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Adding to existing blown-in fiberglass insulation

mg6467 | Posted in Energy Efficiency and Durability on


I am located in climate zone 5, bordering 6. I have remedied sealing and kneewall issues in my attic. I am ready to add additional blown-in insulation to the existing blown in fiberglass (R-38). My plan is to bring the final R value to R-60. If I currently have blown-in fiberglass, is there any benefit to adding cellulose over the top or just top it off with additional blown fiberglass?

Many Thanks,

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "If I currently have blown-in fiberglass, is there any benefit to adding cellulose over the top or just top it off with additional blown fiberglass?"

    A. If both options are available in your geographical area, I advise you to go with the cellulose. A cap of cellulose over the blown-in fiberglass will help reduce air leakage and will reduce the chance of convective looping.

  2. user-6863358 | | #2

    As a matter of full disclosure, I do work for a fiberglass manufacturer. Martin's advice is certainly an acceptable solution, however the weight of cellulose on top of the blown fiberglass must be taken into consideration. The weight of the cellulose (heavier than fiberglass) will compact the loose fill fiberglass to some degree, depending on how much cellulose is installed. The resulting compression of the fiberglass can reduce its overall R value (while increasing slightly its R value per inch), meaning that you may have to install more cellulose than you had planned on.

    If you estimate the current R value of the existing fiberglass as R38, it is not as simple as adding an additional R22 of cellulose on top in order to achieve R60. More than likely you would need to install more than an R22 in order to get to the desired R60. Adding additional fiberglass can certainly be considered without the penalty of compressing the existing insulation.

    Either will work, just providing additional information.

  3. Expert Member
    Dana Dorsett | | #3

    Regarding the "additional" amount of cellulose needed due to the compression of the fiberglass it's "in the noise", not worth calculating.

    The amount of compression R38 fiberglass would get with ~6" i (~R22) of 1.2lbs cellulose is tiny, given that its only a bit more than a half-pound per square foot. You might have to add R23, but nobody is going to be charged extra for that, and nobody is going to measure it. The amount of slop in the air/fiber ratio on the blower has an error that large, and the amount of time it takes to install the "extra" quarter-inch isn't likely to even hit 5 minutes.

  4. mg6467 | | #4

    Gents, thanks so much for the expert advice on this project. I am going to go with the cellulose.
    Best Regards, Michael

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