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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Blown on blown - no (assuming it's in great shape, no rodent issues,etc...)

I would still recommend checking that the air sealing work was done properly before just blowing over it

I would not recommend blowing cellulose on top of fiberglass insulation, as it weighs more than fiberglass and will compress the fiberglass, resulting in an overall reduced R-value. You can though blow fiberglass over cellulose.

While I would normally agree with you (i.e. batt insulation) this is blown FG and it is said to actually improve it's performance (assuming one doesn't go crazy)


That is exactly what you want to do, cover the fiberglass and bring it to R38.




I used to think that the cellulose would keep the blown fiberglass warm and therefore minimize the effect of convective heat loss failure (as per a study by Oak Ridge Nat'l Lab).

But then I also used to think that I'd grow up to be prosperous and handsome.



Thanks - I needed that this morning.  And who's to say you aren't prosperous and handsome?
Thanks to everyone who has replied to this posting.  Very good information and really helped out our Crew Chief!  Thanks!
I could still use some clarification on the subject. I seem to remember reading that fiberglass allows infrared energy through it. It also leaks air.  In my experience I've seen many fiberglass insulation jobs with evidence of rodents yet almost no such evidence with cellulose. That must have some value. Please enlighten me.

George M:  . . . you might want to check out this web site (http://science.hq.nasa.gov/kids/imagers/ems/infrared.html).  Infrared energy is heat.

As it happens, everything leaks heat . . . the laws of thermodynamics insist upon it.  The only exception is the last, unimaginably miniscule hint of heat that makes absolute zero impossible to achieve . . . that last little bit of heat can't be removed because it can only go to someplace colder, and there is no such place!

My heavens this is fun.  Here's a bit of news about Einstein that may be even more fun:  http://science.nasa.gov/science-news/science-at-nasa/2011/04may_epic/.



Ed nailed the infrared & heat part - as for the rodents, it depends on the attic & issues the house has. I dare say you might not notice it as much with cellulose as they love paper & other materials which will blend in more easily. The last house I was at, was all cellulose with about 30 birds nest & way to many mouse droppings, traps,etc... in there 

First check your air sealing.  Wall/ceiling joints, plumbing and electrical penetrations, and anything that is big like flues and dropped ceilings over showers.  If there are ducts, seal the heck out of them and drop them down as low to the attic floor as possible.  Once everything is sealed, asses the condition and R-value of the insulation now that you have mucked around in it.  Then do the math.  In my mid-Atlantic area, R-25 in a natural gas or heat pump house is a loooong payback, but better in a propane or oil house.  If there ar ducts, then it pays to blow over them - and if you are there anyway to blow over ducts, it generally pays to do the rest of the attic.  And how R-38 got to be the magic number I have no idea - each increment at that level means so little.  And when you compress fiberglass, its R-value per inch goes up - not as fast as the number of inches go down, but for what you are doing, the difference is absolutely minimal.  Cellulose has boric acid, a bug repellent. 10 years ago there was a company that would guarantee no roaches if you did the walls and ceilings, but that didn't last long because you can bring roaches in with the groceries and now they can't get out!

Ed Minch 


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