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?

 

Thanks!

George Kopf

Views: 9239

Reply to This

Replies to This Discussion

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)

George,

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

George

nope

 

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 

RSS

Featured Forum Discussions

Energy auditing as a stand-alone business model...

Started by Rob Madden in General Forum. Last reply by Kent Mitchell 16 minutes ago. 11 Replies

Too many BTU's. Too much horsepower?

Started by Steve in General Forum. Last reply by Eric Kjelshus on Saturday. 4 Replies

Stack/wind pressure and flow networks.

Started by Daniel James Grundy in Training. Last reply by Daniel James Grundy on Thursday. 5 Replies

BDT with vermiculite in hollow CMU walls?

Started by Brad Cook in General Forum. Last reply by John Nicholas on Thursday. 2 Replies

Videos

  • Add Videos
  • View All

Latest Activity

Kent Mitchell replied to Rob Madden's discussion Energy auditing as a stand-alone business model...
"Swiftsure Energy has run as consultants, verifiers and raters for 8 years- we audit very few (about…"
16 minutes ago
Horace Douglas Hunt, Jr. commented on Home Energy Magazine's blog post The Elephant in the Room
"poignant and timely story!"
2 hours ago
Horace Douglas Hunt, Jr. liked Home Energy Magazine's blog post The Elephant in the Room
2 hours ago
Home Energy Magazine's blog post was featured

The Elephant in the Room

Last week I attended the annual Home Performance Coalition (HPC) Conference & Trade Show in…See More
9 hours ago
Lucifer Prescott is now a member of Home Energy Pros
11 hours ago
Home Energy Magazine posted a blog post

The Elephant in the Room

Last week I attended the annual Home Performance Coalition (HPC) Conference & Trade Show in…See More
19 hours ago
Diane Chojnowski replied to Quinn Korzeniecki's discussion Don MacOdrum Receives BPI's 2017 Tony Woods Award and Four Others Inducted in to Hall of Fame in the group Building Performance Institute (BPI)
"Congratulations Mike! Thanks for all you do for the home performance industry!"
21 hours ago
Sarah OConnell's blog post was featured
21 hours ago

Home Energy Pros

Welcome to Home Energy Pros – the unique digital community by and for those who work in the home energy performance arena.

Home Energy Pros was founded by the developers of Home Energy Saver Pro (supported by the U.S. Department of Energy) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network. Please honor our Guidelines

© 2017   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service