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What are the pros and cons of blown in cellulose and fiberglass?

ltarn | Posted in Energy Efficiency and Durability on

Is their an inherent problem with moisture in cellulose? What do the experts recommend?

Planning on adding insulation to attic space in Michigan ranch house.

Now have fiberglass batting between floor joist. Keep getting conflicting information between blown in cellulose and fiberglass.


  1. GBA Editor
    Martin Holladay | | #1

    The topic has been discussed many times before in our Q&A section. You might start by reading this recent collection of opinions:

  2. Steve El | | #2

    I asked the same question here not long ago, except I was asking about blowing into wall cavities instead of an attic. As a retrofit over existing fiberglass I have a very hard time imagining any advantage of fiberglass.

    For that application I've heard people argue about weight, Ph, medical hazards, ecological impact of the material, and how the materials respond to moisture if any. I think that's the list. If that's the list, my opinion is cellulose is an easy winner. I'm curious if the conflicting advice you got had reasons to back it up?

    Either way, and you may know all this of course, remember to thoroughly air seal and install soffit venting if needed. You might need blocking on top plates. Also do any other work you have in mind first. I blew cellulose over the fiberglass in my last house. One of the extra things I did first was to cut and refit the poorly laid batts, and insulate my plumbing (they had laid the batts up and over the pipes. Only when I was done with everything did we blow the cellulose.

    Good luck whichever you choose... if you want to see the comments in the thread I started, just click "All questions" and flip thru the first few pages. It will be obvious by the title.

  3. Steve El | | #3

    Oops... you beat me to the "post" button, Martin!

  4. Steve El | | #4

    on more ps.... my mistake... I did not start that thread, I hijacked it.

  5. ltarn | | #5

    In response to Steve's question about "were these the conflicting advice" was the same. Yes, It was.
    Most answers were based on "common thought" rather than science, which however, can get even more confusing. Thanks for your response.

  6. J Chesnut | | #6

    The previous discussion that Martin pointed out focused on applying insulation within the cavity of a wall.
    If you are only blowing insulation onto an attic floor this is a significantly different scenario.
    Fiberglass blown into a pile on the attic floor performs very poorly because of convective heat loss through moving air.
    Most people, if not everyone, will agree piling up the more dense cellulose insulation will give you far greater insulation performance.

    What is the source of the moisture you are concerned with? With moisture through vapor escaping from the interior of the home up through the attic there is no significant difference between using fiberglass and cellulose. If you are concerned with moisture intrusion through the roof into the attic than neither insulation is going to protect you from long term water damage.

  7. Riversong | | #7

    I"ll repost what I had said in the last thread on this topic:

    There is no question of which is better.

    Fiberglass has a high embodied energy and is a known carcinogen (probably moreso with the much finer fiber size of Johns Manville Spider, which is closer to asbestos).

    Cellulose is almost entirely recycled paper with boric acid as a fire retardant. The use of cellulose insulation reduces one of the largest contributors to landfills - newspapers. Cellulose has almost no embodied energy or global warming contribution. The boric acid (don't use product with ammonium sulfate - it is corrosive to metal and can offgas) is an insecticide and fungicide as well as an effective fire retardant (allowing cellulose to function as a firestop) but is completely non-toxic to humans and pets.

    Cellulose deadens sound better than fiberglass, stops convection better than fiberglass (though Spider is close), and it buffers changes in relative humidity as well as protecting the wood framing from excessive moisture accumulation.

    There is simply no comparison in the overall quality of the two materials, though the quality of installation is also critical to proper function.

  8. Laura | | #8

    Thanks for your input everyone. Just to clarify, we are concerned about the moisture we are finding in the attic (which is very tight - crawl space only, part cathedral, part flat). We do not have leaks, it's a new roof and it performs well from the top.

    After reading a lot on this site, we are thinking the moisture is from a combination of leaky ducts and air leaks along with a moderate "stack" effect, plus the fact that the house is in a wooded area with a lot of roof shading. We originally turned away from cellulose because we thought it would exacerbate the moisture problem and become soggy and moldy. We thought blown fiberglass would "wick" moisture up and away from sheathing and joists.

