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Old 11-10-2011, 10:22 PM   #16
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Attic Insulation: Should I use fiberglass or cellulose?


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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Old 11-10-2011, 10:36 PM   #17
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Attic Insulation: Should I use fiberglass or cellulose?


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Originally Posted by Windows on Wash View Post
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.
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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Old 11-11-2011, 10:03 PM   #18
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Attic Insulation: Should I use fiberglass or cellulose?


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/sickhou...rglassMold.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=3kL...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/m...6/ai_n8582994/

Especially if you skip the most important step in insulating---------- air sealing http://www.jlconline.com/cgi-local/v...170a32100a05c7

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/docum...ty-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: Blown in insulation question?


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


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Old 11-12-2011, 10:04 PM   #19
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Attic Insulation: Should I use fiberglass or cellulose?


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.

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.

Last edited by mnp13; 11-12-2011 at 10:11 PM.
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Old 11-13-2011, 01:35 AM   #20
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Attic Insulation: Should I use fiberglass or cellulose?


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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Old 11-14-2011, 12:10 PM   #21
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Attic Insulation: Should I use fiberglass or cellulose?


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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