Our Crew Chief asked me this question today:


I have an attic with R-25 blown fiberglass insulation.  Is there any reason I should not install blown cellulose on top of it to bring the attic up to R-38?


I can't think of any reason.  Does anyone out there know of any reason not to install blown cellulose on top of blown fiberglass?



George Kopf

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If the mouse enters the attic from the fiberglas side (the living space side, in this example), the boric acid will not touch those little mousey feet; a hungry cat would be better than a cellulose layer on top that keeps the mice warm in winter and cool in summer.


If, however, the critters arrive from the top, then the cellulose primary fire protection ingredients (boric acid and aluminum sulfate) burn Mr and Mrs Mousie's little feetsies, and they'll exit the way they entered, usually a soffit vent or a hole in the facia. So as Ed M has pointed out, the air sealing effort is beneficial not just from the air leakage standpoint (and it IS) but also from the critter prevention perspective.


Best solution: air seal attic floor; feed the cat very little during cool autumn days, blow cellulose over the too thin layer of fiberglass, and here's one for the record book, leave a CFL in the attic ON, maybe two if there are corners, in order to inform the critters analyzing your attic that 'there's something happening here.'


Two 13 watt CFLs ON  24/7 will cost $2.25 a month at $0.12 a kwh; that's cheap compared to removing squirrel, lizard, bat, 'possum, raccoon, grizzly bear and mousie poop . Plus there's savings in cat food!

If you look @ the a cellulose manufactures web site (applegate, I'm not associated with them) you can see fiberglass losses 1/2 of its R-Value at temperature extremes ! Fiberglass can also be "fluffed" by turning the air up on the machine, cellulose cannot. Cellulose is denser and it will air-seal the area you cannot get to once it settles (usually about 1/2").

I have only blown cellulose for 28 years for these reasons... Hope this H.E.L.P.s

Ed: This discussion makes me think that you were right in 2008: having an article explaining the heat transfer mechanisms in fibrous insulation would be helpful. It is tempting, but time is a problem.

But think of the fame and glory.  I have seen many articles quoted by either glass or cellulose proponents saying how bad the other is (see above) but I don't trust any of them.  And the ASHRAE 518 (if I recall correctly) standard is just for heat transfer - no radiant or convective component - at a specific delta T.  So something definitive is needed.

Ed Minch

Ed M.: Thanks for the comment. My reply was directed at Ed Voytovich.


Ed V. approached me after a talk encouraging me to write a white paper on this. It never happened... Ed V.: from the many instances when this became an issue recently, you were right about the need.



Air seal first.  Find top plates and penetrations, spray with 2 part.  


Where are you?  Why would you only go to R-38?  Remember, code is building the crappiest building legally allowed, why would you just go to code?  Seems an "always behind" way to do things.


Cellulose is cheap!  Since you are there, might as well go to R-60 and really bang it home.  



tedkidd makes a good point about exceeding code requirements from the time value of money and time value of time issues. Once you've air-sealed the attic floor, shopped for cellulose, brought it to the site, set up the blower, remote, lighting, ladder, baffles and are blowing old news, WHY NOT do a bang-up job and reduce the transmission of warmth, coolth and sound to absolutely negligible?


The Economist in us will reply something about the diminishing return of investment somewhere between 6 and 12 inches with current rates, but the Historian points out the rising costs of fuels and the relative ease of continued delivery of fluffy stuff now that we're here delivering the goods, and then the Psychologist concludes that just for peace of mind if not for immediate financial ROI, the extra ten or twenty-three bags and one hour or so will deliver a minor gain in comfort, a tiny dollar savings down the long road but a heap of satisfaction knowing that we did actually kick some ass up there and that a U-value of .016 is WAAY better than a U-value of .026!!


But selling it and billing it to others won't be quite the same as doing your own house. Still, it's worth a try to at least present an option. Marketing experts would suggest a name like the "The Alaska Attic" or "The Denali Option" or perhaps "Max Comforter" 


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