    After reading your opinions, and reading other threads, answer this: Would cellulose with an added resin help to reduce the stack effect and limit air movement and therefore moisture? Of course, we would do our best to air seal and duct seal - yes, we have lots of ducts in there, add baffles and soffit vents. Would we be able to lower the amount of condensation in the small attic better than if we blew fiberglass in and hope to "wick" the moisture away?

    Will cellulose be able to dry out well enough, even with moderate roof shading?

  9. Charles | | #9

    There is a difference between the 2. Cellulose is better in my prospective do to the fact fiberglass is twice as harmful to your body. Not to mention cellulose washes off easier than fiberglass.

  10. GBA Editor
    Martin Holladay | | #10

    If you have lots of ducts in the attic, then you are thinking about insulating the wrong area. You don't want to insulate your attic floor; you want to insulate between your rafters -- or, better yet, on top of your roof sheathing.

    Bringing the attic into the home's conditioned area will address your moisture problem; it will make your attic dryer.

  11. Steve El | | #11

    Regardless what you do, I'd try to stop the water vapor from getting in there in the first place. So first things first..... get a couple pair of chem gloves and a bucket of duct sealant and go slop mud on your duct connections to seal them. Watch those sharp edges. For really clean and straight spots maybe use metal tape but make sure its UL listed. Next up, read about air sealing against stack effect and do that too. It's messy, but not complicated. Finally, pay attention to insulation and weatherstripping at your attic hatch. There are articles about all these online. I would do all those things regardless what approach you take to insulating.

    For what its worth, I'm just an interested homeowner, not a pro. But I'll babble a bit anyway. it might be possible to build "retaining walls" out of scrap just where your ducts are, and fill the space between the walls with cellulose so the ducts are still properly insulated to your desired R value. The duct layout in my last house made this the cost effective approach over insulating under the roof decking. "cost effective" means material only. I invested a lot of extra time doing this.

    BTW, where the roof pitch cut into the ideal cellulose depth I insulated those spots instead with closed cell foam (with soffit vent chute). I'm sure the technical experts could poke holes in what I did, but I was very happy to be the last house with snow still on the roof in the spring, and the heating bills went way way way down

  12. Steve El | | #12

    my approach wont help with the cathedral part's r value I don't think. also you may not have clearance in your crawlspace for what I did, but maybe you can use something

  13. Steve El | | #13

    sorry about the triple post, but I had an afterthought

    Martin, does the approach you suggested alter a homes heat-load calc? If the attic becomes conditioned space can that cause a change size furnace or AC a house needs?

  14. jbmoyer | | #14

    "For what its worth, I'm just an interested homeowner, not a pro."

    Then perhaps you shouldn't be giving out advise.

  15. Riversong | | #15


    Fiberglass does not wick moisture - it can hold moisture by adsorption (surface bonding) but after a certain concentration, it merely drains to the lowest point.

    Cellulose, because it's highly hygroscopic (like the trees from which it's made), does wick moisture extremely efficiently. That allows it to quickly redistribute moisture to minimize local concentrations, and it even draws moisture out of wooden framing, thus protecting it from saturation and consequent mold and decay problems.

    Cellulose, unlike any other insulation, can safely absorb and release up to 30% of its weight in water, so it creates a diurnal moisture buffer and can even store excess humidity seasonally until release. The boric acid used as a fire retardant is also an excellent fungicide and insecticide, so few things will live or grow within it (and rodents don't like it either).

    However, ducts don't belong in an unconditioned attic. They not only leak heat and coolth and moisture but also create pressure imbalances in the conditioned space which can exacerbate heat loss and moisture movement. At a minimum, they must be well sealed and insulated.

  16. J99aAMQzYo | | #16


    I know you mention the 30% factor for cellulose to absorb & release moisture fairly often. Where does that number come from? Was there a report or study done that determined this? I'm not doubting you, but before I provide this info to a client, I just want to know that I would have a source from which to back-up the claim if they asked for it or challenged it. I tried a couple of Google searches and couldn't come up with anything. If there's a link or PDF you can share that would be great.

