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Blown-in insulation for existing walls

user-6026880 | Posted in Green Products and Materials on

My circa 1900 house in West Virginia (Zone 5) has hardwood paneling throughout the interior and wood siding on the exterior with no insulation in-between the actual 2×4 walls. I’m pondering either blow-in cellulose or open cell spray polyurethane foam applied through holes in the siding. (Should I consider other options?) Both open-cell SPF and cellulose show an R-value of 3.2 to 3.8/inch. I understand the cellulose will settle over time leaving some gap at the top and foam will not. I like the idea of an air barrier provided by SPF between the leaky wood interior and exterior surfaces. However, I understand understand that open-cell SPF is not a moisture barrier. As their is, and can be, no moisture barrier on either side of my leaky wood walls, will I have a moisture problem with either option? Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    You need to talk with a local insulation contractor or weatherization contractor.

    In most of the country, walls like yours are insulated with either dense-packed cellulose (the preferred insulation for this purpose in New England) or blown-in fiberglass (the preferred insulation in most Southern states).

    Most open-cell spray foam contractors don't install spray foam through holes in a wall. They want the stud cavities to be totally exposed, either on the interior side or the exterior side, before spraying the foam into the stud bays.

    A few contractors install so-called "injection foam" into closed stud cavities through small holes. The work is tricky and can sometimes result in blow-outs, so most contractors steer clear of injection foam. It isn't worth considering unless you have a contractor in your area familiar with the product.

  2. GBA Editor
    Martin Holladay | | #2

    You wrote, "I understand the cellulose will settle over time leaving some gap at the top,"

    You're wrong. Properly installed (at a density of 3.5 pounds per cubic foot), cellulose won't settle.

    For more information on this topic, see How to Install Cellulose Insulation.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "As there is, and can be, no moisture barrier on either side of my leaky wood walls, will I have a moisture problem with either option?"

    A. Probably not, but every house needs to be inspected to determine moisture risks. To assess the risks, we would need to know answers to a lot of questions: Does your house have any sheathing, or is the siding simply nailed to the studs? How wide are your roof overhangs? Is it a one-story house or a two-story house? Is your siding painted? Have you ever had any signs of water entry near your windows? Are your windows equipped with any type of flashing?

  4. Expert Member
    Dana Dorsett | | #4

    The air tightness (or lack thereof) of the interior side hardwood paneling is a concern in a zone 5 climate.

    A better description of the wall stackup is needed to say what works (if anything) and what doesn't.

    Is re-siding an option?

  5. user-6026880 | | #5

    Thank Martin and Dana for your advice! Permit to elaborate in response to your questions.

    I've consulted with two insulation contractors. One who recommends cellulose and one who recommends open-cell foam (both only have equipment do do what they recommend).

    All the walls inside is diagonal 4" lapped (I'm not sure on the correct term, but a narrow leading edge fits under trailing edge) hardwood boards.

    The exterior siding in 4" wide, 1 (3/4) inch deep lapped siding nailed directly to studs.. Overhangs on the 2-story house are ~16 inches. Siding will be repainted. New windows will be flashed. I have not discovered any water entry around windows or elsewhere save for the middle of the house and that will be permanently remedied by a new roof.

    The open cell foam (the contractor, who specializes in foam, told me that blowouts occur mostly with poorly affixed drywall and that solid wood panelling won't be a problem, save perhaps for some leaks of foam that can be wiped off.

    Would not the foam be a good air barrier to mitigate the inherent leakiness of two single layers of nailed boards on studs?

    Is the preference for cellulose in the north and fiberglass in the south related to climate, the Mason-Dixon line or something else?

    Thanks again!

  6. GBA Editor
    Martin Holladay | | #6

    It sounds like your walls on the interior are finished with diagonal shiplap hardwood boards. That's fairly unusual (and quite leaky).

    I wouldn't recommend either type of insulation for your walls. Ideally, your house needs a layer of exterior sheathing and a water-resistive barrier (WRB) before you consider insulating the walls. That work is expensive, because you would have to remove the siding to do it.

    If you want to go ahead with your plan in spite of my advice, the cellulose makes a little more sense than the foam.

  7. Bigrig | | #7

    Question, do you have knob and tube wiring or has your electrical been upgraded? I would not recommend installing insulation over knob and tube. In fact the National Electrical Code expressly forbids it (NEC 2014 394.12(5)). However I do not know of any cases where it has caused a serious issue/fire.

  8. Expert Member
    Dana Dorsett | | #8

    Clapboards nailed directly to the studs with an air-leaky plank interior wall is a problem stackup. If you filled it with cellulose any wind-driven moisture that gets into the wall wicks in, soaked up by the cellulose like a sponge and there will almost certainly be places where it would be wet enough long enough that structural rot would set in. The paint will fail first , since the clabboards can't dry to the back side resulting in high vapor pressures when the sun warms it up, causing blistering & peeling, but that's just a symptom. Wintertime ,moisture drives from the interior would be redistributed by the wicking nature of the cellulose but with no way to dry it would result in high moisture content throughout.

    Slow rise open cell foam wouldn't be much better- blowouts would not be your primary concern, but the exterior side of the foam would collect wintertime moisture from vapor diffusion from the interior in the siding and in the foam (not redistributed), leading to even faster failure of the exterior paint.

    It's a real pain in the wallet, but the only way to safely insulate this wall is to add exterior foam sufficient for dew point control and add new siding, or to stuff it full of cellulose and add a housewrap detailed as an air barrier with new back ventilated siding (vinyl, or furring + new siding providing at least a 1/4" gap between the new siding and the housewrap, aka a "rainscreen" type assembly.)

    Simply filling the cavities is guaranteed to fail.

    Cellulose vs. fiberglass has nothing to do with the Mason-Dixon, but cellulose provides a bit of wintertime moisture buffering in cooler climates that is sometimes useful (or necessary). Under hot southern roof decks on attic floors cellulose performance is superior to fiberglass in the cooling season due to it's opacity to infra-red radiation (fiberglass is somewhat translucent to IR, and the hottest layer in the stackup is an inch or two into the fiberglass layer, hotter than the attic air). The difference at IRC 2015 code min R-values is somewhat academic, but when there's only 6" of fluff the performance difference is pretty real. Cellulose having a large recycled content, higher air retardency than fiberglass at open-blown densites makes it pretty much a universally better solution for attics. When dense packed into walls you get a slight performance edge out of 1.8lb or higher density fiberglass, due to higher R/inch and comparable or better air retardency than cellulose.

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