# Attic Insulation: Should I use fiberglass or cellulose?



## Windows on Wash (Aug 30, 2011)

Cellulose is much better.

Borate only stabilized cellulose at that.


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## HomeSealed (Jan 3, 2008)

+1.... The real question is, is there any reason whatsoever for you to use fiberglass in your application? The answer is no. Cellulose is less costly, will offer superior performance, and you already have an existing layer... Seems like a no-brainer.


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## Gary in WA (Mar 11, 2009)

Long as your drywall/span can handle the added weight: http://www.energyguide.com/library/EnergyLibraryTopic.asp?bid=austin&prd=10&TID=17324&SubjectID=8375

http://michaelbluejay.com/electricity/insulation.html

http://www.cellulose.org/userdocs/T.../ConsumerUpdate02-InsulationEffectiveness.pdf

http://docs.google.com/viewer?a=v&q..._kPOR4&sig=AHIEtbQGp1pSKFVQrXO3MrFpuTdu1ubwtA

Or f.g, though I'm not a big fan: http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/

Check for your Code required minimum, per location: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec001_par001.htm

http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002.htm

Because.........http://www.buildings.com/ArticleDetails/tabid/3321/ArticleID/6061/Default.aspx

Gary


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## strategery (Jul 18, 2011)

GBR in WA said:


> Long as your drywall/span can handle the added weight: http://www.energyguide.com/library/EnergyLibraryTopic.asp?bid=austin&prd=10&TID=17324&SubjectID=8375
> 
> http://michaelbluejay.com/electricity/insulation.html
> 
> ...


Good info GBR.

Is there a simple way to determine my drywall thickness/span so I can see how much more weight my ceiling can tolerate?


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## Gary in WA (Mar 11, 2009)

½” regular drywall, 24”o.c.--------1.6#,

5/8” board, 24” o.c.-------2.2#,

½” at 16”o.c.---- 2.2#,

 Table #4, page #9: http://gypsum.org/pdf/GA-216-2010.html

Check at the attic access (scuttle-hole) edge. Look in the attic to see spacing of the ceiling joists.


Gary


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## coutain93 (Nov 5, 2011)

*Only roof vents, HELP*

There is a roof vent on both peaks of the house. There is NO insulation in the soffit area along the base of the attic floor. I am going to put pink rolled batting over the existing old blown in. My question? should I put the pink in the soffit areas???l Oh I am a woman age 63 and love to do it myself. I know to remove the paper off the pink batts for the added insulation. IF I have to put in the soffit area do I remove the paper?? Thanks in advance for any and all help. Did the crawlspace floors over the summer, pink down, paper up against the floor!!!


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## Ironlight (Apr 13, 2011)

Do not put insulation in the soffit area or you will block the intake of air into the attic. The peak vents are where the air goes out...the soffit is where the air comes in, provided you have soffit vents, that is.


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## Gary in WA (Mar 11, 2009)

Correct, I agree. Good that you know to remove the facing (if over existing b.i.) *or* the blown-in and reinstall it over the faced batts. You should add baffles to direct the air up the roof sheathing away from the insulation to degrade it's R-value (and the edge of the f.g. batt). http://www.bergerbuildingproducts.com/productsAccuvent.html

Gary


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## coutain93 (Nov 5, 2011)

*insulation in attic*

Thank you for your help. Insulated the attic above the bathroom, pantry and kitchen yesterday. High winds today and colder. Sounds silly to say, but rooms already feel warmer. Bath and Pantry hit directly from the Northside and No wind block. Thanks Again! Carol


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## mnp13 (Jan 16, 2007)

Windows on Wash said:


> Cellulose is much better.
> 
> Borate only stabilized cellulose at that.


Why? If cellulose gets wet, it compacts and basically turns to mud. It also loses the fire-resistance after it dries out. So it's useless and a fire hazard. Fiberglass doesn't absorb water, and if it gets wet, can be "fluffed" after it dries. 

Can you explain _why_ it's "much better"? 

R-30 is R-30 regardless of what is doing the insulation, yes? No?


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## Windows on Wash (Aug 30, 2011)

From a company standpoint, cellulose is much safer for my installers and the residents if they need to get into the attic.

From a performance standpoint, cellulose has a much higher density and therefore does not suffer for the convective looping issue that fiberglass does and the resultant loss of R-Value.

If you have a 20 degree temperature differential from attic to inside living space, blown in fiberglass will only perform at roughly 60% of its indicated R-value once you factor in the convective air looping and striped R-value.

example:
10" of blown in fiberglass is (for the sake of argument) and R30
10" of cellulose is also and R30

20 degree difference makes the fiberglass perform at R18 whereas the cellulose is holding steady at an R30.

I don't think you should choose an insulation based on which one is more resistant to water leaks as that is not its intended purpose.


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## mnp13 (Jan 16, 2007)

Interesting... definitely something to think about there.

I understand what you're saying about the blown in, but what about the rolls of batting? Assuming they are cut and fit in correctly of course. 



> I don't think you should choose an insulation based on which one is more resistant to water leaks as that is not its intended purpose.


Well, when you have to consider what would have to be done after a water leak in the attic (certainly not unheard of) I think it does factor in.

If the cellulose gets wet and dries, it is no longer fire resistant, so once it has been wet it needs to be removed. Now, granted, if fiberglass gets wet, you still have to figure out how you're going to get it completely dry; but once it's dry and assuming it's not compacted, it's back to what it was.