    On a side note, I know you mention that the boric acid is "completely non-toxic." I had a meeting recently with a couple of scientist from the EPA and they're telling me that they have some concerns that borates may be acting as endocrine and reproductive inhibitors. They weren't able to show me any final data yet, but they thought it was worth putting on my radar screen so I thought I'd pass it along.

  17. GBA Editor
    Martin Holladay | | #17

    Steve El,
    Of course, bringing an attic with the conditioned space of a home increases the home's conditioned volume and increases the area of the thermal envelope. Although these factors theoretically increase the heating load and cooling load slightly, their effect is overwhelmed by energy savings attributable to bringing the attic ductwork within the home's conditioned envelope.

  18. Steve El | | #18

    Thanks Martin, that is pretty obvious now that you got me thinking about it.

  19. Riversong | | #19


    Wood fiber (as in trees or structural lumber) becomes saturated at approximately 28%-30% MC by weight - beyond that the water is "free" or within the lumens (voids). Similarly, finely ground wood fiber in the form of cellulose insulation can reversibly absorb 30% of its weight in water. At 3+ pcf cellulose density, that means a pint of water per cubic foot of wall cavity.

    The 30% figure I use is almost certainly a very conservative number, since damp-spray cellulose is installed at 25%-35% MC (see - scroll down to Installation).

    "A target of approximately 30% moisture content by weight is appropriate. Freshly sprayed cellulose should feel damp, but you should not be able to squeeze water out of a handful if you tried. As the sprayed cellulose insulation dries it stiffens and is very resistant to settling. Sprayed walls should be left open until the Moisture Content (MC) of the fiber drops below 25%."

    - from Cellulose Insulation – A Smart Choice, Department of Building and Construction Technology, UMass Amherst

  20. Laura | | #20

    Thank you for all of your expertise. We decided to use cellulose. Unfortunately, we don't have it in the budget to use spray foam to make the entire attic a conditioned space but we think we should still see an improvement with sealing duct work, using baffles and soffit vents. Tomorrow we blow cellulose in there. I'll send you an update after the work is done. I appreciate all of the dialog for an attic since everything I found was for walls.

  21. Riversong | | #21


    When you talk about "adding soffit vents", does that mean there are already existing exhaust vents, either ridge, gable or roof "mushrooms"?

    Intake vents are useless unless they are balanced with the same net free vent area of exhaust vents, which ideally are as high as possible and continuous (as in ridge venting). It's important not to have more exhaust area than intake area, since that will create negative pressure in the attic and exacerbate any ceiling or supply duct leakage.

    A rule of thumb vent ratio is 1sf of net free vent area (split equally between bottom and top) for each 150 sf of ceiling.

  22. Laura | | #22

    yes, I agree. We had a few mushroom/gable vents but never any soffit vents, so we changed everything. When we did the new roof last summer, we deleted the mushroom vents and made a ridge vent. Now, we are adding soffit vents with baffles.

  23. Jaxson | | #23

    I am going to blow insulation inside an existing interior wall. Home Depot has either Atticat expanding fiberglass insulation or green fiber cellulose. Any reccomendations? The HD guy was pumped that the Atticat fiberglas was the better of the 2 by far. He said that the cellulose will settle and will be not as effective in a few years as opposed to the fiberglass which will be all set forever.

  24. GBA Editor
    Martin Holladay | | #24

    I would choose the cellulose. Be sure that you rent a blower capable of dense-packing. You want to install the cellulose at a density of at least 3 lbs. per cubic foot -- 3.5 pounds is better -- to minimize settling.

  25. Steve El | | #25

    FYI, I've asked about blowers at Home Depots in three different states. None of the stores had high density blowers.

  26. West | | #26

    We are dealing with a low-frequency noise problem in a condo and are not sure whether it's caused by bass or something mechanical. For soundproofing purposes, is the denser cellulose insulation better than the expanding fiberglass insulation? If so, where might we get these high-density blowers? Thanks for any advice.

  27. Riversong | | #27

    Greater density usually means greater sound deadening. Dense-pack cellulose is an excellent sound attenuator. You can often rent cellulose blowers at a lumber yard or tool rental shop.

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