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Polyisocyanurate foam retrofit and rainscreen

johnhon | Posted in Energy Efficiency and Durability on

I have a question about a job I am starting in Amherst, MA. and I would like to hear others chime in about it. This is a whole house exterior renovation (built in 1982) The owner’s want to insulate and install new windows and siding.

Existing wall from inside to outside: 1/2″ drywall, 1″ polyiso., 2×6’s 16″ oc w/ fiberglass batt insul, 1/2″ asphalt coated ‘Beaverboard’ sheathing(OSB on corners), Tyvek ( which has deterriorated) and 1x cedar vertical siding.

I’ll be removing the siding and Tyvek and add 2″ polyiso. board (not taped), 7/16″ Zip wall taped, Homeslicker rain screen and red cedar shingle siding. (Note the cedar shingles requiring a good nailing base)

I’ve done a lot of research and realize that it’s not a good idea to add another vapor barrier on the outside of the wall cavity, but the owners are set on adding exterior insulation. I’m thinking of adding another vent screen under the 2″ polyiso? or switch to 2″ XPS ( a little more vapor permeable, but vapor would still have to pass through the taped Zip wall) or just go with the original plan and not worry about it. 😉

Any other ideas/feedback?

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  1. Greg | | #1

    You could use this product below and it will not crush like the yellow stuff and it cost half as much.

  2. Riversong | | #2

    You'll be making a costly mistake enclosing the wall framing in a sandwich of vapor impermeable materials. Unless the interior air/vapor barrier is perfect (and remains so) and the interior relative humidity is well-controlled (by both spot exhaust and whole-house ventilation), and the exterior cladding layers are perfectly weather-sealed and remain so for the life of the house, you're inviting serious moisture damage.

    Studies have demonstrated that, in the event of a small leakage of environmental moisture, a house with impermeable layers can take months or years to dry out. The exterior insulation will also raise the framing and sheathing temperatures to the perfect breeding conditions for mold and decay fungi.

    In a cold climate, the exterior "skin" of a house should ideally be at least 5 times as vapor permeable as the interior.

    By the way, much of my 30 years of building was in the Greenfield MA area as well as here in VT.

  3. ja9PtwChNE | | #3

    Where does "the exterior "skin" of a house should ideally be at least 5 times as vapor permeable as the interior" come from?

  4. Riversong | | #4

    From those who have been building highly efficient homes in cold climates for decades.

    Why do we pay a lot more for a Goretex parka than for a plastic raincoat? Because it breathes.
    Why do skin and leaves transpire moisture? Because it's essential to the maintenance of life.
    Our houses are our third skin. Like our biological and textile skins, they must breathe.

    This is an essential element of the Bau Biologie movement, which understands the relationship between shelter and biology.

  5. GBA Editor
    Martin Holladay | | #5

    The answer I give will reflect a different perspective than Robert's answer.

    In many cases it's OK to install exterior foam on an existing house with interior foam. But you have to use common sense and your observational skills when opening up the walls to determine whether the exterior foam will work.

    As you take apart the wall, look for signs of water damage or staining. If you see any such damage, it's time to re-think your plan. At that point you need to assess how the water got into the wall, and correct the existing problems.

    If the wall looks perfectly dry, go ahead and install exterior foam. Be sure it's thick enough for your climate; thick is better than thin. You're in climate zone 5, where a 2x6 wall needs a minimum of R-7.5 foam — so 2 in. of polyiso will be fine.

    Do an impeccable job of designing and installing exterior flashing at windows, doors, and penetrations.

    I'm not ready to endorse OSB siding over rigid foam, though. Has anyone out there done it? If so, how did the job turn out?

  6. johnhon | | #6

    Robert and Martin, thank you for your comments. The new addition to this project that I just finished framing has 2x6's 24"oc with Zip wall, 2" polyiso.,and another layer of Zip taped. I secured the last layer of Zip with 4" ext. deck screws 24" oc vert. and horiz. and it worked pretty well. The screws enable you to adjust the amount of compression, but I would only use the last layer of Zip with cedar shingle siding, as I needed a good nailing base. Otherwise for lap siding I would go with furring strips.

    I'm also going to attempt to keep the house under a negative interior pressure through the use of a new central Fantech exhaust fan for 3 baths, and an ERV for the whole house.

  7. Jesse Thompson | | #7


    We used ZIP sheathing over 2" of foam sheathing over 2x6 w/ dense packed cellulose on this project because of the shingle siding:

    We used steel strapping for lateral bracing instead of another layer of structural sheathing underneath the foam, the foam was attached directly to the studs. The builder never complained about working with the zip on the foam. The cedar breather is squirmy enough I bet it wasn't noticeable.

  8. Chris Harris | | #8

    I would love to talk to you more about the project you referenced. I am doing a similar house for my family and have some questions about detailing such a wall system. I took a look at the website. You guys are doing some beautiful work. If you are interested in chatting, please contact me at [email protected].

  9. Chris | | #9


    Can you direct me to where in the code it talks about needing "R 7.5" of ext. foam in Zone 5? I've looked in the state code and the 2009 IECC, but can't track it down.


  10. Chris | | #10

    "Table N1102.5.1 in the 2007 Supplement to the IRC"
    Never mind found the reference....

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