shop sweats

[EddieWalker]

kmdigital,

Thank you for a great, and very detailed explination.

Most of what you say is way over my head or experience level. You have more knowledge here than I do, so I won't argue any points that I know nothing about. And I'm not pretending to be very knowledgable on the points I am argueing. hahaha

My observations are based just on experience on residential slab homes. Basements leak for allot of reasons that can be explained other than concrete being pourous. If a basement was built like a swimming pool, than it wouldn't leak. From what I've seen, basements are full of seams and air pockets. I know there are basements that are many decades old that don't leak without any fancy sealers, and I know that some very new basements leak. With milions of homes, the odds are that some will leak. The ones that support my point, are the ones that don't leak.

The plastic on the ground test is similar to the same one you do on concrete slabs before you put down a laminate floor. There is a period of time that mosture comes out of the slabe as it's curing. Once the curing process is done, the plastic taped to the slab will no longer have any moisture under it. This usually takes a month or two for it to be cured enough to not have any more moisture in it.

I can see where you'd get moisture under a tarp or piece of plastic on the ground in the right situations. One survival technique is to sustpend a tarp over the ground and weigh the center of it with a rock. Put a container under the tarp just below the rock and the air will condense under the tarp. Water droplets will flow down the bottom of the tarp and drop into the container.

A plastic sheet on the ground will have all sorts of things happening. It will raise the ground tempature and force evaporation to happen, and it will increase the tempature of the air trapped under it and create humidity.

I don't think this relates to a concrete slab at all. The problem with the slab is that it's sealed off all the air above the soil under it. The building over it creates an air space over the slabe that is different than what's outside the building. The soil under that slab is no longer receiveing moisture and will dry out to the point of not having any mosture left in it.

Even if I'm totaly wrong about moisture traveling through a concrete slab, there is still no moisture in the ground under it. If there was, than the dirt around the building would be moist or wet to the touch. Homes and buidings are always the highest point in there imediate area. If water is coming up from the ground, than it's gonna come up to the lowest point, not the tallest. If you have water coming up from the ground, or water vapor, than it's gonna do so on the sides of the building.

Why would there be mosture on top of the slab from this water vapor coming up from the ground, but not outside the slab at a lower elevation?

Thanks,
Eddie

 

[CTyler]

I would tend to think that if the ground was indeed bone dry under the slab and the slab is wet then the surface temp is causing it to condense. We can't make one statement for a wet slab, you have to consider each case and evaluate.

Moisture under a slab can go from a low point to a high point and defy gravity. The fancy terms for this is capillary action(wicking) and vapour diffusion(gas). Think of a wick on a oil lantern.

Eddie, I think if you lived farther north you'd get to see this first hand. Since your way down South I'm guessing this doesn't happen to much and its' mostly surface condensation.

Kmdigital, great explanations.

 

[Defective]

Eddie,

I'm still going to back your theory. The plastic on the ground test doesn't really relate unless the plastic is as big as a typical slab and stays there as long as a slab. What you say about the moisture working its way up at the lowest point is dead on. I think I mentioned somewhere in this thread about the spring that developed under my shop. There was NO moisture on top of the slab over the spring. There was, on the other hand, a fairly substantial little creek flowing out from under the slab. It caused me quite a bit of concern for a while. If the slab had been less than 8 inches thick, I'd have been very tempted to break it up and install drainage under the floor. One of these days I really should take a drive up there & see if the current owner is having any problems with that floor...

CTyler,

North vs. South doesn't make too much of a difference in where the moisture comes from. My experience has pretty much parallelled Eddies and this includes places as far north as Moosonee. One difference is how long it takes for the soil under the slab to dry out. In one case, I helped replace a floor in a large shop and we found that the soil under one half of the slab was bone dry (at least in the top 3 feet) and the soil under the other half was slightly moist. The first half of the floor had been in place for about 8 years and the second half had been poured the previous summer.

From what I've seen, I would think that what moisture IS under the slab tends to migrate out around the slab over time.

mudcat,

As kmdigital pointed out, leaving an unsealed airspace between the insulation and the steel is a bad idea. part of the requirement for keeping the steel from sweating is keeping the moisture away from the steel. If there is an airspace, there can still be condensation because insulation is never perfect. That airspace can still be warmer than the outside. If you get condensation in that airspace, not only will the insulation get wet, but the steel can rust out from the inside where you never see it until it's too late. This is why I went with polyurethane. It seals completely.

 

[mudcat]

I don't think I would do this. The reason is that there will be pretty large air gap between the insulation and the metal, and the metal will still likely sweat, only now it will be hidden above the insulation. The R-board will hold the water above it, and it will rot out the purlins. The insulation needs to be against the steel to prevent condensation on the bottom side of the steel. This is why pre-engineered steel buildings always recommend putting the insulation down first and putting the steel directly on top of it.

If the air space were sealed, so that no air could move between the R-board on the bottom and the steel on top this would work, but because the steel is corrugated the air easily moves between the purlins. There isn't any good way to seal this, so it would be better to put fiberglass insulation between the purlins, and then put the R-board under the purlins if you want the finished look. Doing it this way gives you a nice finished look, and will definitely solve your problem.

What if I placed 1-1/2" R-board between the purlins,then another layer of beneath the purlins? Would this be as effective as the fiberglass? I have plenty of R-board.

 

[mike69440]

KMdigital,
In my experience, you are correct.

Eddie,
Your water not getting through a slab theory works where there is not a lot of ground hydrostatic pressure.

