shop sweats

[CTyler]

Would sealing the floor with a polyurethane help at all ?

Dennis

I do not beleive that will not help in this case. The problem is no ventilation causing water to condense on the inside of the building. My BIL has one like this, a metal building with no ventilation. The darn thing will practically rain inside some days!

 

[Willl]

For what it's worth, my shop (pole type) has no vapor barrier in the floor and the only insulation is in the ceiling (under the roof panels).
It don't sweat.

 

[Defective]

In a previous shop with a steel roof, I used to get rained on when the woodstove was running on 40 below days. The solution I found best was sprayed on foam covering the entire inside of the roof. I didn't vent the shop during winter because I didn't want to loose heat when it was really cold out.

That particular shop was built on a piece of land known for the springs that appeared at random all over the place. The closest thing to a vapour barrier under the slap was 4 inches of styrofoam that was placed before the slab went down. No care was taken to seal the cracks in this insulation made up of 2 by 4 foot sheets. The floor was never wet except where equipment dripped on it. (plus that one time when I backed into the slack tub... )

Eddie,
I'll completely agree with you about the water vs. concrete thing. After the shop had been up for a year, I discovered a small creek running out from under one corner. It didn't enter anywhere. Must have been one of those bloody springs. BTW: it's the INSIDE of the steel that developes condensation during the heating season. I know it's easy to get that mixed up when you live in the tropics.

 

[wushaw]

I'm in N.Texas, my 40X50 shop has two roof vents and the roof is insulated, I get very little rust on my tools and no sweating anywhere any time of the year for the past four years.
I can't remember if there was a vapor barier under the slab or not I just remember sand, rebar & forms...no water/condesation on slab.

Eddie the only time I ever saw a sweating slab was in Houston and it wasn't the slab sweating it was 99.9% humidity forming on the slab. This was in a hangar with very thick concrete to hold large aircraft.

 

[jinman]

When the shop is heated, the outside temps are now cooler than the inside temps. This will cause the formation of condensation on the outside of the roof. If you were to go outside late at night or early in the morning, you'll see this all over the place. The roof and hood of your car does it all the time.

Eddie, I believe this is just the reverse of what happens. When I turn the defroster on in my vehicle, the windshield becomes much warmer than the outside air and this causes the moisture to clear. I believe moisture forms on a vehicle sitting outside because the vehicle is slightly cooler than the air, the same as a soda can or glass of iced tea is cooler than the outside temperature and moisture forms. If your example is true, wouldn't a cup of coffee sweat if you took it outside in cool weather?

 

[ovrszd]

Hey Ovrszd,

When the shop is heated, the outside temps are now cooler than the inside temps. This will cause the formation of condensation on the outside of the roof. If you were to go outside late at night or early in the morning, you'll see this all over the place. The roof and hood of your car does it all the time.

There is a formula as to how much water the air will hold based on the tempature of the air. Warm air will hold more water than cold air. If it's a dry climate, there isn't going to be much water in the air and you have very low humidity. Areas with lots of moisture and high temps get high humidity because the air will hold large quantities of moisture.

When the air is cooled, like the soda can example, you get condensation because the air can no longer hold the level of water it is holding. The soda can is cold enough to change the tempature of the air imediately surrounding itself. A warm soda will not have any change. There is a point for the air tempature and the soda can that will cause this. I forget the term, but at this point, humidity turns into condensation.

Fog is the same thing, but on a much larger scale. The ground lowers the air tempature to the point of 100% humidity and condensation forms on the surface of everything.

I'm also of the opinion that your slab has absolutely zero affect on condensation in a building. I'm in the extreme minority here to the point I might be the only person here who thinks this. My reasoning is very simple. To believe that water vapor travels up from the dirt under a slab, means that there has to be moisture under the slab. I've demolished quite a few concrete slabs on one kind or another over the years, and have never found one with moist dirt underneath them. In fact, the soils is so dry, it's almost as hard as the concrete. The other reason I don't believe moisture is traveling through 4 to 6 inches of concrete is that it wont' travel down through it. Pour water on the slab and wait for it to leak through. It's not gonna happen. If standing water wont' travel downhill through concrete, it aint gonna travel uphill through it. Espesially if theres no moisture under the slab to begin with. From what I understand, the reason for a vapor barrier under a slab is to hold the moisture in when pouring the slab. Once it's poured, the vapor barrier does nothing.

Venting the inside of the building will equalize it with the outside tempature. This will stop the formation of condensation. But if it's foggy outside, than it will be foggy inside.

Heating the inside will stop condensation from forming on the inside also, but it's an expensive fix. Circulating the air will help, but only to a limited degree unless you have vents. With vents, you can still circulate the air, but I don't know if it will make a difference or is needed.

