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I just got a new shed built with concrete blocks and a metal roof last month and thing is dripping with condensation from the roof. There is absolutely no insulation in the shed.

How would I get rid of this, insulate the metal roof with spray foam or the normal kind? Plaster the inside and outside of the shed?

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Aircraft Hangar Rainstorm. Ugly in a damp climate like the Pacific Northwest. Ground fog meets cold sheet metal. –  Fiasco Labs Dec 12 '13 at 2:24
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2 Answers

up vote 5 down vote accepted

Because the roof is metal it will get cold and when the (relatively) warm damp air inside the shed comes into contact with it, condensation forms.

You need to make sure that there is a thermal barrier between the inside surface of the roof and the outside surface.

So any of the things you suggest would do, but might cause ventilation problems (say). So a false ceiling would probably work best.

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Thanks for the reply Chris. Could you expand on "You need to make sure that there is a thermal barrier between the inside surface of the roof and the outside surface." a tad more if you have time. –  Keith Donegan Nov 26 '10 at 18:02
    
@Keith - A thermal barrier something that doesn't transmit heat. So in this case it'll be insulation. Double glazing works by having a partial vacuum between the two panes of glass. –  ChrisF Nov 26 '10 at 19:23
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If there is adequate ventilation, then the inside and the outside will be at the same temperature. So add ventilation. (Screen the vents as necessary to keep out all intruders.) Ventilation will also help reduce the humidity inside the shed. I would want to add several vents through the roof, and several more down at the base to allow for good air flow.

It is possible no vapor barrier was used below the floor of the shed, which is one reason the interior is so humid. This is something difficult to change now. Of course, if the floor is made of dirt, then I'd add a vapor barrier, then a properly poured floor.

Next, use insulation on the underside of the roof. Spray foam is perfect here. Pick a day to do so when you can dry out the roof, possibly a fan may be necessary.

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