Sweating metal building

EddieWalker · Mar 31, 2006

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 · Mar 31, 2006

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 · Mar 31, 2006

( The vapor barrier is on the warm side of the house, give me a little credit here. )

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

( There is a vapor barrier (double bubble foil) against the roof, followed by unfaced R-21 (total R-30) insulation on the ceiling. )

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 $$$.

hilld · Mar 31, 2006

The reason for a heat pump is that we need heating and cooling. Gas is not available where I live and I just got done ripping out the underground oil tank (it was leaking - what a nightmare). I looked at an electric furnace with an A/C system, the cost was actually higher for the parts, the cost to operate was much higher with the furnace-A/C combination rather than the high efficiency heatpump. I managed to purchase a 2.5T 12 Seer Rheem system for $1300 + shipping on ebay and it is brand new. It won't meet the new federal regs (min 13 seer) that kicked in in January, hence it was discounted.

The other reason for the "fancy system" is that the building will be used for commercial purposes (photography studio) and the lighting system generates a fair amount of heat and I don't want my subjects sweating lots (unless they are swimsuit models or even nude subjects). Other items that also don't like the heat are food product shots, don't want the stuff to melt.

The ceiling is a special case. The foil insulation gets installed by the builder before the metal is attached, as a matter of fact the foil is attached to the metal with double sided sticky tape. The rest of the ceiling insulation was installed by an insulation contractor, they used the unfaced stuff and covered it with FS paper (they assured me that this is the way it is done), I insulated the walls with the Kraft faced paper before the sheetrock was installed.

This is my first venture in metal buildings, guess they are different enough from conventional construction as far as how they react to climate changes.

I really appreciate all the advice given so far, however, I cannot make up for the lack of a vapor barrier in the concrete, I just have to come up with a solution to address the shortcoming of the construction. It also doesn't help that we have had twice the normal rainfall for this time of the year.

I will try to take some pictures in the next few days to show the building.

Thanks for all your input.

Derek

inveresk · Mar 31, 2006

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 · Mar 31, 2006

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 · Mar 31, 2006

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 · Mar 31, 2006

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 · Mar 31, 2006

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