Yesterday it was raining (slight overexaggeration of course) :-) inside my garage... Hood of the car was covered with nasty orange water spots. It's an attached garage. There's no garage "ceiling" (or attic above) as such, it goes all the way up to the roof. The water was primarily coming from the very peak, but you could see water running partway down the slope. There's fiberglass insulation attached to the inside of the roof and the backing of the stuff up near the peak appears to be quite wet. It's worse over by the outside/cold wall, not bad at all over by the inside/warm wall.

I figured I had a bad roof leak that just finally soaked through (actually hadn't rained in the preceding 24 hours, but before that, we did have a nice soaking all day rain. Was unable to see anything that looked bad at all, so I called out one of the more highly regarded local roofers.

He spent some time looking around. His comment was that the roof doesn't look very old at all (I've only been there a year, so I don't know for sure) and is still in very good shape. He also mentioned that if it was leaking, I should have seen it within an hour or 2 of the rain starting (not 24 hours after it stopped). We did have a very dramatic temperature swing from 20 degrees up to 70 degrees over the last couple of days though. His thought (based on prior experiences with similar setups) was that it was condensation from having the insulation attached to the roof itself. I did not have (or did not notice) the same problem last year, but we didn't have any crazy temperature swings like this either.

So, the plan is to try to find a ladder long enough to get up there and see what I can see. Also, to monitor it during the next rain just to make sure I don't have some freak huge leak in a perfect looking, probably relatively new roof. :-)

If it truly is the insulation/condensation issue, then how do I go about insulating this garage space? My water meter is in there, so I don't want it to get too cold (especially since I'm on a vented crawl and we can see -15 to -20 degrees Fahrenheit in the Winter).

  • The issue is to stop the warm moist air hitting the cold external wall. I would have thought the insulation would have done that. Can you post a picture?
    – ChrisF
    Mar 18, 2011 at 16:54
  • Unfortunately, I don't own a camera. However, I can say that the insulation installation is far from ideal. There are gaps all over the place, especially so up at the peak. I'd like to tear it all down and re-do it, but am concerned that if it doesn't solve the problem that I'll just be covering it up and increasing my chances of major mold issues in the future. Mar 18, 2011 at 17:23

1 Answer 1


Based on your comment I think what you will need to do is replace the insulation.

As I said in my comment condensation is caused when warm moist air hits a cold surface, be it a window or roof. If your insulation was sound then this would prevent this happening, but it sounds like it letting the air through to the roof.

I'm reluctant to suggest what to replace it with as I haven't seen the roof - so it might be worth getting someone in to quote for the job and ask them what they intend to do.

If you do decide to replace then putting a semi-permeable membrane up as well will help keep the moisture off the cold surfaces.

  • The local guy I usually turn to for assistance when I'm out of my comfort zone is tied up in a big project for the next month or so. I'll plan on monitoring it closely for the time being, and assuming no major untoward developments, I'll hit him up for help when he's free. Mar 18, 2011 at 17:42

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