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Community and Q&A

Insulating walls with no sheathing

maeegg | Posted in General Questions on

My 1899 – 2 story house has T&G exterior siding (5/4” x 8” novelty pine) nailed directly to studs (no sheathing). And no ability to drain – no weep holes. Most important is the dimensions of the studs – they used 3”x3” (actual measured dimensions).


Reading articles on how to insulate this type of wall I have come up with the following plan


·     Create a ¾” air space using nailing sticks in the corners of each stud bay, tacked to the exterior up against the siding.

·     Apply 2” polyisocyanurate into each bay R-12. Using adhesive to attach it to the nailing sticks. The adhesive would be Dow INSTA STIK™ Quik Set Commercial Roofing Adhesive 

·     Using same adhesive attach 1” polyisocyanurate (4×8 sheets) across the entire wall to create a thermal break and add another R-6. 


It seems R-18 is the best I can achieve with a stud cavity that is only 3” deep and very roughly 24” on center.


Two articles that where most helpful

1. Insulating walls in an old house with no sheathing

2. All about rainscreens



·     Does the Polyiso end up functioning as an exterior air barrier and a substitute for the missing WRB?

·     Are the lack of weep holes a serious problem, we get about 48” of rain and 47” of snow a year in Zone 5A, New York.

·     With the adhesive used do I still need to spray foam the perimeter of Polyiso. I will probably have a 3/8” gap around the perimeter.

·     In article 1 above it mentions “this type of wall is designed to dry to the interior, so you don’t want to install any polyethylene on the interior side of the wall.” Polyiso has a vapor barrier; is that a problem? If not, can I use multiple layers of Polyiso in this fashion?

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  1. David Argilla | | #1

    Just curious, did you compare the cost to solve this by removing the siding, sheathing, and insulating externally? Is the interior plaster removed already? Your house sounds like an exact duplicate of our 1906 2 story on the west coast, no sheathing with a beveled siding as the only air and water barrier. We determined that the cost benefit for the most effective solution was to remove the siding and sheath, insulate, and air seal from outside. Of course our siding was in need of stripping and and re-painting and some areas needed repair, and I also really prefer the original plaster walls and trim. If the interior is in good shape I would think you would get a much more effective and valuable result from insulating from the outside.

  2. maeegg | | #2

    I have removed all the interior plaster. The house siding is in excellent condition and is a true 1" deep and not made any longer. Removing all this from a timber frame house and following your suggestion might be cost effective but my preference is not to disturb a well built house, i.e. over built. In addition going to that next level will involve permits and building inspections on a 1899 house.

  3. Jon R | | #3

    Consider mineral wool (hydrophobic) in the cavity and then putting all your rigid foam on the interior side. Do test with a blower door.

  4. Expert Member
    Zephyr7 | | #4

    +1 for mineral wool in the cavity. In some of my attic walls I replaced the fiberglass that had been damaged by nesting critters with mineral wool. I then put 3" of polyiso over the attic-side of the walls (there is no structural sheathing on these walls).

    If you have to put the rigid foam on the inside of the studs, make sure you do a very good job of airsealing everything. Tongue and groove is generally pretty leaky, so your wall should be able to dry to the exterior.


  5. Expert Member
    Dana Dorsett | | #5

    What is the stud spacing?

    How deep are the roof overhangs?

    Is it balloon framing, with open cavities running from the foundation sill to the attic?

    It doesn't take 3/4" to have adequate drying- half that is plenty even in the temperature zone rain forests of the Pacific northwest. In zone 5 NY even 1/4" would do it. If it's too uneven to get a consistent 1/4" using flat rigid stock for the exterior side air barrier cut up strips of 1/2" foil faced polyiso as spacers.

    The lack of weep holes isn't an issue with t&g- it leaks plenty of air at the seams. But if you'd sleep better, installing a pair of 1" round screened vents per stud bay, one near the bottom another near the top would do it (probably overkill but I don't think they make them smaller than that.) If the cavities vent to a vented attic, just the bottom vents would be enough, except where vertically blocked by windows and other framing.

    1. maeegg | | #6

      Stud spacing is 24" OC with 3" x 3" studs. So insulation must fit a 21" space.
      rood overhang is 15"
      It is not balloon framing

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