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What do you think about this approach to insulating in a 100-year-old gut rehab?

BostonBIC | Posted in General Questions on

I’m in the middle of a major rehab project in the Boston area gutting a 100-year old 2-story house and putting it back together. Trying to sort out our wall assembly and rapidly getting (very) overwhelmed by the options out there and hoping to solicit some feedback and advice. The existing structure is the original board sheathing (with lots of holes and spaces) over the dimensional framing.

First, a couple of philosophical statements, to let you know how I’m thinking about this:

(1) I am trying to balance energy efficiency with cost, and with indoor air quality.

(2) I am going to have a 14kW solar array on the roof, which should cover much of our heating and cooling (a heat-pump based system). This also means I am willing to sacrifice somewhat on the energy efficiency/insulation performance side of the equation because the energy for HVAC is coming renewably from the roof.

(3) I don’t actually want a super tight house because I’m a little anxious about moisture and indoor air quality

(4) We will use spray foam in the attic and sparingly in the basement, but I want to keep it out of the living area — which is the part of the house I’m thinking about here

An now, based on conversations with my builder, architect and an energy consultant, here is the current working plan:

(1) caulk the major gaps and cracks in the sheathing, then cover with builder’s felt

(2) blown in mineral wool in the cavity for the exterior walls

(3) fiber cement siding exterior

So given this, my hope is that some of the knowledgable folks on this forum can weigh in on whether this represents a reasonable compromise approach that will perform just fine, or whether I’m making some terrible mistake(s). Or, yes, that I’m somewhere in between. If it matters, the attic will be a combination of closed and open cell foam to hit R38.


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  1. Expert Member
    Dana Dorsett | | #1

    >"(3) I don’t actually want a super tight house because I’m a little anxious about moisture and indoor air quality"

    That's an argument FOR a a super tight house, combined with active ventilation. With a leaky house there's no control over the path by which air gets into your house, and a surprising amount can be coming through the foundation. There is also no control over putting the ventilation where it's needed the most, in the correct quantities. For optimal indoor air quality you want is a balanced, ducted ventilation plan to start with AND a super tight house- that way you have control.

    Unless the attic insulation at the roof deck AND unvented, it's silly to use spray foam. If you're re-roofing as part of the rehab, installing 4" of reclaimed roofing polyiso on the exterior and up to 8" of open cell foam on the interior would be preferable to a layer of closed cell foam under the roof deck followed by open cell foam. With a nailer deck above the exterior foam it means a fatter roof and a new facia board for the foam, but that can usually be accommodated without dramatically changing the aesthetics of the exterior. That would also be more like the R49 code min.

    The structural aspects of the roof need to be verified before installing the PV up top, but a foam-over done right would improve, not reduce the overall structural capacity. If the racking will be affixed to the new nailer deck that may change the fastener spacing requirements for the nailer deck in those areas.

    A full-dimension 2.0" x 4.0" cavity filled with rock wool won't hit code-minimum, even if it's a balloon framed/low framing fraction type of wall framing. Installing an inch of foil-faced polyiso on the exterior would get you to code-min performance, mprove air tightness if the seams are taped, and top/bottom edges caulked. At R6 it's marginal for dew point control for colder parts of MA, but adequate for Boston. Installing a smart vapor retarder on the interior would be cheap insurance.

    Alternatively, installing 3/4" polyiso on the framing edges on the interior and 1x (3/4") strapping aka Bonfiglioli strips would bring it up to code-in on an center-cavity R value basis (=R20 or more, at center cavity), giving up 1.5" of interior side space. With even 1/4" of rainscreen gap between the fiber cement and plank sheathing the assembly would not need an interior side vapor retarder, and would dry in both directions.

  2. BostonBIC | | #2

    Thanks Dana Dorsett for the response. A little over my head, but let me try and clarify a few things. Also, just for the record, where I live the codes for renovation are just to fill the cavities, no R value requirement.

    >>Unless the attic insulation at the roof deck AND unvented, it's silly to use spray foam.

    Can you explain that? We aren't putting on a new roof (aside from shingles) but I don't understand why foam wouldn't be a good option here.

    As for the wall assembly, I asked my builder about adding rigid insulation to the exterior, but he thought it would be prohibitively expensive and time consuming to do this over the existing sheathing. I recognize that we're leaving a lot of insulation efficiency on the table but I'm more concerned with having a setup that will provide some respectable level of insulation (better than the zero insulation that was there before without needing to go crazy) and not cause any active problems in terms of water infiltration/condensation. Does installing a smart vapor retarder on the interior make the exterior building felt redundant? Should I be using tyvek or something similar on the exterior?

    I recognize these questions are seemingly ridiculous -- I'm in over my head and trying to sort it out! All help is appreciated.

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