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Community and Q&A

Fiberglass vs. Cellulose Insulation in Attic

Cortland15B | Posted in General Questions on

I read an article on Finehomebuilding that Martin Holliday wrote and he said there was a study that showed a 30% reduction of the effectiveness of fiberglass insulation in the attic when attic temperatures dropped below 0 degrees F. The suggested fix was blowing at least 2” off cellulose over the fiberglass to stop convection currents. I live in zone 7 and was planning on adding a third layer of fiberglass batts and now I’m rethinking cellulose might be a better option.

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  1. Expert Member


    That's interesting. can you find a link? Was it primarily due to convection, wind-washing or temperature?

    1. Cortland15B | | #3

      It was convection they said. Here’s the link, it’s talked about at the end. This may be the study of the loose fill fiberglass performance before they changed how they produce it?

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Fibreglass batts and loose fill fibreglass have very different properties. The batts do not have the same propensity for convective currents, and don't experience anywhere near the 30% reduction in effectiveness that the study found in loose fill.

        If you are thinking of "adding a third layer of fibreglass batts", I'm assuming the attic now has two existing layers. Since you won't have any loose fill fibreglass, it isn't something to worry about.

  2. Expert Member
    BILL WICHERS | | #2

    Fiberglass batts are usually said to INCREASE R value slightly in cold temperatures, and I'd have expected loose fill fiberglass to perform similarly. I would definitely like to see a link to that article!

    I am aware of fiberglass being less effective compared to cellulose for high temperatures though, since IR radiation can penetrate fiberglass more deeply. I've never seen anything say it was near 30% though -- typical estimates of reduced performance were much less than that.


      1. Expert Member
        BILL WICHERS | | #6

        Wow. Surprising results. I've always preferred blown cellulose over blown fiberglass anyway (I hate the itchy fiberglass!), but cellulose also weighs a good amount more than fiberglass per unit R value, so sometimes I'd use fiberglass due to saggy ceiling concerns.


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