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Community and Q&A

Choosing attic insulation

GBA Editor | Posted in General Questions on

I have a project at hand that includes installing approx. 900SF of insulation on an attic floor. The home is two-story and the roof pitch is 12:12. It’s a large attic with the usual soffit and ridge ventilation as well as a couple of gable vents. Existing insulation is 6″ unfaced fiberglass laid between the 2×6 truss chords, circa 1995, and probably not very effective at all. It has becomes contaminated with bat droppings and needs to go. Climate is coastal Washington, generally mild, approx. 5100 HDD.

At the moment I have two proposals from insulators in hand. One is for a fiberglass batt installation, with R-21 laid between the chords and R-38 laid perpendicular over that. The installer is well known to me and he is meticulous. The other is for loose-fill, either cellulose or fiberglass, at R-60. In order of cost, the batts are lowest, followed by the blown FG and then the cellulose.

My understanding is that all of these materials will suffer some degree of convective “heat stripping” in an open attic installation. My question is, which of these installations will perform the best, and by what sort of margin? I realize this may not be easily quantifiable, but would appreciate any comments. People in this market are accustomed to seeing minimally-effective batt installations in almost every situation, and few have any sense that improvements can be had, or what they’re worth.

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  1. Riversong | | #1

    The cellulose will reduce convection more than the fiberglass options because of the greater density (1.47 pcf @ 16" settled R-60). In addition, it will reduce the potential for condensation damage by distributing any moisture away from the trusses, and it will offer the least hospitable environment to rodents and insects.

    It's well worth the additional cost. And it's a recycled product with extremely low embodied energy, global warming contribution and ecological footprint. Just make sure that the cellulose is treated only with borates and not ammonium sulfate which can corrode metal truss plates.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Robert. In order of desirability: cellulose, blown-in fiberglass, fiberglass batts.

  3. Dan Kolbert | | #3

    Just make sure you air seal well before laying the new insulation - wire & pipe penetrations, top wall plates, etc.

  4. Doug McEvers | | #4

    Cellulose also has good soundproofing properties and is the insulation of choice for airport noise abatement.

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