Metal Shed Construction (Condensation)
I have to build a 30 x 60 metal building for a customer (uninsulated). Building it with 6 x 6 post and horizontal purlins for my metal walls. Concrete floor. Roof will be 4/12 pitch with trusses on 2′ center. Will have purlins on roof on 2′ centers for metal roof. This building will basically be holding maintenance equipment for a golf course. My concern is with an uninsulated building and being fully enclosed is the condensation forming on the roof panels. I was thinking about installing a titanium vapor barrier over the trusses and then installing purlins over titanium then the metal roof. This way i will have ventaltion between the vapor barrier and metal roof panel, drawing air from the soffits to ridge vent. Leave opening at top of titanium venting the interior warm air out the ridge vent. Do you think this would fix any condenstion problems?? I just want to do this project right from the beginning. I know people build this type building without this vapor barrier all the time. Just don’t want to take any chances on them thinking they have a leaky roof. I have never had a whole lot of experience with this type of construction. Just wanted to get some thoughts from more experienced contractors on this subject. Thanks for any help.
Replies
What's the source of moisture that you're worried about?
I am sure at times in the winter, when they have to do maintenance in this building on their golf carts. They will probably run some type of heater. The lighting could put off some heat. The concrete could be putting off amounts of moisture, especially during the curing process. The building itself during the days where it just heats up on warm days and then at nights the temp. out side drops.
But no cooking, no bathing, only a few humans exhaling in a poorly-sealed building.
Heating alone is not a problem -- it will actually reduce the possibility of condensation.
So your anwser would be no need for any vapor barrier? Metal walls on purlins and metal roof on purlins?
Some types of heaters can introduce water vapor - like the torpedo propane heaters for instance. Otherwise vapor from combustion is vented out.
The concrete should be well drained from underneith, no water there.
If the air is humid outside, then it cools down - the outside will have condensation.
Here in the Pasific Northwest humidity in the air condences on the inside of metal buildings. The aluminum canopy on my pick-up rains inside. Weather reports include the "Dew Point". That's where temps and humidity reach the point of causing condesation. In a dry climate, it's not a problem most of the time. I would add a vapor barrier. Jim
How will a vapor barrier do any good?
It may not
do a whole lot of good, but the problem is definitely real. Condensation forms on the underside of metal roofs up here, and drips down on whatever's underneath. Worst cases I have seen are very low slope with metal installed directly over purlins. Building can be enclosed or open (happens in carports). I think closed is more likely to be a problem due to ground and/or slab moisture. A lot of metal buildings have "bag and sag" insulation, wide fiberglass with a plastic vapor barrier under it, rolled out over the purlins, with the metal laid directly over, and moisture finds its way up through seams in the plastic, condenses on the metal, and then forms ponds on top of the plastic. My guess is that rigid foam insulation might be enough to solve it, but maybe not. Here's a shot showing a pond on top of the plastic in an airplane hangar.
If the condensation is coming from outside air, and you install a vapor barrier and ventilate above it, there will still be outside air in contact with the metal roof. In addition, there will be outside air in contact with the vapor barrier, which will be nearly as cold as the roof. How is the "vapor barrier" an improvement?
The successful solution
would be one that keeps the bottom of the roof assembly (whether that's the roofing itself, a vapor barrier, a deck, or whatever) above the dewpoint.
A VB itself won't do this. In some cases the dew drops roll down along the top of the VB instead of dripping inside.
Question:
So, as I understand it, this moisture condensation is caused by the temperature drop both inside and outside with high water vapor content both inside and outside. So when the temperature everywhere drops below the dewpoint, dew forms on every physical object it can find. Or does it just form on the ceiling of buildings? If so, why only there?
Google
"night sky radiation"
Surfaces facing the sky on clear nights can be quite a bit colder than the air temperature. The roof gets colder than the rest and condensation forms on one or both sides of it.
Interestingly, the best fix for potential interior condensation on a metal roof is a "hot roof" of sorts -- one that has an insulating layer in direct contact with the roofing. The insulation prevents any radiative cooling from conducting down to the "ceiling" surface, and the lack of an air gap prevents condensation on the back of the metal.
The insulating layer doesn't have to be much, of course -- 3-5 degrees difference is all it takes. A layer of spray undercoating would likely work, building paper somehow laminated to the back of the metal, etc.
One more comment from someone else in the NW who knows about condensation in metal pole barns. We had one built not so long ago and the contractor quoted the building with the plastic covered insulation SO AS TO REDUCE condensation. Then he started pushing to use plywood sheathing under teh metal roofing instead of insulation. He wanted about $1000 for an up charge. I did not wish to pay this. He wished to do the plywood so badly he did for the cost of the material. I don't know if the insulation is a pai in the but for the guy or if if he did not want complaints about condensation (leaky roofs) or what.
Well, the plywood looks great making for a beefy looking building. No condensation. ANd I do know the insulation sags over time, and BIRDS LOVE TO TEAT IT OUT AND BUILD NESTS IN IT! Think about that mess.
Give the customer the option...
If you can present options and potential consequences to the customer, then they will understand what they are getting and what they are paying for.
I live in an area with relatively low hunidity (Central Montana), but extreme temperature fluctuations in winter. (60 above to -35 F) When the temps outside drop 50 degrees overnight, we get some condensation issues in uninsulated metal buildings. Most shops around here are built with just the frame and purlins, as you described. Some end up spraying foam on the underside of the metal, and others just live with the occasional "indoor rain."
Since you are already planning to have trusses at 2' on center (many will go with trusses at 4' on center with 2x6 purlins) you could easily just sheath the roof with ply or OSB. Then put a good synthetic underlayment (Titanium or similar) and screw the metal to that. Makes a very solid roof structure and any condensation on the underside of the metal drains down the underlayment and out the eves. Occasional condensation on the walls doesn't present as much of a problem, at least in my climate where everything gets dried out pretty quickly.
Give the customer the option, and explain how condensation works. Then they can make an informed decision...
That why i thought it might be best to set trusses and then titanium overtrusses, then purlins, then metal roof. Cold air would still be over titanium but atleast warm air will not come in direct contact with metal 1st. Figured the space above the titanium would have some sort of chimney effect between the soffits and ridge vent and would cause the condensation to be less and would dry faster (more air flow acrossThe bottom of metal roof). What did condensate on roof panel would drip on titanium and run down to soffit, instead of dripping on floor.
Give the customer the option...
If you can present options and potential consequences to the customer, then they will understand what they are getting and what they are paying for.
I live in an area with relatively low hunidity (Central Montana), but extreme temperature fluctuations in winter. (60 above to -35 F) When the temps outside drop 50 degrees overnight, we get some condensation issues in uninsulated metal buildings. Most shops around here are built with just the frame and purlins, as you described. Some end up spraying foam on the underside of the metal, and others just live with the occasional "indoor rain."
Since you are already planning to have trusses at 2' on center (many will go with trusses at 4' on center with 2x6 purlins) you could easily just sheath the roof with ply or OSB. Then put a good synthetic underlayment (Titanium or similar) and screw the metal to that. Makes a very solid roof structure and any condensation on the underside of the metal drains down the underlayment and out the eves. Occasional condensation on the walls doesn't present as much of a problem, at least in my climate where everything gets dried out pretty quickly.
Give the customer the option, and explain how condensation works. Then they can make an informed decision...