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Community and Q&A

Insulating old house 2×4 walls

smrtk | Posted in Energy Efficiency and Durability on

Old 2×4 Wall Insulation Plan

Zone 5 – Southeast Michigan
Hot and humid summers, cold dry winters
I purchased a 1930 cottage that I am in the process of gutting and renovating. 2×4 construction all around (including roof rafters). The house was only insulated in the attic with asbestos (professionally abated). The exterior is shiplap siding on 1×12 horizontal sheathing.
I would like to insulate the walls (and roof rafters eventually, probably another Q&A post after this one) and after researching I have arrived at what I think to be a pretty good plan. I lack experience here though and would like to throw it out to the GBA community for input. Plan section attached.
General Assembly Description
  • In the cavity I plan on adding 3/4″ x 3/4″ blocking on each side to create a drainage (or drying?) plane between the siding and the to be installed rigid insulation
  • On top of that blocking I plan to cut and install used 1″ polyiso rigid foam paper backed. (cut and cobble)
    • Is the paper backing going to be a problem?
    • Do I need to foam all the edges or can I get away with construction adhesive (assuming that it does not attack the foam) acting as a gasket preventing moisture from getting further into the wall?
  • Given the shallow cavity to start and the even more shallow cavity after installing the rigid foam I was looking at methods to increase the depth while avoiding at all costs using any 2×2 from any lumber yard. Came across an article in FHB from April/May 2015 illustrating a technique used by builder Stephen Bonfiglioli (LINK). 1″ rigid foam strips nailed to 1×3 furring strips then nailed to the framing. His method seems interesting as it would both build out the depth of the wall (right to where I would need it for R13 fiberglass batt) but it would also seem to help minimize/eliminate the thermal bridging of the framing.
    • With the rigid foam nailed to the furring strips and the final assembly nailed to the existing framing with a framing gun I am assuming that the finished assembly will act and be as strong as a standard framed wall (drywall hanging, picture hanging, etc.). Has anyone worked with this assembly before and/or have any opinion on it? 
  •   Once assembled I could then lay in my R13 batt and hang the drywall. 
Some reasons why in case you were wondering:
  • The 1″ paper faced polyiso is available locally used (CL) for about $6 less than new plus it will be repurposed. I still need to go and check the condition and confirm (through this Q&A if the paper backing will be a problem.
  • The R13 fiberglass batt is something I have a lot of on hand due to insulating the rafters during the winter to keep the pipes from freezing.
A lot of words here (sorry), hopefully my write up and drawing are enough to get my intent across. If not please let me know any additional information that you would need to weigh in on this approach. 
My appreciation in advance for any insight on this. 


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi David,

    I'd say that if you have no intention of remodeleing the exterior any time soon and are unsure of the existence of a water-resistive barrier between the sheathing and the siding, this is a conservative approach that will allow for some inward drying of the siding and sheathing at certain times of the year. If you were planning to remove the siding in the near future, then it might make sense to consider other options, working from the outside.

    It's probably a good idea to shoot for R-5 for condensation control with the cut-and-cobble rigid foam, to detail it in an air-tight manner with tapes, caulks, and/or canned spray foam, and it may be a good idea to detail the drywall and top and bottom plates in an airtight manner as well.

    I recently was speaking with Joe Lstiburek about a home that had stone veneer tight to the sheathing (no air space) and he recommended a similar approach as you are considering. One option he offered is to use a 3/8 inch drainage mat inside the stud bays with the filter fabric facing in. In this case you save a bit of space and could press fibrous insulation against the drainage mat. You would probably want a class II or variable perm vapor retarder also have to use the drywall (or vapor retarder) as your air barrier.

    That said, if you have a source for recycled foam, I'd probably stick with your plan.

    1. smrtk | | #2


      Yes, we are trying to avoid residing the house which poses the challenge of working from the inside. The 1" polyiso should be R-6 so hopefully good there. Would you have any concerns about the paper backing? I can buy new if I need to (or keep searching) but would prefer to reuse if possible.

      Thank you


      1. Expert Member
        Akos | | #3

        You want the polyiso with paper facing as it is permeable. This would allow for the wall to dry in both directions. For sealing, canned foam is the easiest, just cut the foam so it is loose in the bay and fill the gap.

        Your assembly should work well and with the foam strips and end up with a pretty decent whole assembly R value.

        I haven't done this assembly but I've tried 1x3 for siding which tends to be problematic as it can easily split, you might have the same issues with the foam strips on the inside.

        Would be better to rip some 3/4 plywood or if you can get use R6 ZipR for the strips. The OSB on ZipR hold screws reasonably well.

    2. smrtk | | #5

      What are your thoughts on a similar approach for the roof? Make the cavity between the underside of the roof deck and the polyiso 1”. Still using the cut and cobble paper backed and then back filling with fiberglass. Would run this continuously from the soffit vent to a ridge vent running the whole length of the house (minus the overhang). Then just need to figure out building up the depth (currently 2x4) to 7.5” overall. Would shoot for something around R24 which I believe is the state minimum. I don’t think I can get any more than that without worrying about the structure as a whole.

      1. Expert Member
        BILL WICHERS | | #6

        If you have a vented roof your roof assembly will be much more forgiving. 1.5” is better for a ventilation channel, but 1” meets code. As long as you have proper ventilation at both the soffit and the ridge you should be ok.

        I’m pretty sure R24 won’t meet code in any situation in the roof. R38 minimum in some cases, R49 is current code.


        1. smrtk | | #7

          I’m trying to figure out how to build out the depth I would need for code. It’s an old cottage with 2x4 rafters. I would also like to use the attic as an office so part of the conditioned space. I need to give it some thought. The walls were easy. This is not proving to be. Maybe I use thicker polyiso. Will keep working on it.

          Thanks for the feedback on the venting. 1.5 should not be a problem.

        2. smrtk | | #8

          I’ve been avoiding it because I am not as familiar with the process but I could put the polyiso on the roof deck. Non vented assembly. 4” on top, 4” below right to the bottom of the sheathing. Would need to fur out the 2x4’s a 1/2” but that is probably better than trying to build them out to 9.5” on the interior to achieve the same depth for polyiso. I don’t think I’ll ever get there with fiberglass. A late night thought I’ll probably talk myself out of in the morning.

  2. smrtk | | #4

    Appreciate the information on the paper backing. I have a lot to learn. Good idea on the ply as well, more stable and straight I would imagine.


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