Condensation on Vapor Barrier


Old 10-14-05, 03:19 PM
Thread Starter
Join Date: Oct 2005
Posts: 13
Condensation on Vapor Barrier

Hi Everybody,

I just insulated a new 6-inch exterior wall with unfaced fiberglass bats. The wall sheathing is Ĺ-in plywood, then tarpaper then cedar clapboards. On the inside, I put up a vapor barrier with 6-mil poly. I sealed it all around with sealant. I did this on Wednesday night. Thursday morning the building inspector came and complimented me on my job. Today (Friday) I see that I have quite a bit of condensation on the interior (inside the wall cavity) of the vapor barrier. The fiberglass batting is wet as well. I canít figure this out. It was raining the day I sealed it so the air inside was humid. Could that be the issue? No actual rain got into my wall.

The wall in question is south-facing. There are east-facing and north-facing walls that were also insulated at this time and they do not have any condensation. I take it that the sun heated up the south-facing wall and caused this.

Thanks for any advise.

Steve Prescott
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Old 10-14-05, 06:06 PM
Join Date: Mar 2005
Location: USA
Posts: 6,126
Condensation on Vapor Barrier

As little as 1/2% to 1% moisture can cut the insulation value of fiberglas by 50%.

You could have had a lot of moisture in the fiberglas when you installed it. Same for the studs. With the recent temperature and moisture swings a lot of thing could have happened.

Best to dry it out some way.

Old 10-15-05, 08:05 AM
Thread Starter
Join Date: Oct 2005
Posts: 13

Thanks for the ideas. We decided to go with fiberglass because we could not find a foam insulation contractor who was willing to do the work. The day I finished insulating we received a written proposal from one firm I had contacted. I am now inclined to tear out the fiberglass and get them to come and install foam.

As you say, it could have been a lot of things. The problem is that, not knowing the exact cause, we cannot know for sure that whatever caused it will not recur after we have closed up the walls.

Thanks again. This is a good forum.


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