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Community and Q&A

Blown Cellulose vs Blown Fiberglass

DRILLINDK | Posted in Energy Efficiency and Durability on

Hello Everyone. I own a four family apartment building in St. Louis and I’m considering having insulation installed in the attic, first floor ceiling, and basement ceiling. I

The attic contains original R-9 (roughly) insulation. The first floor ceiling probably contains little to no insulation and I’m looking more for noise reduction. The basement double as garage parking for the tenants with existing unfinished drywall installed. There is no existing insulation underneath the drywall.

I’ve spoken to four different contractors and have received four similar but unique proposal bids. Three have proposed cellulose blown insulation. They would drill holes the size of door knobs throughout the existing ceilings and blow in the cellulose. One mentioned dense packing the cellulose.

Another contractor, whom one of my coworkers recommended and I consider trustworthy, suggested using blown fiberglass. Same technique as above. They would drill holes in the ceilings and blow in fiberglass. He said he prefers the fiberglass because it doesn’t settle like cellulose. However, he said he would do whatever I like.

Does anyone have any advice? I feel like my head is spinning with these choices.

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  1. GBA Editor
    Martin Holladay | | #1

    Either product will perform well if properly installed. Here in New England, the preference is definitely for cellulose, but I know that in other parts of the country, common practice differs.

    In your case, the most important factor for you to consider is the skill and reputation of the installer, not the material being installed.

  2. user-974013 | | #2


    Cellulose is the best answer for your needs. Sound suppressing is best done by cellulose, second in line is open cell spray foam. For the money and performance you will want to go with cellulose in this situation.
    If done right you can get sound suppression, insulation, and draft blocking between your 2 floors.

    I will recommend that you have your insulation contractor air seal before insulating the attic flats, and a product i like to use for the skuttle hatches (attic access) is called an energy guardian door cover kit. This will air seal and insulate your access to the attic and act as a dam for the blown cellulose.

    I would recommend you stay away from fiberglass.

  3. DRILLINDK | | #3

    Thank you for your responses. Being roughly a 60 year old building I'm concerned of the state of the electrical wiring thoughout the place. Would the fiberglass be a better options for something like this b/c from what i can gather on the internet the cellulose is treat with acid, but still flammable.

    How messy of a job is this? Would I need to wait until tenats move out of the apartments before having the first floor blown? Could I do garage and basement commmon areas and attic okay without making too big of a mess. Basement contains some of the tenants personal belongings.

  4. Expert Member
    Dana Dorsett | | #4

    If you have degraded electric connections heating up to the kindling temperature of paper in the attic, it's not as if fiberglass is going to save you! The fire retardents in cellulose can withstand direct blowtorch flame for many seconds or even minutes before it has vaporized enough of the retardent to light off, and even then it's usually self-extinquishing when you take the flame away.

    Dense-packed to 3lbs cellulose won't settle, but in an attic situation the amount of settling that occurs even in low density blows is miniscule compared to what happens in 2-story balloon framed low-density blows. Dense-packing is a must in walls, but not important at all in attics. If a foot of cellulose sags to 10" before it reaches it's final sagging density (which it might), it's lost about 17% of it's total loft, but only ~10% of it's total R (it's R/inch rapidly increases with density when starting out at 1.2-1.4lbs density.)

    The borates used as fire retardent's aren't very acid, but they DO kill off the gut-flora of wood-boring insects (ants/bees/wasps/termites), which prevents the insect hose from being able to digest wood fibers, killing them by starvation. Many cellulose products also contain sulfate fire retardents which are fairly acidic and corrosive should they ever get wet. Insist on a "borate only, sulfate-free" version (all cellulose vendors make some- the "stabilized formula" used for wet-sprayed goods is almost always sulfate-free.). Borate-only goods don't stink or corrode metal when wet, and are tougher on wood boring insects.

    Dense packing would increase the air-tightness, and if you stopped at 2.8-3lbs density it would be pretty near it's max-R. But dense-packing increases the mechanical coupling between the surfaces, and increases the acoustic transfer over what a low-density blow achieves. For noise abatement purposes keep it under 2lbs density.

    All insulation blowing makes a certain amount of dust. With blown fiberglass that dust remains suspended in air, and is a health hazard. With cellulose the dust is primarily the fire retardents, which has low toxicity and doesn't remain suspended in air for long. It's a nuisance dust for the installers, but presents no longer term issues for the occupants. But air infiltration coming in from recessed lighting in fiberglass attics bringing in micro-fiber particulates is a fairly common well documented indoor air quality problem for less than air-tight houses, but careful air-sealing can mitigate that issue.

    Drop cloths and diligent vacuuming would be necessary no matter which fiber you use, but most installers use a cut-off sweatshirt sleeve or something where the blowing nozzle or dense-packing tube enters the hole (to avoid exessive blow-back in their face, even if there were no other reason.) It's only going to be HUGE mess if they're sloppy about it.

    Cellulose (even low density cellulose) is more air-retardent than low-density open-blow fiberglass, but the ultra-fine dense-packing fiberglass blowing wools like Spider and Optima can hit or even exceed cellulose-type air retardency if dense-packed to 1.8ls density or higher. It's good stuff when dense-packed, but it's usually more expensive than cellulose. At only 1lbs density (which is a legitimate rated installation density for most of them) it's EXTREMELY air-permeable, and would need a top-side air barrier to perform well in the attic.

  5. DRILLINDK | | #5

    Wow, thank you Dana. That was very informative.

    Just to recap what I need to ask the contractors.

    1. Ask for borate only, sulfate-free cellulose.

    2. Find out if they are going to dense pack the cellulose and to what density. 2.5lbs-3lbs is ideal.

    3. Do they offer fine dense packing fiberglass blowing like Spider or Optima.

    Anything else???

    On a side note. Only one company has suggested in the proposal bids that I seal the attic before blowing in insulation. However, for an additional $1700, I don't think I can justify this for a rental. I think installing the insulation is going to make such a drastic improvement anyway. I want to improve the living conditions, but it also has to be cost effective.

    Thank you for all your help!

  6. GBA Editor
    Martin Holladay | | #6

    It is essential that you air-seal the attic floor before you install cellulose there. Any insulation contractor who is willing to blow insulation on an attic floor without performing air sealing work is ignorant. Run away from that type of contractor, as fast as you can.

  7. Expert Member
    Dana Dorsett | | #7

    What Martin said- the air sealing is worth it- not from a tenant comfort point of view but from a long term maintenance point of view, the costs of which are borne by the landlord. Think of it as an "ounce of prevention" kind of deal.

    Air transported moisture is an order of magnitude bigger problem than vapor diffusion, as much focus & discussion as there is on vapor barriers, etc.

    And bulk water incursions are an order of magnitude bigger issue than air-transported moisture.

    But to keep your attic dry and less prone to wood-fungus & insect issues, air sealing before insulating is a priority, especially on a multi-story house.

  8. DRILLINDK | | #8

    Thanks for all the input. I appreciate your help.

  9. belmanliving | | #9

    Hi Rick,
    I think cellulose is much safer and most suited to your needs. It not only fills the space between the framing wall but also offers a slightly higher r-value per inch and hence reduce conductive heat transfer. It is also found that cellulose insulation limit air movement which cuts down on heat loss through convection which is not found in Fiber glass insulation. So will definitely suggest you to go with Cellolose insulation by a professional and get worry free installation.

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