Sweating metal building

[EddieWalker]

Derek,

No venting of any kind would definetly lead to a green house effect!!! You have air at one tempature surrounded by air at another tempature. This will creat condensation on your walls.

Knowing this, it sounds like the cause of your moisture to me.

Condensation is formed when the water vapor that is held in the air reaches a point of 100 percent and turns from vapor to liquid. The air holds a certain level of moisture at all times. The warmer the weather, the more moisture in the air. When that tempature drops, the amount of water vapor that the air holds decreases. This is also how fog is created.

Not know how your insulation is installed, nor how your rafter work makes this hard to solve over the net.

How tight is your vapor barrier? Is there anyway to seal it up better to keep the air in your shop seperate from the air in your insulation?

This is where you problem is most likely occuring. Where the outside air makes conctact with the different tempature air on the inside through the metal of your roof.

Heating the inside air will allow you to hold more moisture in the air and stop the condensation on the inside, but it's an expensive battle going about it that way.

Venting the air above your insulation and sealing off your vapor barrier better seems like the solution to me with what I know so far.

Can you remove a section of the peak and replace it with a vented peak?

Can you create air pockets between the insulation and the roof to circulate air and vent it outside without affecting the integrity of the insulation?

Eddie

 

[TwinWillows]

Did you lap & tape all the seams where the double bubble foil sheets overlapped? That's an important step & is needed to keep the air off the underside of the metal roofing.

 

[Cord]

</font><font color="blue" class="small">( The vapor barrier is on the warm side of the house, give me a little credit here. )</font>

Actually, I came to that conclusion because of a earlier statement:

</font><font color="blue" class="small">( There is a vapor barrier (double bubble foil) against the roof, followed by unfaced R-21 (total R-30) insulation on the ceiling. )</font>

In your climate, primarily a heating climate, the vapor barrier should go on the warm side, which in this case would be behind the drywall, not against the metal. The dew point will occur in the insulation which is why it's wet. The building it's self isn't putting off the moisture, it's just that the moisture is condensing on the building. This often happens in the springtime in unheated buildings so it's only temporary. The long term problem is the wet insulation. The batt will hold the water for months. Yes, air flow and heat will help solve the moisture problem (but it won't fix the vapor barrier). To answer your other question, I'm really not sure why you are putting such a fancy heating system (heat pumps and heat recovery gear), when a regular force air furnace with some make up air will do the job for much less $$$.

 

[inveresk]

Condensation is a consequence of a number of factors.

1 The lower the temperature the greater the risk that any moisture vapour in the air will hit 'dew point' and condense out. Hence we see some things in winter that we don't in summer - dew on the grass, condensation on cars and insides of windows, etc. When it gets really cold, things can drip. This was one of the reasons (strength for ice breaking being another) that the ship "Discovery" that was custom designed to take Scott to the Antarctic was made with a double timber hull with the cavity filled with rocksalt. It was also designed with an open bridge - no sissies on the high seas in those days!!

2 If you have a sealed building without ventilation, the moisture vapour will be trapped inside and condense out on cold, inner surfaces when the temperature gets low enough. Breathable structures help and there are "breathing wall' buildings now being designed which have no vapour barrier. They work on a dynamic insulation principal which allows warm air suffusing through the walls to partially heat incoming air doing the same thing. Liken it, if you will, to wearing cotton clothes and a string vest as opposed to oilskins. With or without a breathing wall, ventilation certainly helps.

3 The purpose of the vapour barrier is to prevent the movement of moisture from one area to another, particularly from within wet rooms to within the structure. That's why the barrier is always on the warm side of the insulation - it's within the insulation, or just beyond it, that are generally the likely dew points in a structure. If the vapour barrier isn't entirely sealed, moisture laden air in bathrooms, laundry rooms, etc. can get into the wall or roof structure and so increase interstitial condensation. Current practice is to have a vapour barrier on the inside and a breathable membrane under the cladding which repels water from the outside but allows any trapped moisture within the structure to diffuse to the outside.

4 Heating a building moves the dew point within the structure. The higher the temperature, the more you'll move the dew point towards the outside. That's why in a cold car your breath will cause condensation on the inside face of the windscreen. Turn up the heating and the dew point moves outside and so you don't get condensation on the inside of the screen any more.

Some buildings like swimming pools pose particular problems and so need specialised solutions. At other times and places, the climatic conditions can be almost reversed to those we experience in the north. An example might be a very hot and humid climate where outside temperatures are higher than inside and where humidity outside can hit 100%.

The ideal is to design a structure that is forgiving in all inside and outside conditions. Warm roofs (where the insulation is placed outside the structure) is a good solution for this and means you don't need to ventilate the rafter void. The same can be done in walls. I'm currently working on a remodeling project for an office building in Vancouver where there's some evidence of moisture damage within the structure. When I ran a few models through the computer, calculations showed that by adding 2" semi-rigid mineral wool slab to the outside then overcladding with metal (with a ventilation air gap behind), we move the dew point outside the structural area where damage is currently a problem to a point which is at the other side of the breathable membrane.

With your building you want to solve with ventilation and/or some space heating, not by altering the construction. However, the ideal situation is to run a few construction models through the computer at design stage. That way, it's easier to tell if there's likely to be a problem or not.

 

[hilld]

I can't look at the foil anymore, it has been covered up, but if I recall, when I checked, I had to pry the layers apart, there was tape in there.

I cannot ventilate above the insulation as the insulation is attached to the metal so to speak, the only ventilation I can do is to ventilate the entire building to replace the air in the building.

 

[hilld]

This is the confirmation I was really looking for. All my research has let me to a efficient ventilation system combined with a climate control system for the building. I cannot change the way the building was constructed (you would assume a builder that builds these buildings all the time would know the "gotchas" of this type of construction). Given the fact that I hired a builder to build me one of these buildings knowing what I was going to be using the building for. He has never encountered a building that has this much moisture in in, however, many of the buildings he builds are not sheetrocked and are used for commercial warehouses that probably have some sort of ventilation.

After hooking up the air handler and auxillary heat last night, I noticed a dryer (actually feeling - not measured with a moisture meter) building and will it run the rest of the day before going back. I will crack a window to facilitate some sort of air exchange. Going forward, I will install an air exchanger system of some sort (still researching) and will update you on the progress.

The problem I am having is almost identical to others having a damp basement in their houses and the solution there are to install an air ventilation system - at least that is why I read and I did stay at a Holiday Inn Express last night. /forums/images/graemlins/grin.gif

Derek

 

[Cord]

If you building was built buy a builder that was responsible for the entire project, then **he built a defective building**. He needs to come back, remove the drywall, remove the insulation and remove the incorrectly installed vapor barrier. He can then reinsulate with dry insulation, correctly install the vapor barrier and rerock. All of this would be done at his cost.

 

[hilld]

The builder built the metal building, poured the slab and did the roof insulation. He did provide the kraft faced batts for the wall, but did not install it. He did not rock the building.

Derek

 

[Cord]

That's unfortinate. Ok, he needs to fix his error in the roof construction. You will then need to fix the walls. Sorry for the bad news.

 

[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.