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Community and Q&A

Attic insulation

Windyofhighakers | Posted in General Questions on

A 16′ x 26′ addition, one story, partially vented attic(soffit and gable end vents).
Presently 6″ fiberglass batts between 2″x6″ceiling joists 16″ oc.
Can I blow in cellulose insulation to add to the R factor, making sure not to    
    block soffit venting between the rafters?  How much in terms of depth and 
    should it be borate treated?

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  1. Expert Member
    Dana Dorsett | | #1

    Soffit vents work better by closing off the gable vents, and adding a ridge vent. The area of the ridge vent should be smaller than that combined are of the soffit vents to something like a 40/60 proportion to prevent the ridge vent from depressurizing the attic in winter, drawing humid conditioned space air up into the attic.

    Before installing new insulation roll back the batts and get serious about air sealing every seam, hole, duct/plumbing/flue/electrical penetration.

    It's fine to re-use the existing batts and blow cellulose over them. To hit R49 (= IRC code min for climate zones 4 and higher would take a combined depth of 14-15" (initial depth after blowing-it'll settle some over about a decade). To hit R38 ( code min for zones 2 & 3) takes about 11", for R30 (zone 1) it takes about 9". Set up some depth gauges, and rake it smooth at the target depth as you go.

    All cellulose insulation sold in the US has some amount of borate fire retardent, but many also contain ammonium sulfate as a fire retardent, which becomes corrosive when wet. Ideally it's better to use borate-only/ sulfate-free goods.

    It takes an overblow of at least 3" to be sufficiently air retardent to make the fiberglass batts perform to spec. Even if the additional weight compresses the fiberglass it's fine, as long as the combined depth target is maintained.

    Odds are pretty good you don't have sufficient clearance between the top plates of the exterior walls and the roof deck at the soffits to hit R49, but cut'n'cobbled rigid polyiso foam board is used for the depth chutes you can get R6/inch out of it. With 2x6 joists you can usually get at least R30 (and often R38+ ) over the top plate and still have the mandatory 1" air space between the insulation and roof deck.

  2. Jon_R | | #2

    Don't worry about your gable vents. As Lstiburek says "Where the air leaves isn’t as important — whether it’s a ridge vent, or mushroom caps, or gables.".

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