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I currently have fiberglass batt insulation of about 6 inches (in some spots less than that closer to 3 inches) due to a cleanout necessitated by rodent activity which has been hopefully remedied.

I have encountered the following recommendation:

"If you have cellulose, fiberglass above is ok, never let anyone blow cellulose over fiberglass. Batts or blown."

Can you please explain the reason for this and what is the best choice to put over the fiberglass batt. My research concludes that blown-in cellulose would provide better overall insulation vs. fiberglass for both temperature and sound factors. I am also on a slope and more exposed to fire risk and the cellulose has a borate fire retardant whereas I would not get that protection with fiberglass.

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  • Makes no sense to me; fiberglass installed over an irregular layer of cellulose would have air gaps all over the place. I'd remove the fiberglass and just do cellulose (that's what I did in my own attic three years ago).
    – Daniel Griscom
    May 19, 2016 at 13:07
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    I'm not going to offer this as answer because of the phrasing of your question, but I believe a reflective radiant barrier will be your best option.
    – bigbull15
    May 19, 2016 at 14:57
  • No reasoning is given with that quote, so I disregard. I'd add cellulose and keep the fiberglass. Adding another inch or two of cellulose to account for compression of the fiberglass is probably cheaper (and much easier) than pulling out all those dusty batts, presumably through your house.
    – isherwood
    May 19, 2016 at 15:43

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It's the opposite, in fact: in terms of thermal performance, cellulose is better on the top layer. The reason for this is that compared to fiberglass, cellulose is more dense and more opaque to infrared radiation than fiberglass is. Most attic heat gain is caused by infrared radiation, and convection through the insulation plays a strong role in heat loss. Having a product on the top layer that's denser and more IR-opaque helps to prevent convection within the insulation and slows infrared radiation more.

Cellulose is fire-resistant because of the borates, but by no means fireproof. It is still paper. I've heard of attic fires caused by smoldering electrical work done wrong but buried in insulation. Fiberglass of course is basically fireproof until the fire is so hot that it causes the fiberglass to melt, which is somewhere around 1,100 degrees Fahrenheit. Much less risk.

A superior product compared to both is mineral wool. It's got all the advantages cellulose has over fiberglass (in fact, it's even better), but it's inorganic and thus fireproof. You'd need to install this product in batt form, since it doesn't generally come loose like cellulose and fiberglass. And it is much more expensive than those. But if you want optimal thermal performance as well as fireproofness, it's hard to beat.

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  • In the event of a fire, cellulose will burn very, very slowly, while fiberglass will melt quite quickly. Fiberglass can't start a fire, but it will allow it to finish much faster than cellulose will.
    – Zhentar
    May 19, 2016 at 15:05
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i am not sure entirely what your post is asking, but if you are looking for a reccomendation, choose mineral wool fibre (like roxul) first, fiberglass second, and cellulose last.

mineral wool doesnt hold water, get eaten by bugs, burn or lose its loft

fiberglass holds water, doesnt get eaten by bugs, doesnt burn, and loses its loft fairly slowly

cellulose holds water, will burn (thus the retardant), will get eaten, and loses its loft fairly slowly

hope that helps

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  • What bugs eat cellulose, in your experience?
    – iLikeDirt
    May 19, 2016 at 12:41
  • i dont know - i am not an entymologist, biologist or mycologist. however, anytime we do a place with cellulose insulation (very uncommon here in ontario), there is always significant breakdown of the cellulose. this has to be from something biological, unless i am mistaken.
    – personal privacy advocate
    May 19, 2016 at 12:54
  • Interesting. "Breakdown" how? Into pure dust? I know from personal experience that termites won't eat the stuff. It's the borates, but perhaps the borates leach out over the years? How old is this cellulose? Is it really ancient stuff?
    – iLikeDirt
    May 19, 2016 at 14:14
  • age wise, i would say generally any cellulose we see is 20yrs plus old (nobody has used it here for years due to our building code requirements - its still available, but nobody uses it because of the mold, settling and weight issues). we dont really have termites here as an issue, so the degradation we see must be something else. could be fungal or microbial. could be simple time based disintegration. either way it pretty much just decays into powder.
    – personal privacy advocate
    May 20, 2016 at 3:38

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