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Community and Q&A

Insulating a 100-Year-Old Farmhouse

PBMoran | Posted in Green Building Techniques on

Hi, I’m a contractor working out my second renovation project. It’s an OLD 200 sq ft farm house in King George, Va, and I am value engineering a green build, while incorporating as many passive house principals as economically feasible. I also plan to pursue LEED certification.

The exterior wall on the original structures includes rough cut 2×4 ~24″ OC on top of a 5×8 timber sill. Original clapboard wood siding with vinyl siding installed on top of that. I would like to keep the vinyl siding and am designing the exterior wall system.

I have a ton of salvaged .25″ fan board XPS. I am thinking of installing two layers of XPS in the wall cavities against the inside of the wood siding (R-2 and water barrier). I would then use strong-ties to double up the rough 2×4’s to create 2×8 interior walls. I would then install either 8″ inches of Ultratouch Denim Insulation R-30 or 8″ of rockwool. I would then install CertainTeed Membrain on the double stacked 2×4’s to provide an air barrier. On top of the Membrain, I would seal the gypsum walls.

By stacking the 2×4’s I realize there will be a thermal bridge, but with the membrain installation, I would mitigate condensation on the exterior side of drywall.

The existing structure had log joists, which I am replacing with an insulating concrete slab (4″ of concrete, 2″ of Foam (XPS or EPS or Foam), 4″ of gravel. In the concrete, I would install radiant floor heating.

Will this work?

Appreciate feedback, thoughts, or alternatives.

Thank you!


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  1. 1869farmhouse | | #1

    Do you know what kind (if any) WRB is below the vinyl siding?

    With an old house, maintaining an adequate drying path is going to be key. I’d be leery of putting the XPS on the interior. Rock wool is great in that it allows drying to the interior, but it also allows air flow. If you must keep the vinyl, I’d personally focus on extensive air sealing, and then double up on 4” rock wool batts.

  2. PBMoran | | #2

    Thanks, Austin.

    There's no barrier behind the vinyl other than the original wood siding installed directing onto the exterior wall studs. With that setup, I figure air sealing at this point will be impossible. The best I can do would be to insulate the interior wall by building a double stud wall or doubling up the 2x4's to allow for 7.5" batts. Then I would install a membrane sheet behind the drywall

    Since the salvaged XPS would only be in the cavities between the studs, the wall would still be able to breathe through the break where the wood siding and studs meet. But it would serve as a dry backer for the batts and would also help close air gaps between the clapboards.

    Later on, I would hopefully make good on passive by removing the vinyl and doing a REMOTE type wrap.

    What do you think?

    1. 1869farmhouse | | #4

      Seems like it would be OK in theory. In practice, having retrofitted a few century homes now, I’m extremely conservative in my approach. It really has to be all or nothing or you risk mold, mildew, and rot.

      Personally, I’d scrap the foam and just load up on rock wool (keeping in mind the attic is by far more important) or scrap the vinyl siding and do a full exterior WRB. These homes survived as long as they did by having the ability to dry all of the moisture that inevitably ended up in the assembly effectively.

      1. PBMoran | | #9

        Austin, I really appreciate your help and specific experience. I am impressed with the rockwool product, and I agree that it makes sense here.

        Have you done anything with hempcrete? I'd love the opportunity to support the industrial hemp industry if performance proves similar.

        For the roof insulation, I am planning to insulate it from the outside apart from a 5" foam board installation that I would install between the old trusses directly to the original roof sheathing.

        I then plan to follow up on the roof with several layers of foam and rock wool (still locking in specifics).

    2. seabornman | | #10

      What are you doing with windows and doors? Are the interior finished of the exterior walls not salvageable? Can you afford to lose the space required for a double wall?
      If you're going to remove the vinyl someday why not now? I'm redoing an 1830s house similar to yours now. We are removing all of the sidings down to the studs, installing batt insulation where missing, sheathing with zip, and adding exterior insulation and all new windows and doors. The interior disruption is less than I thought it would be and it gives a chance to remedy issues with sill decay, missing framing, etc. We've already got the zip, insulation and windows up, and the house is tight and warm.

  3. GBA Editor
    Kiley Jacques | | #3


    Have you worked with Ultratouch Denim Insulation before? If so, I'd like to hear your thoughts on the product.

    1. PBMoran | | #7

      Thanks, Kiley.

      This would be the first time. I am concerned though about the moisture retention that can happen in cotton.

      I have a hempcrete firm reviewing the plans for a potential spray-in hemp insulation. I like the cement nature of it with it's ability to resist moisture while still being vapor permeable, which would help things dryout.

  4. Expert Member
    Dana Dorsett | | #5

    Using batts often requires a lot of trimming to fit antique full-dimension lumber, often not on standard 16" o.c. spacing. As a rule blown cellulose would be a better fit, and would be fine to use up against your double-layered XPS.

    A critical detail to get right before any insulation is to retrofit adequate window flashing. Empty wall cavities dry easily even with the occasional bulk water incursion, but with ANY fiber insulation in the cavities that will no longer be the case.

    Instead of 2x4s stacked on the existing studs with Strong ties, running them laterally "Mooney Wall" style would deliver a higher thermal performance due to the dramatically reduced thermal bridging. The R1.2/inch wood robs the R4/inch insulation of it's performance, even more so with full-dimension lumber, which delivers a higher framing fraction. If using batts use R15s in a Mooney wall configuration, not R30s in a deepened cavity approach.

    If deepening the cavities with vertical studs is still your preferred method, consider using 2" wide 1" polyiso strips on the original stud edges and 2x3s (finger-jointed 2x3s if higher flatness is a design goal) to achieve at least some thermal break on the framing. This is similar to the "Bonfiglioli strip" approach that uses furring instead of dimensional studs:

    1. 1869farmhouse | | #6

      Hi Dana, I’ve been reading the forums here long enough to know the depth of knowledge and experience you have. I certainly do not begin to approach that level, especially when it comes to building science - but I do have years of hands on construction experience, much of that in historic homes.

      I’m curious about the ease at which you can recommend cellulose in a century home. My admittedly (comparatively) small sample size that is my own career has led me to believe that the cellulose insulation that filled so many walls in the Midwest through the 50’s and 60’s was the kiss of death. Almost every time I’ve opened a century home this was done to, I’ve found mold and rot. Homes on the same block that were left hollow/empty were often times perfectly fine. What are your thoughts on this and how can it be done better in 2020?

    2. PBMoran | | #8

      Thank you, Dana. I appreciate you taking the time to review my situation and to share your thoughts.

      I had researched the Mooney walls a bit and agree that they would be a good solution here. I am concerned though that a loose cellulose may retain too much moisture. When it came time to remove and replace the vinyl siding and potentially the clapboard siding, I would lose all of the cellulose, where as a rigid insulation could either stay put or be easily staged and reinstalled.

      Likewise, the "Bonfiglioli strip sounds like a great installation.

      Have you had any experience with using hempcrete products? Depending on their installers analysis for either a spray-in installation or formed concrete, I like the idea of having a insulating, vapor permeable organic concrete mass. It would also allow the existing clapboard to serve as a form for the hempcrete installation. Ideally this would stay in tact if it was ever removed.

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