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Community and Q&A

Exterior insulation retrofit suggestions?

Andy Saemann | Posted in Energy Efficiency and Durability on

I have purchased a house in Vermont and it needs new siding. This is the perfect opportunity to add more insulation!

The current wall construction from the inside to out is: 1/2″ gypsum, 4 or maybe 6 mil poly, 2×4 with R-11 fiberglass batts 16″ OC, 1/2″ plywood sheathing, some sort of building wrap, and vertical wood siding.

The reading I’ve done on GBA has made me very nervous about adding polyiso to the exterior of the wall, due to the presence of the poly on the interior. Can anyone suggest an alternative insulation retrofit to the exterior of the house? If it makes any difference I’m looking at cedar shingles for new siding, but am also considering Hardie Board.


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  1. GBA Editor
    Martin Holladay | | #1

    This question comes up a lot. Here's my standard answer:

    When your existing siding is being removed, carefully examine the sheathing for signs of any moisture problems. If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible.

    If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, do an excellent job with your new WRB and flashing details.

    If your wall sheathing shows moisture problems, your house may be a poor candidate for exterior foam. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

    Tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and these Canadian homes are not experiencing wholesale failures or problems.

  2. Aj Builder, Upstate NY Zone 6a | | #2

    Ridgid foam guys my way always use two offset well taped layers of foam. Martin, someone should link the roxul exterior alternative thread.

  3. GBA Editor
    Martin Holladay | | #3

    AJ's comments are sometimes cryptic, but I think he is advising you that mineral wool panels are more vapor-permeable than polyisocyanurate.

    Here is the article he is talking about: Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

  4. Andy Saemann | | #4

    Thanks for the responses. I tried to find them by searching the archives - honest! Must not have been searching the right keywords. Oh well. What do you recommend for a nailing surface for shingle siding?

  5. GBA Editor
    Martin Holladay | | #5

    There is no easy way to install cedar shingle siding over rigid foam. I know of only two ways to do it:

    1. Install a layer of plywood or OSB over the foam. (Some builders use nailbase instead of a layer of foam followed by a layer of OSB).

    2. Install horizontal 1x3 nailers, 5 inches on center, over the foam.

  6. Albert Rooks | | #6


    I also like the mineral wool option with a rainscreem overlay. Out west we have a shingle panel product called "shaker town". 8' strips of 3/8" plywood with prehung shingles on them. Expensive as I recall, but beautiful. This option would allow the panels to be hung on verticle furring strips.

  7. Albert Rooks | | #7

    By the way... In case anyone cares, we finally ordered the Heco therm screws shown in the thread: They will be here late December. We tried them further and they work great but are a bit expensive... They only make sense at 4" or more of exterior insulation. They make better sense the thicker you get.

  8. Aaron Vander Meulen | | #8

    These are shingles such as Albert mentioned. IIRC, the upfront cost was more expensive, with the offset of reduced labor costs to install. Also, remember if you chose horizontal furring you should cut scores/dado's to allow for air movement behind the shingles.

  9. GBA Editor
    Martin Holladay | | #9

    I disagree with your recommendation to cut kerfs in horizontal furring. That's a bunch of wasted labor. You'll get a tremendous amount of ventilation behind cedar shingles, and kerfs aren't going to add any useful benefits. Cedar shingles aren't airtight, and normal wind effects and changes in temperature will lead to regular air exchanges and drying.

  10. Albert Rooks | | #10


    As soon as I put away the computer after posting the about the shingle panels, I got a little nervous about warping on a rainscreen application. If one was to choose this method, I would check with the manufacturer to see if the recommend, or have experience with rainscreen applications. Pay close attention to a 4 sided finish if specified and use good ring shank nails.

    Thanks Aaron for suggesting a source. I'll leave the venting issue between you and MH.

  11. Aaron Vander Meulen | | #11

    Guess I don't consider gang cutting kerfs as a huge time investment. Depends on how big the project is I guess.

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