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Open-cell spray foam in historic retrofit

akhuntia | Posted in Green Building Techniques on

Hi, we are going to be renovating/adding to an 1888 Victorian to achieve “pretty good” near net-zero standards in the Chicago area. We are currently debating the type of spray foam to use in the existing structure (open cell vs closed cell). We will be using dense-packed cellulose with staggered stud walls in the addition.

We are currently leaning towards open cell in the old part of the house primarily because of its flexibility in the event the house needs to “flex” a little. The house has a large, tall flat front and often creaks under moderate winds (the front face is 45′ wide x 35′ tall from grade level to top of the gable.

Is the flexibility the right reason to choose open-cell? Do old houses need to “flex”?

thanks for any thoughts.

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  1. GBA Editor
    Martin Holladay | | #1

    In my judgment, you should choose your insulation based on other criteria, not flexibility.

    Many builders would provide an opposite argument: that closed-cell spray foam adds strength to a wall, which is beneficial.

    But personally, I wouldn't get hung up on the flexibility vs. strength issue. Choose the insulation that you feel will perform best from a thermal perspective, or the insulation that is most cost-effective.

  2. Expert Member
    Dana Dorsett | | #2

    Before using ANY cavity insulation on the old walls it's important to know the siding/sheathing stackup.

    Does this house have retrofit OSB or plywood sheathing, plank sheathing, or is the siding nailed directly to the studs?

    What is the siding type?

    What is the depth of the framing cavity ( to the nearest 1/8th inch)?

    Will you be adding insulating sheathing on the exterior?

  3. akhuntia | | #3

    Dana, sorry for not providing more of that background information initially...

    Although we haven't done full exploration on all walls, we believe we have plank sheathing nailed on the original studs that are approximately 3.75-4" deep. There are multiple layers of siding on the exterior that will all be removed. Interior walls are original plaster on wood lathe and will be fully gutted.

    Since we will/can not do exterior rigid insulation, our plan was to build in a staggered stud interior wall to both allow a true and plumb interior wall and create a thermal break. the plates of the new interior wall will be notched around the existing studs. We expect the total cavity depth to be around 6 inches.

    New siding will likely be LP Smartside unless we find the original wood clapboard is worth saving. (it is underneath layers of aluminum and asphalt shingles) Assuming we remove all of the exterior siding, we will use some sort of vapor barrier housewrap on the exterior sheathing.

  4. Expert Member
    Dana Dorsett | | #4

    Since you have multiple layers of siding you can probably put something an inch or so of continuous insulation on the exterior without changing the appearance, no?

    With the staggered stud configuration and a 6" cavity it's approximately the same as reducing the framing fraction from the mid-20s to about 18-20%.

    At an 18% framing fraction with R3.7/inch half pound foam you're looking at about R18 "whole-wall R" after thermal bridging. With most half-pound foam it's safe to install that in one pass, two at the most. It would run about $2 per square foot.

    Changing that to R6/inch 2lb foam raise that to about R22.5 whole-wall. It would have to be installed in three lifts separated in time by a cooling/curing period (for both quality and fire safety while curing). At about 0.15-0.20 perms the sheathing would be protected from interior moisture drives, but it would have no drying capacity to speak of toward the interior. The foam would run about $6 per square foot.

    The $4 per square foot difference could finance a number of alternative builds such as:

    *Adding 1.25" Roxul Comfortboard IS to the half pound foam wall would raise it to about R23 whole-wall, and give it considerable drying capacity toward the exterior. Cost varies by distributor.

    *Adding even 3/4" of XPS siding underlayment to the half-pound would give you nearly R22 whole-wall, and you'd still have over 1 perm of drying capacity toward the exterior. It would run about 50 cents per square foot, installed.

    *Adding 1" of unfaced 1.5lb density (Type-II) EPS would give you R22.2. whole-wall and about 2.5-3 perms of drying capacity toward the exterior. Costs around 40 cents per square foot.

    For any of the alternate builds it would be prudent to either use half-perm "vapor barrier" primer on the interior gypsum/plaster or install 2-mil nylon (eg Certainteed MemBrain) at about 20 cents per square foot (installed) under the gypsum board.

    Any of the alternative builds will be cheaper than 6" of closed cell foam by half more more.

    Any of the alternative builds could be done with 0.7lb water blown foam without being too vapor tight, adding a bit over R1 to the whole-wall R. A 0.7lb cavity fill with even a thin 3/8" fan-fold XPS siding underlayment would come in around R21 whole-wall, with better drying capacity than the 2lb foam wall.

    The higher the actual framing fraction, the narrower the performance advantage of the closed cell foam becomes.

    Is a 10% (or less) improvement in performance worth 2x the cost?

    Unless you use LaPolla's HFO-blown foam, most 2lb polyurethane is blown with HFC245fa, which has a 100 year global warming potential of about 1000X CO2.

    At 3" or greater thickness the closed cell foam becomes structural, but it's not more structural than cut in bracing or shear panels at the corners.

  5. akhuntia | | #5

    Thanks for those detailed comments Dana, I will review them thoroughly. How do you come up the the "whole wall R after thermal bridging"? Is that a rule of thumb?

    If we chose to only fill the 6" cavity with half pound foam, this should dry to the interior, correct? And the Drywall will be the final air barrier?

    What would you recommend for the Vapor barrier on the exterior sheathing?

  6. Dana1 | | #6

    I maintain spreadsheets for calculating U-factors/whole-wall R based on the material stackup. In your case I had to make some guesses on the thickness & R value of the siding & sheathing, as well as the actual framing fraction and the thermal performance of the framing, etc. This can't be done with rules of thumb.

    A decent 3-pager on calculating U-factors/whole wall R lives here:

    With half-pound foam it can dry in both directions. You don't need or want a vapor barrier on the exterior, but you DO need a weather resistant barrier. (Standard housewraps or #15 felt are fine.) As long as you build out the siding as a rainscreen, with as little as a 1/4" ventilation gap to the next layer it can also dry toward exterior as well as the interior. If that's not possible it will dry primarily to the interior, which is why using 2 mil nylon is preferable to vapor barrier paints.

    The foam-in-framing is already an air barrier, but the 2-mil nylon can also be detailed as an air barrier. Depending on the framing details you will likely have to caulk some seams between framing & subfloors, etc. since the foam only touches the interior of the wall cavities, sealing the sheathing to the framing, and while sheet goods like 2-mil nylon can cover over seams in doubled-up framing elements, they don't really seal the 90 degree turn to the subflooring well.

  7. akhuntia | | #7

    thanks again Dana. Great info and I will use this guidance as we review our choices.

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