Retrofit Vapor Barrier from the exterior?

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  #1  
Old 03-19-18, 04:45 PM
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Retrofit Vapor Barrier from the exterior?

Hi folks, I am wondering if anyone has some words of wisdom concerning, installing a vapor barrier in a home from the exterior. Here is the situation. This home was built in 1991 in a northern climate (Montana). WE noticed this past summer that the siding on the north wall was warping/waving on a two story field of siding. We just removed the siding for replacement and found the underlying sheathing was showing water damage, warped and registered as wet by meter. This section of exterior sheathing is right where 2 stacked bathrooms are in the house. We removed the sheathing, and fiberglass insulation (soaking wet) and found that there was no vapor barrier installed under the interior gypsum board, which is code and common practice in this region. -Ironically their are 6 mil. vapor barriers installed on both sides of the bathrooms (bedrooms). We need to install a vapor and I would prefer not to have to remove the interior gypsum board and Bath/shower Unit to do it. The wall is a double wall construction (2x4 interior, 2x6 exterior aligned @ 24" O.C.) Here were my thoughts.
1. Over cut strips 28" wide strips (or so) of 12 mil vapor barrier, shove them into the stud bays against the interior gypsum board and nail/staple off the flanges to the studs, and then insulate the cavity with a compressed BIB insulation system to keep the VB flush and secure against interior gyp board.
2. Spay 2" or so of closed cell foam insulation into the interior wall cavity covering the sides of the studs and the gyp. Board. Again finish off an R-23 or so with a BIB's system.
Like any of those options or have you got a better solution?

BTW Yes I am installing new high capacity exhaust fans, on humidistats, for each bathroom!

Thanks in advance
 
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  #2  
Old 03-19-18, 08:32 PM
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Although vapor barriers are required in some regions those writing the codes need to understand that air leakage is a much greater mechanism for transporting moisture.
https://www.energyvanguard.com/blog/...rrier-Probably

And here is another related article: What?s More Important, Air-Sealing or Insulation? | GreenBuildingAdvisor.com

I suspect all of that moisture came from interior air seeping into the wall cavity and finding a cool enough area to form condensation, thus review the links above.

You didn't mention what the siding was and what was used for a house wrap. In a cold climate you want the vapor retarder on the inside but also want the assembly to be able to dry to the outside.

I'll let you read.

Bud
 
  #3  
Old 03-20-18, 07:19 AM
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In Montana you do need a vapor barrier on the inside face (warm side) of the wall and ceiling. As an architect that specializes in rehabbing/updating/remodeling affordable housing units as well as homes, w/ 25 years experience. I have seen first hand, dozens of times, the problem of condensing moisture when warm air hits sub 32 degree exterior building material. Even here in western Montana where the climate is considered arid it is a major problem both in moisture and mold. I have seen projects that are only 2-3 years old with soaked insulation and major mold problems when a vapor barrier was not installed properly. Venting, both natural and mechanical, is also a major component in battling condensation, but with out both you might as well kiss the livability of a home good by.

For the record: The exterior siding was pre-finished composite wood siding over Tyvek house wrap (although it was in rough shape in the area of moisture problem over 1/2" OSB sheathing. Inside the sheathing was an inch of bead board loosely installed (most of the boards had fallen loose and were at the bottom of the wall cavity. There was also R-19 fiberglass batts shoved in the 8 1/2" wall cavity, soaking wet when we opened up the wall, but dry in the area where the vapor barrier was installed properly.
 
  #4  
Old 03-20-18, 10:32 AM
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The first step is to decide where that moisture came from, my guess is air leakage because as the article says, very little moisture moves through painted drywall. In a 1991 house air sealing would not have been as high on the list as it is today. Combine that with the 2x4 mated to 2x6 wall structure and you have the potential for extra horizontal air movement. If those members were glued together then not a problem, but still other paths.

Your two options, the plastic or the closed cell foam will both work, if you carefully detail every seam, penetration, and electrical box (and more). Totally eliminating all inside air from reaching those cavities and allowing them to dry to the outside should resolve most of the problem.

I avoided saying "all of the problem" as there can still be positive pressures inside the house trying to make things worse. If forced hot air be sure there are sufficient returns especially in those bathrooms.

I would also be concerned, if I read it correctly, with the wall cavities not being completely filled, just invites air circulation is each space. I also prefer Roxul over fiberglass.

Bud
 
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