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I'm renovating a living room in my 1940 two-story home in Central, NY which involved adding insulation where there was previously none.

Before the drywall goes up I decided to run some electrical wiring for future recessed lights.

After pulling back the insulation, which I installed in September, I noticed this damp spot. The insulation was slightly damp on the surface as well. The dampness dried up by the time I was done running the wire.

Similar dampness was found in a stud bay about 10 feet to the left. When the stud bays were exposed during hard rain I never saw water infiltration. The siding is cedar shingles.

Is this just condensation because the internal envelope hasn't been sealed or is this indicative of other problems?

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MonkeyZeus
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  • It could be moisture from an ice damn that occurred in the winter months. The lack of proper insulation makes that a distinct possibility. Even if it's not the top floor, water could migrate down. – Steve Wellens Jan 10 '22 at 16:50
  • @SteveWellens I know it's hard to see but the dampness is limited to the red circled area. This face of my house has no gutters, only rake. Above this is a bedroom and above that is a full stand-up attic. – MonkeyZeus Jan 10 '22 at 16:58

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It's Winter in Central NY. You have no vapor barrier in place. Warm moist interior air cools and deposits water on the cold siding. Building Science 101.

"But I have Kraft paper backing" - if the detailing of the Kraft paper we can see is typical, you have nothing approximating a useful vapor barrier. Moist air will happily move through all the gaps.

If you can deal with the agony of redoing the trim, covering the whole face of the wall (studs and all) with a sheet of foam insulation (taped at the joints) will provide a real vapor barrier, more insulation, and cut down the thermal bridging due to the studs.

Ecnerwal
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  • Re-doing what trim? The door and window jambs? They stick past the studs about 7/8" because the walls were rock-lath. I admit I'm not a pro insulator but won't the drywall press tight enough against the studs to seal from bay to bay? – MonkeyZeus Jan 10 '22 at 15:58
  • Could I achieve a good barrier using a plastic sheet like they do for [unfaced insulation](https://d3fgmcoixbear.cloudfront.net/s3fs-public/vapor_retarders_-_resize550_0.jpg)? – MonkeyZeus Jan 10 '22 at 16:01
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    Yes, the door and window jambs, as it's typical to use an inch or more of insulation sheet. I guess if you have adequate space you could go down to 1/4" fanfold or 1/2" sheet and perhaps not have to redo them. And yes, Visqueen (6 mil poly vapor barrier) is the normal way to get a good seal with no additional insulation/space. Drywall is not a vapor barrier. – Ecnerwal Jan 10 '22 at 16:04
  • I had 3/8" and 1/2" drywall, mud, tape, and screws delivered two weeks ago and it is waiting for my drywall guy this weekend. Really not looking to delay things any further; my wife would possibly literally kill me. In general do you agree that the moisture issue is likely due to the lack of proper vapor barrier? – MonkeyZeus Jan 10 '22 at 16:08
  • I didn't mean to suggest that drywall is a vapor barrier but I was rather suggesting it would press against the kraft paper on the studs and create the seal. – MonkeyZeus Jan 10 '22 at 16:11
  • I think I said exactly that the issue is lack of proper vapor barrier in the first paragraph. I have doubts that the drywall is going to make magic happen with some of the kraft edges I can see. – Ecnerwal Jan 10 '22 at 16:11
  • I appreciate this answer and am thankful that my theory about vapor barrier isn't complete lunacy. To the Kraft paper's credit the moisture is limited to the upper portion where the insulation technique is sub-par. The moisture pattern is almost identical in the other stud bay that I mentioned in my post. I'll consider 6 mil poly but it seems like overkill, no? Would 2-4 mil suffice? – MonkeyZeus Jan 10 '22 at 16:52
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    6 mil is standard. Thinner might work, never tried it myself. – Ecnerwal Jan 10 '22 at 16:59
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I’ve recently changed my thinking about vapor barriers and compressing batt insulation on walls.

