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What insulation can I add to my attic?

greg1258 | Posted in Energy Efficiency and Durability on

I currently have blown-in loose-fill fiberglass in my attic. Do I have to add the same product, or can I use cellulose?

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  1. GBA Editor
    Martin Holladay | | #1

    Yes, you can add cellulose insulation on top of your existing loose-fill fiberglass. Not only can you -- you should.

    Adding cellulose on top of loose-fill fiberglass insulation is the best way to overcome the inherent problems of loose-fill fiberglass (especially the degradation in thermal performance due to convective loops in fiberglass). A cellulose "cap" not only warms up the top of the fiberglass, cutting down on convective looping, it also reduces air leaks through the fiberglass.

  2. Expert Member
    Dana Dorsett | | #2

    It takes at least 3" of cellulose over-blow to "restore" the full R of the fiberglass layer. If you add 6" or more the weight of the cellulose will compress the fiberglass a bit, but that gives the fiberglass a higher R/inch (if a lower overall R of the fiberglass layer by itself.)

    The overall performance will be a function of the total settled depth of the cellulose + fiberglass, which will average about R3.5 per inch over a wide range of temperature conditions (some manufacturers claim R3.7/inch, which it probably does in a lab-test blow), as long as there is at least 3" of cellulose over the fiberglass. Less than that would still yield some convective losses at the low-temp extremes.

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