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Community and Q&A

Is this fiberglass dense packed?

springtempersteel | Posted in General Questions on

We had a contractor insulate the raked (half cathedral) ceiling of a “bonus room” addition in climate zone 3B. He said it would be “dense packed” fiberglass. Later, we decided to replace the leaky inward-tilted clerestory windows at the high end of the ceiling. This involved removing a 10″ wide strip of plywood across the upper edge of the ceiling, revealing the rafters and the insulation. 

For most rafter bays, the insulation freely tumbled out and could be easily shoved back down into the rafter bay. I discovered one rafter bay that held the insulation tightly and could not be further compressed without a good deal of force. The difference is dramatic.

Should all of the bays be as dense as this dense one?

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  1. Expert Member
    Dana Dorsett | | #1

    Carefully cut or scoop out a section of known dimensions in the less-dense insulation bays and weight it. If it's not 1.8lbs per cubic foot (about a quarter gram per cubic inch) or higher it's not sufficiently dense-packed.

    1. springtempersteel | | #2

      Thank you so much, Dana. I was looking for a qualitative response and you (rightly) asked me to quantify. I expect no less from the GBA Q&A!

      Hard to cut out a known dimension from loose fiberglass, but my best estimate is the looser bays were filled to ~1.0lb per cubic foot while the dense bay was 1.95lbs per cubic foot.

      Now I know where I stand at least. Just live with it is probably the best option on the table.

      Thanks again, Tim

      1. Expert Member
        Dana Dorsett | | #3

        It's possible to re-pack blown 1lb density fiberglass to bring it up to snuff, but whether it's "worth it" depends on how expensive or complicated it becomes. A density of 1lbs per cubic foot is not a disaster- it a standard (not dense packed) blowing density. It's usually pretty easy to thread a dense-packing hose through 1lbs fiberglass in a framing bay unless there are other obstructions (plumbing, wiring, etc) in the cavity. (It's also possible to dense-pack it out using other materials such as cellulose, if you like.) Unlike cellulose, under-spec low density fiberglass isn't likely to settle leaving big gaps, but at 1lbs it does allow some air convection currents within the fiberglass layer to undercut performance at the temperature extremes, and it isn't very air-retardent for slowing down air leaks (about an order of magnitude less air resistant than 1.8lbs + goods.)

        The most important performance aspect here (for both heat & moisture) is to make the assembly as air tight as possible on the sides that you have access to.

        1. springtempersteel | | #4

          Thanks for the good advice, I can see that you could dense pack through the 1lbs fiberglass but when I consider the expense, hassle, and having to do another round of drywall patching and painting, I think the cons outweigh the pros. I definitely pay attention to air sealing at every opportunity. Cheers, Tim

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