Sweating metal building

[Alan L.]

I have a metal building with 3.5 inch vinyl backed fiberglass insulation. The insulation came in rolls and was applied (on a still day) over the purlings and then the metal skin was installed. Its the same in the roof and walls. I doubt it is the most efficient it could be since it is squeezed flat over the purlings, but it is effective and my building stays fairly warm in winter and does not get near as hot as an uninsulated building in the summer.

My slab does not have a vapor barrier either and the floor sweats sometimes. I was told this is not moisture coming up through the concrete, but rather condensation from the air inside the building. I believe this is correct because it only happens on humid days.

I guess the vinyl backing toward the inside of the building would act as the vapor barrier. Don't know if there is any moisture behind the insulation because I can't see it.

 

[hilld]

Sounds almost identical, the only problem here is that is has been humid for the last 4 months continuously. I can't wait for a dry week or two. I know, it is the grass is greener, but I can't pave or pour concrete until it dries up a little.

Derek

 

[RichNJKubota]

To find out if moisture is coming up throught the concrete you can out a piece of poly sheet on the concrete floor for a day or two. Hold the edges down with some boxes, etc.
If there is water under the poly it very well could be coming up through the concrete.
I do disagree with other on this topic - From my POV concrete under 6000 PSI allows water vapor to go thru.

Rich

 

[EddieWalker]

Rich,

Taping a piece of plastic to a concrete slab or floor is to measure if the floor is dry enough to put a flooring down.

A few weeks after the slab has been poured, it's still drying out and the moisture from the mix is still evaporating upward.

Let's say you just had a slab poured. Take the plastic and tape it to the slab. The next morning the concrete under the plastic might be wet, or just darker in color. Do this every week and there will be less and less change in the color until there is no change. This is when it's safe to install your flooring.

I'm not positive on the exact time of this, but concrete reaches most of it's hardness in a day, but takes several months for it to get almost all the way dry, and years to be one hundred percent.

After a few weeks, it's time to start testing it if you want to install the flooring. In new homes, we rarely have a problem since the whole house has taken 3 to 4 months to get ready to the point of putting down flooring.

The notion that you get moisture up through a slab, poured on top of the ground isn't true. This is different than a basement or something poured under grade, but that's a totally different situation and has nothing to to with this topic.

Eddie

 

[hilld]

Just an update, the builder is coming out again tomorrow to check every screw on the roof and the wall just to make sure. The moisture on the concrete floor is isolated to a few specific areas at the perimeter walls (only on 2 walls though). After heating the space for the last 6 days and nights, the moisture level in the building has dropped, even though it has been raining cats and dogs every day, except today.

I still think getting ventilation into the building is one of the keys in making this whole thing work, I will update you after the builder "makes his corrections"

Thanks,

Derek

 

[Clyde]

Hopefully for you, your contractor will do his inspection honestly and identify the source of your moisture. I mean that sincerely! Any mistakes on his part will be a huge cost to him and easily shuffled off. Right now I have my pole shed up without any concrete floor but I will install the thickest mil plastic I can find to isolate the concrete from the earth below. Best of luck to you - Clyde

 

[Parttimer]

Concrete is not water proof, it can act as a sponge if the soil below in contact with it is moist. This is the reason we pour on a well drainable base such as sand, stone. This minimizes capalary rise and contact with the concrete. The Vapor barrier thing is a long standing argument amoung designers, by the way the vapor barrier is usually torn or punctured during the pouring and leveling process or by the person who is in a hurry for it to set up. By using a higher compressive strength concrete say 6,000 to 8,000 or even higher it gets to be more water resistant, but special additives would be needed to achieve water proof. I can personally attest to this for in college we built a concrete canoe and rode in it. fun fun fun. There are products out there to increase the water wicking through the concrete if this is the case "DRY LOk" makes one but there are many. But I agree with the others in the case that the building is not correctly constructed for your environment. Listen to the dew point explanation again, he is correct. Blocking the vapor is the first attemp, but ventallation is the primary. I have investigated many mold problems in my area and they are mostly due to high moisture inside of a very tight house/building and poor ventallation above.

This site has such a wide range of people and ideas, it has got to be the single best site i have read yet.

 

[slowzuki]

1. Concrete wicks water from the soil, sort of, it is driven out of the soil because water wants to go from wetter to dryer areas (vapour pressure).

2. Air holds more water the warmer it gets.

3. Anywhere water/moisture can get too it needs to be able to get out off.

4. Moisture that has condensed stops contributing to vapour pressure, it has to evaporate again to do that.

So if you have warm air that has been allowed to pick up extra moisture in contact with cold surfaces you get condensation.

Kraft paper is only a so-so vapour barrier. Metal and I suspect the foil insulation is a perfect vapour barrier. moisture can move into the insulation and condense making a low vapour pressure area to continuously draw moisture out of your building.

You need to stop this by a better inside vapour barrier and someway to dry the insulation to the outside or,

Alteratively, you need to really control the humitity inside the building so your building pulls moisture out of the insulation. This would mean limiting influx of moisture by sealing the floor, air-exchanging and probably dehumidifying.

These are the two fundamental approaches although the 2nd isn't traditional in residential buildings.