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Old 06-08-2018, 01:39 PM   #1
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"Dense Pack" Cellulose in Attic w/ "Cold Roof"


Building a new home, and was considering insulating the attic myself. I was planning on keeping the roof sheathing un-insulated, with ridge and rafter vents for airflow.

I know I won't truly achieve "dense pack" with one of the blowers rented from HD. But if I calculate how many bags of insulation I SHOULD need to hit my target R-value (60), and I actually blow it all in, does this even matter? So I know to cover the 1800 sq ft of the attic with 16" of "dense pack" I'll need 200 30lbs bags or so. If I just blow all that in, even if the actual height is 30", do I really care (besides ensuring the rafter vents aren't covered). Or am I missing something?
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Old 06-08-2018, 01:58 PM   #2
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


I don't have a lot of experience with using cellulose but I do energy audits (retired) and in an attic we would use the depth of the insulation multiplied by some standard R-value per inch. Looking at my "Residential Energy 4th edition" I'd use about 3.5 per inch. One can go the more complicate route as you are suggesting but you need to be sure your local authority is up to speed.

Since you would be well above the minimum code required I think you are correct, it doesn't matter to the inspector. From my point of view the energy efficiency would be very good with that much insulation. The only conflict would be if you were wanting to obtain some form of certification and I've been away long enough to not know what is out there.

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Old 06-08-2018, 05:58 PM   #3
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


"Dense pack cellulose" is done in closed cavities, like walls, and goes by the weight of cellulose per cubic foot. You're not doing that. If I'm understanding you situation correctly, you're blowing loose fill and like Bud says, what counts is the depth of the cellulose. It really doesn't matter that much what blower you use for an open attic.


HOWEVER, usually the blowers you get at HD are pretty wimpy, and 200 bags is a lot of cellulose to blow. If you have rental yards around, see if you can get a bigger blower. We used to have a Force II, and in an open attic situation, it could blow one bag of cellulose in 45 seconds! Before that we had tried a blower from Menards, and we were days blowing an attic.
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Old 06-08-2018, 07:02 PM   #4
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


Just to comment, before you blow in that cellulose check out Roxul. I did a 4,000 ft² attic with 6" batts to fill flush to top of the joists. I uses batts not rolls and the job went fast and was very tight. The owner was going to add a second layer across the tops. Not sure how tick Roxul is currently available, back then 6" was my thickest choice. But the stuff is dense with other good properties. Probably more expensive than cellulose but IMO, better.

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Old 06-09-2018, 06:37 AM   #5
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


Wow, 6000 pounds of attic insulation.
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Old 06-09-2018, 09:26 AM   #6
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


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Originally Posted by Gregsoldtruck79 View Post
Wow, 6000 pounds of attic insulation.

I did a commercial job once that had IIRC 24" of blown fiberglass in the open attic. The architect specced fiberglass because it's lighter. He said that cellulose would have been too much weight for the drywall lid.



Code there (northern MN) is 16" of cellulose, which I have done many times without a problem.
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Old 06-09-2018, 12:17 PM   #7
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


I haven't done the diminishing returns calculations in awhile but they aren't difficult, but the added savings for going from 16" to 24 or 30 or whatever decreases to where the extra cost is money better spent elsewhere.

Roughly, 1800 sq ft of attic at R-49 code min for most of NH would require 44 gallons of fuel oil per year. Repeat the calculation with R-60 and it is 36 gallons. The increase in insulation from 4-49 to r-60 reduces the heating cost by 8 gallons. At $3 per gallon that is $24 per year. So, how many years will it take to pay back the added cost of the added insulation?

I made some assumptions but you get the idea.

Bud
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Old 06-09-2018, 05:05 PM   #8
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


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Originally Posted by Bud9051 View Post
I haven't done the diminishing returns calculations in awhile but they aren't difficult, but the added savings for going from 16" to 24 or 30 or whatever decreases to where the extra cost is money better spent elsewhere.

Roughly, 1800 sq ft of attic at R-49 code min for most of NH would require 44 gallons of fuel oil per year. Repeat the calculation with R-60 and it is 36 gallons. The increase in insulation from 4-49 to r-60 reduces the heating cost by 8 gallons. At $3 per gallon that is $24 per year. So, how many years will it take to pay back the added cost of the added insulation?

I made some assumptions but you get the idea.

Bud

I wouldn't argue with that calculation Bud. I only did it because it was in the spec book. The architect was into super insulation. Funny thing was, with all that insulation, the building was always too hot--the boiler system always circulated hot water (every radiator had its own thermostatic valve) and that residual heat was enough to heat the place much of the year. So they had to run the AC to compensate! Poor heating system design from a very egotistical engineer. I didn't have anything to do with the plans, and it was the kind of setup that had a spec book, developer, architect, and engineer and I felt like I couldn't so much as fart without calling for approval.



Loose fill insulation is like snow, the more you add, the more compress the lower layers. Not sure how that factors into the r value.
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Old 06-09-2018, 05:17 PM   #9
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


The home you describe is far from typical and to say you don't agree with my calculations is really a poor fit. The numbers and the calculation I used are standard for the majority of homes in America. I can get more specific if someone needed but just wanted to offer a basic example of diminishing returns where at some point it becomes impractical to keep increasing the insulation.

Sorry, but the numbers are correct. Glad to look at your calculations if you would like to post them.

Bud
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Old 06-09-2018, 05:46 PM   #10
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


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Originally Posted by Bud9051 View Post
The home you describe is far from typical and to say you don't agree with my calculations is really a poor fit. The numbers and the calculation I used are standard for the majority of homes in America. I can get more specific if someone needed but just wanted to offer a basic example of diminishing returns where at some point it becomes impractical to keep increasing the insulation.

Sorry, but the numbers are correct. Glad to look at your calculations if you would like to post them.

Bud

Gee Bud, reread my post. I said I wouldn't ARGUE. That means I agree with you that 24" of cellulose is likely folly.
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Old 06-10-2018, 05:42 AM   #11
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Re: "Dense Pack" Cellulose in Attic w/ "Cold Roof"


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Gee Bud, reread my post. I said I wouldn't ARGUE. That means I agree with you that 24" of cellulose is likely folly.
My apology, my bad. I think I read that as "won't agree". Thanks for the correction. The little hints of old age (or bad eyes) keep creeping in.

Bud
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