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I'm in the UK.

My garage has a corrugated metal roof with no insulation. In the winter, when it rains the underside of the roof drips. Not massively, but just enough to be annoying.

The roof appears to be in good condition and not old, so suspect this is condensation. Although I can't rule out a small leak.

As I do not have strong DIY skills I have been looking for a solution which is within my means. I have settled on staple-gunning sheets of aluminium bubble insulation membrane into the beams, spanning the distance between two beams, as close as possible to the underside of the roof.

aluminium bubble insulation membrane aluminium bubble insulation membrane

  • Is this a reasonable solution or will it cause more problems?
  • I'm concerned that covering the whole roof tightly will trap damp above the bubble-wrap, causing mould on the beams.
    Or if I leave gaps for air to circulate above the insulation then that defeats the point.
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  • I'm happy to give this a try and monitor it over the winter to watch out for damp collecting. – Swesus May 25 '20 at 11:09
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    Where is the water the collects on the aluminum sheets going to run down to? I assume the roof is pitched/slanted? – SteveSh Jun 2 '20 at 10:55
  • Condensation is a result of temperature differences between two sides, and could happen even if it was not raining. Does it happen when it is cold outside and not raining ? If it only gets wets when it rains then it would suggest a leak. I would not add the insulation until you have figured out the cause for sure. – Alaska Man Jun 2 '20 at 15:35
  • @SteveSh I would call it a Pent roof. I.e. it is slanted in a single direction. I would need to attach the aluminium at an angle so that the condensation will collect down in one corner. Maybe a bucket at that point? – Swesus Jun 2 '20 at 17:00
  • @AlaskaMan I've run the hose over it for 10 mins and there's no sign of wet inside. I did spot a hairline crack in the metal letting some light in, but it is much too small to justify the amount of water I was getting last winter. So I'll just put some sealant over that. – Swesus Jun 2 '20 at 18:40
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+50

That material looks very similar to what I used to cure a condensation problem with my air ducts. I live in a humid area and the attic is not insulated from the outside. So using the a/c would cause condensation to form on the ductwork and drip on the drywall below. Your situation sounds similar. If you line the metal roof with that material, it should reduce the amount of moist, inside air coming into contact with the cold roofing (where it sounds like it is condensing).

I'm not sure how easy it'll be to apply a flexible material like that to the underside of a corrugated roof, however. You might want to look for a hard foam instead. As for the mold concern, if you seal all the edges of the insulation with tape (use the silver metallic kind used for HVAC), that should prevent air exchange. The temperature between the underside of the roof and the roof-side of the insulation will be less extreme than the inside of the garage or the exterior of the roof and less likely to produce condensation. That said, it might still be a good idea to inspect it occassional. And all of this is assuming that you really do have a condensation problem and not just a leaky roof. - Good luck.

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