If I pile drove a 15' pipe in the ground 10 feet deep and water filled the pipe above ground level, then you have to do the following to the slab.

Crushed stone base with perimeter drainage.
Vapor barrier.
Insulation so the slab may stay a bit warmer than the ground.
Topcoat sealer. (Sealer will not work on slabs where water pressure pushes up!)

Metal building need vent space and insulation in roofs. The walls should have some plywood on them to absorb moister an insulate.

 

[EddieWalker]

Mike,

I'm not familiar with pile driving pipe, nor the results you speak of. It sounds interesting and I'd be interested in seeing pics of it if you have any.

I have a small understanding of hydrystatic preasure and the force of water on opjecst put into the ground. Swimming pools and septic tanks are both good examples. If you put them in the ground and leave them empty, water preasure in the ground will in some instances force them to the surface.

This may be what's happening with yoru pipe, but that's just a guess. A cement slab on top of the ground is not under any hydrostatic preasure. The water in the ground is still in the ground, it's undesterbed and there is nothing that has changed the preasure on the surrounding ground table. Basements are affected by this, but a slab is not.

I don't see anything that suggests water will rise to the highest point of land and then penetrate a cement slab, but not leave water on the ground that is lower in height than the dirt around the building. If this happens, then why doesn't water rise to the ground all over the place? Why does this only happen in order to penetrate concrete slabs?

Another example that I thought of today is the drain lines around bathtubs and showers. When you build a house on a slab, you leave an open area to set the trap and drain. It's just about impossible to get them right without having the tubs and showers on hand. Different brands and models have different locations of the drains too.

In order to install the trap, there needs to be a space that isn't poured. Usually half a square foot, more or less, is normal. So to be clear, there is no concrete poured in these locations. It's just dirt, or whatever is under the slab.

Now if water is traveling upwards to the slab, wouldn't it come through these openings in the slab before traveling through solid concrete?

I live in a sub tropic location. We get more rain than most places in the country and have a very high water table. 20 feet down and your in water. Yet I have no vapor barrier or anything under my slab besides dirt. I do not have any moisture in my shop or on my slab. I do have good ventalation in my shop and good insulation in my home.

Water does not come up through the ground and enter my home through the opend dirt area around my tub drain. It doesn't happen in any homes that I've ever heard of.

I'm willing to concede that concrete isn't 100 percent water proof, but the amount of moisture that gets through it is so minimal as to be non existant.

The one explination I do have for wet slabs is that the ground is cold enough to lower the tempature of the slab and bring the air in that space down to it's dew point. This will cause condensation to form on the surface of a slab.

Eddie

 

[kmdigital]

What if I placed 1-1/2" R-board between the purlins,then another layer of beneath the purlins? Would this be as effective as the fiberglass? I have plenty of R-board.

That should work, just get the insulation as tight as possible to the metal so there aren't too many air gaps.

I don't know that adding a layer of R-board under the purlins is really beneficial, since I am assuming that you have 2x4 purlins. This will leave a 2" air gap between the two layers of insulation. By leaving the bottom layer off you could tell if you still have condensation problems because it would leak out along the edges of the purlins, while installing the bottom layer would tend to hide this which could cause big problems. Assuming that you aren't heating the space, the added insulation won't provide any benefit in regards to helping prevent condensation.

 

[CTyler]

Defective,

I keep going back to my personal experiance with the 5 houses on a cirlce and the one house with the vapor barrier, his garage slab would stay dry.
I sited other examples in earlier posts. There are a number of cases as well where you don't need a vapor barrier.

I also keep going to experiance with basements even walkout basements.
A walkout basement is basically slab on grade. You WILL get sued if you don't put in a vapor barrier and waterproof the walls. Why because you will have a damp smelling basement. I also keep going back the the engineers stating the need for a vapor barrier, again not every slab needs one.

 

[PineRidge]

To control your dew-point problem install one or two electric gable vents and tie them into a humidistat that will cycle them on as the humidity level increases. This is the very same principle used to control damp attics.

 

[Defective]

I keep going back to my personal experiance with the 5 houses on a cirlce and the one house with the vapor barrier, his garage slab would stay dry.
I sited other examples in earlier posts. There are a number of cases as well where you don't need a vapor barrier.

I'd be inclined to look at other possible factors. This house being the only one with a vapour barrier under the slab...by chance was it also the only one with insulation under the slab?

I also keep going to experiance with basements even walkout basements.
A walkout basement is basically slab on grade. You WILL get sued if you don't put in a vapor barrier and waterproof the walls. Why because you will have a damp smelling basement. I also keep going back the the engineers stating the need for a vapor barrier, again not every slab needs one.

Sorry, gotta disagree here. A walk out basement still has walls below grade. Therefore, anything at the bottom of those walls is, by definition, below grade. Any basement can be damp for the simple reason that, once you go below grade, all bets are off. Water will enter through the walls and under the walls. Even sealing the walls and floor will not guarantee a dry basement. Drainage is also mandatory.

hhhmmm...

I just recalled an excellent example of basement water issues. The house I currently live in was built in the 1850s. It has a full basement with a stone foundation.

When I moved in, there was a brand new drainage tile around the outside of the foundation. The previous occupant had spent every spring fighting standing water in the basement.

I have not had so much as a damp floor yet.

The only damp concrete floor here in the time I've lived in the house has been the back porch which is 2 feet above grade. The porch is weather tight and the floor is completely wet every time we get a good rain.