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Exterior Insulation Retrofit Vapor Drive Questions

sb1616ne | Posted in General Questions on


I am in the middle of a renovation and addition on my own home in Northern New England and have a few questions on exterior insulation retrofits regarding vapor transmission and wall drying.

We have a good amount of exterior wall area where we will plan to keep the existing interior of the home intact and not touch much of the sheetrock, interior trim, etc. My goal is too add exterior insulation on these areas as they are just 2×4’s with fiberglass. Currently the assembly is: 1 layer of 1/2″ drywall then unfaced fiberglass batts, 3/4″ t&G 1×8 pine sheeting, tar paper, then pine clapboards.

The previous owner neglected the house to the point I plan to re-side a good portion of it rather than dealing with the weathered, pealing and cupped pine clapboards.  These areas do not have many windows or doors so my plan is to build some sort of screw on window buck when we replace windows and re-side to add roughly 4″ of continuous insulation to the exterior. 

My plan is to tear off the clapboards and use (2) 2″ layers of foam(4″ total), then strapping and new siding.  I have tossed around the idea of a Larsen truss sort of setup with dense pack cellulose, but could be alot more work than foam to do it right. 

My question comes on what would folks recommend for adding vapor barriers or exterior WRB for this assembly. I am at a loss on the correct way to do this if we keep the existing interior in wallboard up as most recommend a vapor permeable air barrier on the inside such as Pro Clima product.

I know the perm rating of 4″ of foam is basically zero, but also, the 4″ of foam should ideally keep the sheathing from never condensing.  

I am fairly familiar with most of the modern published information on the ideal assemblies and have been reading through the new Pretty Good House book as well as the 475 E books, but dealing with a 1/2 baked renovation(such as leaving the drywall up) has be scratching my head.

Oh, another option could be rip off the exterior sheathing from the outside and have it spray foamed. I would not have a thermal break due to keeping the studs but this house is never going to be perfect. The studs are true dimension 2×4 so the spray foam crew could get at least 3″ of foam(r21?). Or I could nail on furring strips either vertically to get a cavity for more foam thickness. This could be much faster than exterior foam board!

Any input would be great! 


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  1. maine_tyler | | #1

    Have you given this a read:

    You could also try searching for 'deep energy retrofits' in which it's fairly common to add exterior insulation without disturbing the interior.

  2. sb1616ne | | #2

    Thanks for sharing that, it was a good read! I trust those guys so it seems like I can safely get away with leaving the interior materials as is as long as an exterior vapor open house wrap is used between the foam and the house. It got me thinking is something like this would help:

  3. maine_tyler | | #3

    I believe the gap between sheathing and insulation is a prescription for when there is too little drying potential to the interior.

    From a link on that page ( ):

    "The 2012 IRC sets the minimum levels for continuous insulation to control condensation when not using a Class I or Class II vapor retarder. These are two very different requirements."

    Another good bsi read is 'confusion about diffusion'

    In short. If you're putting 4 inches of foam outboard of a 2x4 wall you needn't worry about your interior' lack of a Class I or II vapor retarder.

    1. sb1616ne | | #4

      Thanks for chiming in here!

  4. brendanalbano | | #5

    Your proposed wall assembly sounds fine. Just make sure you decide where you want to put your wrb and which layer will be your air barrier. Below are two options. Usually the best way to decide on where to put the WRB is to determine which location makes your window details simpler. Option 1 is probably easier if you are going to install the windows with their flanges on the original sheathing (innies or in-betweenies, depending on what you are comparing to). Option 2 is probably easier if you are going to use window bucks and install the window flanges outside of the the foam (outies).

    In either case, the parts of the wall interior to the foam will dry to the interior, and the thickness of the foam means that you are okay with a class III vapor retarder (latex paint).

    OPTION 1:

    1/2″ drywall (latex paint is a Class III Vapor Retarder)
    unfaced fiberglass batts
    3/4″ t&G 1×8 pine sheeting
    WRB (Detail WRB as air barrier, easier to do with a self-adhered wrb)
    4" Foam

    OPTION 2:

    1/2″ drywall (latex paint is a Class III Vapor Retarder)
    unfaced fiberglass batts
    3/4″ t&G 1×8 pine sheeting
    4" Foam (tape seems, detail the foam as the air barrier)

    1. sb1616ne | | #6

      Thanks for the reply! I would like to keep the windows outboard if possible. Have you ever seen a good solution for a screw on window buck that does not reduce the rough opening size of an existing window opening? With 4" of foam and 3/4" strapping I would need to push the windows out 4-3/4". Would anything be stopping me from building a box out of 2x lumber and screwing it with say 8" screws into the existing window opening? Essentially an exterior extension jamb that would support the window. Then the WRB could lap from the existing sheathing out to this for a nice continuous barrier.


      1. brendanalbano | | #7

        Building a buck from 2x lumber and attaching it to the wall seems reasonable. Hopefully someone will chime in with their favorite way to do it!

        Attaching it with angle clips is another option. Not sure whether that is better than your long screws idea.

        Regarding how far to push out the windows, you might want to draw out the detail and see if you want to push the window out 4" or 4 3/4". I generally align the window with face of foam (4" in your case), but I've seen folks do it both ways. Just changes the details a little.

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