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## Windows on Wash (Aug 30, 2011)

I would base my decision on insulation on which product does a better job of insulating rather than its performance after being wet.

Batts are fine if you get a high density batt but will require a much more careful installation. Fiberglass is also much more of an inhalation and health hazard.

I will defer to GBRs thorough write up on fiberglass and the comparison.

http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/

The post is pretty unarguable in this case.

Whatever product is damaged after a leak can easily be replaced with what will likely be covered by one bag of cellulose.

When we have seen a cellulose that has gotten wet, it is normally dried out by the time the repairs are completed. You can break it back up once it is dry and cover it with new cellulose.


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## mnp13 (Jan 16, 2007)

Thanks! I appreciate your time and input (and that is not "said" sarcastically)


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## Windows on Wash (Aug 30, 2011)

My pleasure.

If I didn't believe is what I posted, I wouldn't argue the point. 

At the end of the day, the people that post regularly on these type forums do so because they want the folks like you to get the best product/installation for your application.

Debate is usually a healthy thing.


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## mnp13 (Jan 16, 2007)

Windows on Wash said:


> My pleasure.
> 
> If I didn't believe is what I posted, I wouldn't argue the point.
> 
> ...


Absolutely. And I appreciate the input, if I didn't want to learn I wouldn't ask the questions... and of course, when the answers aren't what I expect I tend to want to dig. 

(I own a dog forum, where we have the occasional member who wants to hear nothing but "you are doing exactly the right thing" no matter what they are doing or how they are doing it.)


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## Gary in WA (Mar 11, 2009)

Just to touch base on a few points.....

F.g. gets wet, usually the paper backed drywall does also, then molds, unless caught right away: http://www.inspectapedia.com/sickhouse/FiberglassMold.htm

So, you would probably throw away any insulation that got wet by more than diffusion, which cellulose handles much better- dispersing it through all. Fiberglass requires an air-tight surface on all six sides to achieve its R-value- exactly as in a lab, hard to do in an open-to-the-outside air ventilated attic. So don't think you are getting R-? because the top layer of blown-in/batt is exposed to air movement/cold outside air = expect a severe reduction. http://books.google.com/books?id=3kLLZBKa5zAC&pg=PA70&lpg=PA70&dq=Silerbsein+1991&source=bl&ots=1JP4IBaEbX&sig=Bh8KD6UW2SZ74R0QxwqeCjlOVLY&hl=en&ei=6Y8NTdb0FpH4sAPp4LDnAQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CB8Q6AEwAg#v=onepage&q=Silerbsein%201991&f=false

A housewrap would help, but again hard to install 100% coverage. 2" of cellulose over the top or an insulation blanket would stop the convective looping. Any air movement can degrade your f.g. up to 40%, gaps at the batts- up to 60%: http://www.aecb.net/PDFs/Impact_of_thermal_bypass.pdf

Cellulose is air-permeable as is f.g. but it handles air leaks much better: http://findarticles.com/p/articles/mi_m0KWZ/is_3_6/ai_n8582994/

Especially if you skip the most important step in insulating---------- air sealing http://www.jlconline.com/cgi-local/...e.storefront/4d17fa04053d6e9327170a32100a05c7

Cellulose and f.g. are subject to wind-washing near the soffit vents (there should be baffles in place), unless a h.w. is installed to the attic in new construction: http://www.buildingscience.com/documents/information-sheets/installation-of-cavity-insulation/

Good picture of ww: http://www.arc.thermaldr.com/res_pg_3.htm

Another attic-cat……. I seem to be repeating myself, lol. At 0.53# density, it matches the twice failed testing of f.g. insulation, and they haven’t changed it yet…….check your density: http://www.diychatroom.com/f103/blown-insulation-question-122993/


ps.- f.g. is great for finding air leaks from below: http://www.finehomebuilding.com/PDF/Free/021105092.pdf


Gary


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## mnp13 (Jan 16, 2007)

Another question on this - can you "dense pack" blown in fiberglass? 

My joists are 2x12's and someone suggested that I put boards down and fill the resulting "capped" cavities with fiberglass. Or the reverse - put bats in the cavities and then put 4x8 sheets of plywood over them. That would be outrageously expensive, but I'm curious about the resulting R-values. 

All theoretical until I have the attic re-wired, but that's just giving me more time to thing of things to drive myself crazy. :laughing:

And back to cellulose - 10 inches of cellulose won't even make it to the top of my joists, I'd need 12 to come even, and then I'm guessing there is a recommended amount that the joists should be buried for even R-value coverage? Am I going to need to fill this 16-18 inches deep? And if so, that's going to be close to 2 pounds per square foot on the back of my plaster and lathe ceiling. Is that going to crack my plaster over time? Would it make sense to put a barrier (plywood or USB) over the tops of the joists, creating a "floor" and then put down the insulation over that so that the load is spread across the _joists_ and not the lathe? That just seems like a lot of weight for all of those little lathe nails.


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## strategery (Jul 18, 2011)

If I'm concerned about too much weight on the ceiling dry wall is it possible to install BOTH blown-in fiberglass and cellulose? Is it possible for me to blow in about 5 inches of fiberglass and 2 inches of cellulose on top of it for the best performance at the lowest weight?


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## Windows on Wash (Aug 30, 2011)

Dense packing fiberglass will ultimately yield you a very low R-value and if you pack it tightly enough...close to that of glass.

You can blow in fiberglass and they blow in a 5" cellulose cap if you are worried about weight. 

I personally have not observed any issues with the weight of cellulose but that does not mean the potential is not there.


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