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| This is what is happening – warm moist air from inside of your building is rising. When it contacts the underside of the cooler roof steel, it is condensing. Over time, not only will the purlins develop unsightly water streaks, but there is also the potential for rot. It is very possible for moisture to collect around the shanks of the roof screws (or, hopefully not, nails) and rust them, causing leaks. Many steel manufacturers will not warranty their products as roofing, unless installed over an insulated vapor barrier.
Ventilation alone, is not going to solve the problem. Even a roof only building, will have the same issues.
How to solve from where you are now….
Least expensive, would be to place an insulated vapor barrier on the underside of the roof purlins. In order for this to work, it requires all of the seams to be tight. I’d recommend a product which has tabs with adhesive pull strips, rather than square edges which must be taped. Look at www.buyreflectiveinsulation.com for more ideas.
As you say the roof is nailed on – removal of it and trying to reuse the steel panels is not going to happen.
Spray foam is probably going to prove to be less expensive than a new roof.
Sad to say this….but for probably about two bits per square foot of roof surface, at time of construction, we wouldn’t be having this conversation.
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Pole Barn Sweating
