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Closed cell foam in attic: can I get away without doing a full Flash & Batt

johnchard | Posted in Energy Efficiency and Durability on

Hi all. A quick question.

House is 1916 farmhouse in zip code 06820 (CZ 5). The attic is made up of rafters that that are not even 5.5 inches (more like 5 inches) and around 23 inches o.c. Its a small attic with only standing room dead on center as the house is roughly 25 x 30 ft. Ridge line runs north-south and roof generally gets plenty of sun, particularly on western side. We plan to add our PV on western side.

I am reticent to use foam (off-gassing, embedded energy etc) but recognize its my best option to tighten up the attic. After reading Martins update on the risks of just using the fluffy stuff with smart membranes, I want to strike a balance between air tightening it and DIY.

Q1. Can I get away with two inches of foam only our are there other risks that arise? I have seen the mentions of ice damming – what are the consequences of this or other issues?
Q2. If I need to do more based on Q1. can I just pile the fluffy stuff on the floor in the attic (we are using one corner for storage) ? 

Thanks in advance for the guidance.


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  1. Jon_R | | #1

    Your plan is for an unvented/conditioned attic or a vented/unconditioned attic? Unless you have ducts up there, I suggest the latter - all insulation on the floor.

  2. BrianPontolilo | | #2

    Hi John.

    As Jon R mentions above, the approaches--insulating the roofline vs insulating the attic floor--are very different. Insulating the roof line (vented or unvented) brings the attic into the conditioned space of the house. Insulating the attic floor keeps the attic outside of the conditioned space (cold in the winter, hot in the summer).

    In your location, R-49 is what the International Residential Code would like to see in your attic or roof. Most high-performance builders shoot higher than that. Since an unvented, insulated roof is a riskier assembly, and closed cell spray foam more costly in many ways, I think insulating the attic floor with blow in fibrous insulation is a better path for you. Of course, after doing some really diligent air sealing. I assume that you have seen these articles, but in case you haven't, check out:

    How to Build an Insulated Cathedral Ceiling
    Creating a Conditioned Attic
    Air Sealing an Attic
    How to Insulate an attic floor

  3. johnchard | | #3

    Thanks Jon and Brian.

    I forgot to mention. Our second floor mini split will be ducted and the air handler and duct work will be up there. Its an old attic with a flimsy timber door and narrow stair case. The attic floor does have cellulose blown in. The efficiency contractors from Eversource have tried some simple weather stripping. This will be our first winter here and I worry it will be like a fridge with a plywood door!

    So given this and the ductwork, I thought foaming it would make sense to at least make it airtight?!

  4. Expert Member
    Dana Dorsett | | #4

    It can sometimes even be cheaper to build an air sealed utility-chase/platform over the mini-duct cassette & ducted region and install R49 above the platform than to put R49 at the roof deck. Installing short ducts to ceiling diffusers at the interior side of the rooms with ceiling diffusers designed to throw air toward the exterior walls/windows can keep the size of that platformed area small. An insulated weatherstripped hatch or door to the utility chase under the hatch can provide service access. That approach would require air dams in every joist bay that crosses under the mini-wall of the chamber.

    Since it's not structural (it's only holding up insulation, not the ceiling, not the roof) it can all be done with 2x4 framing and sealed half-inch OSB on the exterior side of the framing.

    Some thought has to be given to the return path ducting. A common return grille in a section of ceiling (perhaps set up for swapping the filter media without going into the attic) using partition wall cavities as jump ducts to the common areas can usually work. With a grille into the partition stud bay near the floor in the doored-off room and near the ceiling on the common space side of the walls provides better sound & light isolation than door cuts at the threshold, and better mixing than transom grilles or jump ducts penetrating the ceiling.

    1. johnchard | | #5

      Thanks Dana. We have a somewhat awkward little space up there and the flimsiest of attic doors on super old stairs.

      So what I am hearing is-
      1. No ductwork, then R49 the floor
      2. Ductwork, then I could create an air sealed utility chase with R49 outside (ie a room within the room) or I foam and seal the roof deck at 4" at <R49

      Is this right? Is there an option where its roofline PLUS attic floor?



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