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What is best to insulate with over existing blown in fiberglass insulation?

Gerard Timm | Posted in General Questions on

I have about 9inches of blown in fiberglass already and would like to add more.

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  1. GBA Editor
    Martin Holladay | | #1

    Assuming that we are talking about an attic floor, the answer is cellulose. As much as you can afford.

  2. Expert Member
    Dana Dorsett | | #2


    Even if the weight of the cellulose compresses the lower density fiberglass it's the total depth that counts. Compressing the fiberglass layer slightly makes it more air-redardent and more R/inch, but lower inches, lower total R for that layer, but you want at least a foot, and preferably more of overall initial depth. The cellulose will lose about 10-15% of it's loft to settling over the next decade. To hit ~R50 at the settled depth needs to be about 14", which means installing it at 15.5-16", etc. The higher density and infra-red (radiated heat) opacity of cellulose make it a superior performer in this application. It's pretty cheap stuff, and an easy DIY with a rental blower even for newbies if you read up on how to do it.

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