Attic Insulation: 8" Cellulose Vs. 5" Fiberglass? - Insulation - DIY Chatroom Home Improvement Forum

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Old 03-21-2012, 05:48 PM   #1
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Attic Insulation: 8" Cellulose vs. 5" Fiberglass?

Hey all,

I have a question on attic insulation. I'm looking to add insulation to my attic and wanted some professional quotes to see what I needed to do and see what it would cost to just pay someone.

Our master bedroom\bathroom on the second floor has an arched ceiling that is insulated in the attic, however it's always 5-6 degrees cooler in the winter and 5-6 hotter in the summer up there so I think I need to add more.

I had one contractor quote me 8" of Cellulose to get about an R38-R40 level and the second tell me I need 5" of Fiberglass blown in to get R38-R40. I was reading that fiberglass is about R2 per inch and cellulose is R3.5 or so per inch.

I have about 6-10 inches already with about an R19 rating that is the original from when the house was built 19 years ago. (I had an energy audit done on my home about 2 weeks ago).

The prices are nearly identical...

Wouldn't the cellulose be the best option? More R is better right?

I also have an attic fan that one of said is a huge energy waster because it's not insulated and is essentially a big hole. He recommended R-19 unfaced fiberglass bat insulation on that. This makes sense unless I want to use the thing and I have to move the insulation.

Any recommendations are appreciated as I am confused as to what to do now.




Last edited by fg12351; 03-21-2012 at 05:54 PM. Reason: added energy audit info.
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Old 03-21-2012, 06:27 PM   #2
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I'm not a professional, just a homeowner.

Having said that, your quotes don't make sense to me. 5 inch of fg isn't going to give the same r-value as 8" of cellulose. Cellulose will settle some but it still will perform better than fg.

What climate are you in? You need to air seal your living space to your attic first air sealing helps manage convective energy loss. Insulation will help manage conductive energy loss.

Once your attic is properly sealed from the conditioned space, you might be able to keep your attic fan to facilitate with moisture migration (i.e. prevent condensation, frost, mold, ice damming). Most people seem to think passive attic ventilation (usually ridge & soffit vents) is the best.


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Old 03-21-2012, 07:17 PM   #3
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I am in Indianapolis, IN.

I have all new soffits on the house.

I agree, the quotes are confusing. I think more would be better in this case but I agree I should check for air leaks.
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Old 03-21-2012, 07:31 PM   #4
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Both have roughly the same R-Value per inch.

That being said, the cellulose is the better insulator in every capacity.

Just get a premium blown in cellulose that is borate only stabilized.
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Old 03-22-2012, 03:43 PM   #5
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to add an explanation, fg batts are poor for attics (especially. they are not great ANYWHERE. OK, but not great) because any air currents will pass through them and carry away the btu's (but leave behind the bugs and rodents that like fg).
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attic insulation , cellulose , fiberglass , r value

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