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Questions on insulation and air sealing an attic (new construction)

b4316207 | Posted in General Questions on

Questions for all the great contributors to GBA: Building a new home in Zone 5, Philadelphia area.

Framing is done. Roof is finished. I was talking to the insulation sub, and he had some concerns.

My roof is a 4/12 pitch, that is now set up as a ventilated roof, and because of that in most places it’s more of a crawl space then a walkable attic, and because of that I do not think it would be possible to add plywood as my air barrier in the attic before drywall and and can lights go in.

He recommends one of 2 options:

1) How to insulate this roof to R-50 or higher? Installer recommends closed-cell spray foam @ 6″ and make it unvented (and make the attic part of conditioned space). The problem: this is very expensive.

2) Use high density batts (R-49) and in some areas add additional blown-in insulation.

The problem with this option: I do not know how to create an air seal in the attic, because drywall with can lights will not work. Can anyone give me some suggestions, please?

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  1. GBA Editor
    Martin Holladay | | #1

    For the benefit of GBA readers, I'll repeat what I have often said: The time to make decisions on insulation is at the design stage, before construction begins. Once your house is framed and roofed, it is much too late to begin thinking about air sealing and insulation. That basic concept is explained in this article: Plan Ahead For Insulation.

    (I'm sorry for this detour, Boris. I know that this information doesn't help you at this point. But I can't help asking: Who designed your house? What were they thinking?)

    If you want to air seal your attic, here is my advice:

    1. Get rid of the recessed can lights. Switch to airtight electrical boxes (you can Google "airtight electrical boxes" -- there are several manufacturers) and choose either surface-mounted fixtures like track lighting or the new pancake-shaped thin LED fixtures that fit in a shallow electrical box.

    2. Follow the instructions in this article: Air Sealing an Attic.

    3. Install a deep layer of cellulose insulation on the attic floor, following the instructions in this article: How to Insulate an Attic Floor.

  2. GBA Editor
    Martin Holladay | | #2

    One more thing: If your goal is R-50, as you wrote, you should know that 6 inches of closed-cell spray foam won't get you there. The 6 inches of closed-cell foam will have an R-value of about R-37 or R-38.

  3. jrsevy | | #3

    I have a similar build next year in zone 6A with a single story plan. I plan on 2x4 wall studs, Zip R-12 with rain screen and will insulated the wall cavities with R-15 mineral wool for R-27 walls. The attic will have a 4/12 roof, so I am planning on blowing in either fiberglass or cellulose to around R-50. To make an airtight envelop, the ceiling sheetrock will go over a poly film and where can lights are added, they will be covered with a sealed enclosure prior to blowing in insulation. Since I'll have roof trusses, foaming the roof deck in the attic makes little sense for conditioned space that we'll never use. I may also do air mist sealing of the home prior to sheetrocking the walls. If there is a better option for attic insulation with roof trusses when using Zip-R, I'd like to see that.

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