Condensation will also form in the attics of wood built homes. This is the reason for attic vents. Some people think attic vents are to cool off the attic, but from what I've read, they make no difference in the amount of heat you have in your attic. They are there to equalize the outside tempature to the inside and cut down on the amount of condensation you get in your attic. It will not eliminate it, but the average home can survive without any problems with the minimal ammount of condensation that it gets every year.

Eddie

Another great, detailed response!!! You are on a roll here Eddie. We have obviously hit on something you are very understanding of. I'm on your side about the concrete!!! My concrete floor only sweats when we get a fast warm up in spring or fall and the air temperature quickly exceeds the concrete temperature. Then it goes thru the pop can theory. But the moisture does not rise from the ground thru the concrete. And I, like you, certainly hope that the dirt under my concrete is extremely dry. If not we have bigger issues!!!

My shop has a flat insulated ceiling with attic vents at each end. I also have an air barrier between my wall insulation and the outside metal. My building rarely displays the moisture on the outside that you explained. I've saw plenty of metal buildings that do though.

Some very good information here. Thanks Eddie!!!

 

[EddieWalker]

Hi Jim,

If the inside and outside of the shop or car cooled at the same rate, than I don't think you'd get condensation. That would only happen when fog is being created. But with a car or a shop, you have a warmer space than the exterior space, which will hold more moisture. When these two areas equalize in tempature, the warmer one will form condensation. An enclosed space will be warmer on the inside than the outside air. This area will aslo hold more moisture and heat draws moisture to it. When this space cools down, the level of moisture increases in the air and forms condensation.

The soda can might have been a bad example because it's full of soda. I was hoping to point out that the air holds water in it and when you lower that tempature, water will from. The air around a cold soda can drops in tempature enough to form condensation. With high humidity levels, it doesn't take very much for this to happen.

I wonder what happens to a soda can in South Texas? On a very hot day, does a soda can form condensation? I think the level of humidity there would be very low to begin with, so it would have to be a very, very cold soda to create condensation.

Anyway, it's all dependent on the humidity in the air outside the building, compared to the humidity inside the building and the change of tempatures. Venting the building will equalize the tempatures and the humidity levels, which will eliminate condensation. The trick is to get enough ventilation. Not enought and you will still have the problem.

Heating the building will raise the humidity level inside the building and allow the air to hold more moisture. As long as the air is warm enough to hold the moisture in it, you wont have any condensation either.

Eddie

 

[EddieWalker]

Hi Ovrszd,

Your welcome, but I'm not educated or any sort of an expert in this. I'm just offering my opinion on how I understand it to work. Those who disagree with me on the concrete allowing moisture to come up through the ground have some good arguments against me. I just don't believe it, even though I realize that they are better educated and smarter than I am. It could be that I'm just stuborn, but until it makes sense to me in my brain, I'm of the opinion that moisture does not come up from the ground and pass through solid concrete. If it's traveling up against gravity anyway, why wouldn't it just take the path of lease resistance and come up outside the slab?

I don't know if my explinations are accurate, but it's how I understand how it works. I don't even know if I'm spelling half the words I'm usinging correctly. hahahahaha

Thanks for the kind words,
Eddie

 

[slowzuki]

Some good replies, I'll add a few words to help if you are looking for info. Water comes out of air at the dewpoint. So if you have a weather forecast, 88% relative humidity and 70 F, if you cool that, eventually it will hit 100% relative humidity, can't hold more water, condenses out.

So in a shop you have a variety of angles that can work and some problems.

An uninsulated metal building gets very cold at the steel skin, if you keep introducing water via breath, exhaust, floor etc it will keep condensing out on the cold surfaces, lowering the overall rh% making it easier to suck more water out of floors and outside.

Air circulation can work like what Eddy says if you keep the metal close to the air temp and you aren't introducing lots of water into the air. Works well for big storage buildings for example.

Heating an uninsulated building, direct fired gas or oil is a terrible idea for this as it dumps water directly into the air and will condense out as soon as it touches the cold steel skin. Wood heat is better as it is vented outside and draws lots of air through the building. Problem is storing wood inside introduces lots of moisture in the wood that is slowly releasing it as it dries.

Dehumidification can work well but needs warm temps to work well and if the outside temps are really low you will have cold temps right at the metal surface causing condenstation even if the warm air in the building is very dry.

A layer of insulation is great even for unheated buildings as the surface temperature can (the insulation inside surface) can track the air temperature of the building much better and not provide condensing temperatures. If you heat and occupy the building you need to insulate or just provide local radiant heating and lots of ventilation.

 

[CharlieTR]

I think if it was a vapor barrier missing from under the slab, it would just be a little moister on the floor only. Here is a link to a pdf file from the Metal Building Mfg. Association. They talk about condensation in great detail and you should be able to cure the problem for sure.

http://www.mbma.com/pdf/condensationfactsheet.pdf