Installing a plastic sheeting vapor barrier (on inside of wall) in Canada or the most northern parts of the U.S. is probably ok. However, anywhere else it’s probably going to be a problem. Here’s an article that explains it:

https://www.energyvanguard.com/blog/75130/What-Happens-When-You-Put-a-Plastic-Vapor-Barrier-in-Your-Wall

Basically the study says moisture will get trapped in the wall and not be able to escape when seasons change and moisture starts moving the opposite direction. (Kraft paper seems to be fine.)

Likewise, I now think it’s ok to compress batt insulation in a wall. The critical thing is that the wall is full (no voids). If there’s a void then it can allow moisture to accumulate (like your wall) and leave a wet spot. Here’s an article about that:

https://www.energyvanguard.com/blog/compressed-fiberglass-insulation-really-so-bad

We vent attics because there’s a void above the insulation. However, we don’t vent walls because there’s no void.

Make sure you have no voids in your wall and eliminate the plastic sheeting. Without voids, the exterior sheathing, etc. becomes part of the thermal barrier.

Lee Sam
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  • In many cold-climate jurisdictions, suggesting that no 6mil poly vapor barrier is fine is not only a code violation but will cause huge amounts of mold and rot in a house with batt insulation. – RibaldEddie Jan 11 '22 at 04:49
  • Fundamentally this is about the movement of moisture-laden air through the wall and the plane against which warm and cool air meet. The idea that you can get sufficient density from batt insulation to prevent warm moist air moving through the wall to the cold dry air at the exterior of the building is a pipe dream. Spray foam can do that, dense-packed cellulose also, but not batts. – RibaldEddie Jan 11 '22 at 05:06
  • Mold will grow in the batts themselves. – RibaldEddie Jan 11 '22 at 05:10
  • @RibaldEddie If you read the OP you’ll see that there was no insulation in the wall (and no vapor barrier). This is not a location you’re describing “in many cold climate jurisdictions it’s a code violation” or they would have had a vapor barrier of some kind. Yes, as I said, “in Canada and very cold climates a plastic sheeting is fine.” Read the article and you’ll see why I’ve recently changed my mind about plastic sheeting. Did you read it or revert to your old thinking? – Lee Sam Jan 11 '22 at 06:23
  • I read the article. I’ve read that article years ago. The reason the OPs house was fine for many decades without a vapor barrier is that it had no insulation so the condensation plane was the exterior of the building. The requirements for vapor barriers came into effect at the same time that insulation requirements did as well. Anyway I have seen plenty of moldy insulation due to holes / gaps in vapor barriers. I also lived through the building envelope crisis in my city in the late 90s. – RibaldEddie Jan 11 '22 at 06:36
  • And if the OP is in climate zone 6, which includes central NY, then yes they can put poly on the interior side of the building. – RibaldEddie Jan 11 '22 at 06:47
  • @RibaldEddie You’re wrong and don’t understand the affect of what happens when the seasons change and moisture flows in the opposite direction and it gets trapped in the wall every year. Downvoting something you don’t understand can bite you back. – Lee Sam Jan 14 '22 at 22:20
  • you’ve got to be kidding me. https://www.nyserda.ny.gov/-/media/Files/EERP/Residential/Programs/build-better-guide.pdf – RibaldEddie Jan 15 '22 at 04:49
  • @RibaldEddie You need to reread your article. On page 35 it says to use retarders not barriers, and the chart clearly states poly barriers are to be used in most severe conditions. We all need to change with new information. – Lee Sam Jan 15 '22 at 05:59
  • climate zone 6, Central NY, is one such zone where poly barriers are appropriate. I agree that poly isn’t used much anymore given the popularity of spray foam. But if you’re using unfaced fiberglass batts, you need poly. With faced batts you can likely skip the poly but there’s no room for error with the paper or you’re going to run into problems. – RibaldEddie Jan 15 '22 at 06:10