this on their dime is <1%, IMO. Also, very lax on code enforcement of any kind, generally speaking, which can be good or bad depending on the situation.

[Snipe Hunter]

I just wanna chime in as a lifelong IN resident, for whatever reason it is not common practice to put vapor barriers under slabs of detached outbuildings around here and the chance of getting any builder to correct this on their dime is <1%, IMO. Also, very lax on code enforcement of any kind, generally speaking, which can be good or bad depending on the situation.

It probably isn't common practice anywhere aside from dwellings where it's required. You probably won't find it under the garage floor either. It's something you'd have to ask for. We just had an addition built with a crawlspace underneath. I was surprised when the contractor wasn't going to install the vapor barrier. He did when I was requested it but otherwise he wouldn't have done it. It is required by code but that doesn't mean code enforcement will make them do it. Even the crawlspace ventilation. It's required but they didn't plan on putting it in. He told me the inspectors don't really care if it's a small addition. It's only 300 sq ft. But he put them in for me along with the vapor barrier and didn't come after me for a change order.

I don't think people really realize how damp soil is and the problems it can cause. That moisture needs a way to escape or be encapsulated so it stays in the ground. Encapsulated crawlspaces are becoming more common and they eliminate the need for ventilation.

[Cabinet Monkey]

 You probably won't find it under the garage floor either. It's something you'd have to ask for. We just had an addition built with a crawlspace underneath. I was surprised when the contractor wasn't going to install the vapor barrier.It is required by code but that doesn't mean code enforcement will make them do it.
I don't think people really realize how damp soil is and the problems it can cause. That moisture needs a way to escape or be encapsulated so it stays in the ground. Encapsulated crawlspaces are becoming more common and they eliminate the need for ventilation.

This is why a set of plans AND specifications are so important for any building endeavor.  

Builders need to build what is specified, not what they think is needed, and not what they want or feel like building.

[iublue]

Yes there is empirical evidence. It has always been a problem in home construction. Most jurisdictions require a 6 mil, impermeable barrier under the slab and still it isn't 100% effective. Any moisture evaporating from the floor, even small amounts will get stuck in the air and accumulate without any external ventilation. You've built a greenhouse with a constant moisture supply.

Have you put a moisture meter on the slab? Anything approaching or above 19% MC (moisture content) is very high. Ideally, you want to bring it down to about 5-7%. But even 5-7% will evaporate into the air in the building. I get readings on basement walls around 5-7% and higher on the floor routinely, even if the home is brand spanking new and I know the slab has a moisture barrier under it and the exterior of the foundation is sealed with a moisture barrier. But the subfloor above the basement is very permeable so much of the moisture escapes and you'll see it condensing on the windows on cold days. We need some moisture in the living areas or live is miserable. Even potted plants put moisture in the air, breathing puts moisture in the air. But most of it comes from the basement walls and floor. If it's built right, we might have to add moisture with a humidifier.

You know the moisture isn't coming through the walls or the roof and there's been no rain. It's winter so humidity levels outside are low. That leaves only 1 possibility for moisture intrusion... the floor.

Ok, there is observational evidence which is one component of something being empirical evidence.  I should have used measurable evidence.
I do not doubt that the vapor is coming through the slab but I am puzzled as to why the plastic experiment did not yield any evidence of it.
With my science background, I want to prove the hypothesis!

I am well versed in the ventilation issues in home building.  I built my super insulated home forty years ago.  With all the measures that I took in my home, it is too tight and that is why I have three air to air heat exchangers.  Those exchangers allow fresh air to be brought in and stale air vented out and a majority of the heat to be saved.  All three run for around two hours a day. 

As you have stated, my mistake was not factoring in the spray foam insulation.  If I would have thought about that aspect I would have put down a vapor barrier but that is a moot point now.  As you stated a 6mil barrier is not 100% effective but darn close.  It has a perm rating of .o6.  Something that has a perm rating of 1 will pass 1 grain of water per square foot at a vapor pressure difference of 1 inch of mercury.  There are 7000 grains of water in a pound which is roughly a pint of water.  Evidently the vapor pressure difference is not all that great since about 9 pints of water a day infiltrate.  I am sure it could be calculated since a 4" slab has a perm rating of .8.  I am not going to do it but I imagine the math works out.

There were no plans for this building outside of what the building manufacture used to cut all the metal.  I really don't know how a plan would have helped because as I have stated, in my area it was not a common practice of putting a vapor barrier underneath a concrete slab.  Maybe it is now but I don't really know.  I was the construction supervisor for a multi-million dollar theater.  Obviously an architect drew those plans but there was no vapor barrier specified under the slab.  Many of the homes around me have a slab for the lower level, ditto on no VB.  I was going to build a conventional building and if I would have, I have no doubt that this would have not been an issue since it would not been as tight.  When lumber went nuts and I pivoted to a metal building, moisture issues never crossed my mind.

The bottom line is I will have to use a dehumidifier during heating months.  Not a terribly big deal and on the positive side, the shop will never get bone dry in the winter months!
 

Thanks for all of your responses.

[Snipe Hunter]

Empirical evidence detector for $29.00

There are more accurate moisture meters than this one but this will be sufficient. You're just looking for MC/Moisture content at the slab surface.

The cost to operate a radon fan is a lot less than a dehumidifier

[iublue]

Who designed / specified this shop build ?

Was it inspected by an authority or private engineer ?

You do know that water vapor is an entirely different animal from water. Right ???

Your issue is your slab, plain and simple.    Propane ain’t helping, and your dehumidifier is clearly undersized for  your volume.  Along the lines of big game hunting with a squirtgun.

Do the test, and start looking for companies that apply commercial vapor barriers on floors.  Your issue is not all that unique.

I did the test as stated in another post.

Why do you think that the test was negative for water after six days?