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Community and Q&A

Cellulose or BIBS fiberglass in 16-inch ceiling cavity?

mike78 | Posted in Energy Efficiency and Durability on

My house addition has an 800 sq ft ventilated attic. Ceiling joists are 16 inch I-Joists on 24″ centers with 5/8 drywall underneath and 5/8 plywood attic floor on top.

The plywood attic floor is removable everywhere except the perimeter so that insulation can be blown from above into the 16″ deep cavity. The current plan is to remove a row of plywood sheets on each side of the attic centerline. That will allow insulation to be blown from above and a maximum of 8 ft horizontally between the drywall and unremoved plywood. Then Insulweb would be attached over the I-Joist openings to allow some amount of dense packing in the removed plywood areas

I have insulation quotes for blown cellulose and BIBS fiberglass. Density of either insulation would be in the 1.7 – 2.0 lb/cu ft range. The cellulose would be somewhat packed to reduce settling but could not be a true dense pack of 3.5 – 4.0 lbs / cu ft (5/8 drywall ceiling might bow at that density). Fiberglass would be a true BIBS dense pack of 1.5 – 2.0 lbs / cu ft, not the typical blow-in density of 0.5 lb / cu ft. My goal is R60 in the 16″ cavity. Cellulose would be close to R60 depending on how much it settles. BIBS fiberglass would probably reach R60 and would not settle.

Any opinions on the best approach: cellulose vs BIBS fiberglass? Once the insulation job is done, the removed plywood attic floor panels will be reinstalled and I wont see the insulation any more.


  1. user-1097046 | | #1

    Cellulose installed at >1.6 lbs/cubic foot will settle little, if at all.

    I think you'll get a good result with either product, as long as the ceiling plane is well sealed prior to insulation. The BIBS will give you a little higher R value, on paper at least. The cellulose will have lower embodied energy.

    If you decide to go the cellulose route, be sure to get an all-borate product, not one that uses ammonium sulfate as a fire retardant. Ammonium sulfate is corrosive and smells funky if it gets wet.

  2. mike78 | | #2

    Jon, Thank you for the reminder about ammonium sulfate in cellulose. Both of the companies that I have cellulose quotes from are using insulation that contains ammonium sulfate. I have asked them if they can obtain cellulose that is all borate. My ceiling cavity is about 800 sq ft. At 16" depth and a density of 1.7 lb / cu ft about 1800 lbs of insulation will be needed, either cellulose or dry-packed fiberglass. I have quotes in the $1200 -$1400 range for cellulose and $2000 - $4000 for packed fiberglass. Are the material costs for cellulose and fiberglass that different ?

  3. GBA Editor
    Martin Holladay | | #3

    Considering the price difference, your choice seems like a no-brainer to me. Go with the cellulose.

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