Metal roof--condensation or leakage?
This is not so much a green building question as a building science question. I know a lot of you are quite fluent in this area and would appreciate your comments.
I was asked to look at a building with a "roof leak". It is a slab-on-grade metal building, a bolt-together red iron frame with metal siding and roofing, and "metal building insulation" behind the siding and under the roofing (wide, rolled fiberglass between two sheets of heavy, white plastic sheeting). It is a very large building, roughly 50' x 250' and has a very low-slope roof, at least for metal, about 1:12. The metal roofing is a high rib profile and has no pipe penetrations or obvious openings. It does have some dried-out sealant along the ridge cap, but it didn't strike me as a big deal.
The first bay of the building I looked at had water dripping on the floor, but that wasn't the only area of the insulation holding water. The obvious drip was near the ridge and also near the exterior wall. Another area of water (not yet dripping) is along the opposite wall a little further from the ridge. OK... so maybe some water blows in at the ridge cap and runs down along the top of the plastic until it finds an opening, and then ponds up on the lower plastic sheet, soaking the insulation.
Later, the user of another bay mentioned that he also had water on the floor. This one is near the bottom of the roof slope. There are no penetrations anywhere near this. The gutters are clean and working. There are two other "ponds", one also near the gutter, and a second one up higher, next to an exterior wall.
The entire building is unheated. Yesterday the dew point was about 32F inside, and the air temp was close to 40F. No condensation on anything. No sources of moisture in the building at all, aside from the slab itself and an area of concrete wall that is below-grade outside the building. In wintertime we get overnight temps in the 30s and the sky is sometimes clear.
Thanks for reading this far. Any thoughts?
Posted Tue, 02/01/2011 - 08:39
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