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Condensation

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Guest User
5/1/2005
We have a 14x70 mobile home and about 10 years ago had someone build a new roof over it. The new roof consists of 2x6 rafters, 2x4 purlins on top of that, then ribbed metal roofing on top of that. In the attic space there is fiberglass insulation layed on top of the old mobile home roof. There is plenty of ventalation consisting of soffit vents, ridge venting, and gable vents. On cold frosty mornings you will get a layer of frost under the metal roofing. Once the sun comes out and warms the roof the frost melts and drips in the attic. It doesn't seem to drip tremendously bad or soak the insulation. Perhaps alot of it evaporates before it has a chance to drip. Whats interesting is when the roof is covered with snow, there is no condensation, no frost develops under the metal roofing. Where we live, our roof is covered with snow most of the winter. Also there is no condensation problem during the warm months. Could you explain to me why this is so? Particularly why a layer of snow on the roof prevents any condensation problems. Also could you offer any solutions? I was told that the metal should have been put down over some plywood sheathing with black felt paper over the sheathing. If that would of helped can you explain why that would help? I know alot of the posts say that many metal roofs should be put over plywood sheathing, but they never make clear whether that helps prevent the condensation or is that just a structural recommendation? Like I said the problem doesn't seem to be causing any major moisture problems, but its just a little irritating knowing that its not right. Please forgive me if my questions are repeating what every one else is asking, but I just can't seem to understand the why's of the problem and what possibly could be done to correct. Thanks
Todd Miller
Isaiah Industries, Inc.
5/1/2005
This is the result of warm moist air hitting a cool surface and condensing and then freezing in this case. Reducing the warm moist air will help but that is done through things that you have already done -- ventilation and insulation on top of the ceilings. The reason that snow helps is that the snow actually acts as an insulator and makes the roofing a bit warmer than it could be if it was exposed to the outside air.
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