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Old 11-09-2012, 02:24 PM   #1
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Attic Insulation & Sealing planning


I am planning an attic sealing and insulating project and need to make sure I'm not missing anything;

The attic is very simple - some wiring, 2 bathroom fans that I have replaced already with proper 6" insulated ducts and a vent stack. Electric only, no gas, so none of those worries. No other ducting.
Roof is very basic shape. Joists are 24"OC 2x4s (this really rubs me the wrong way). I assume the ceiling is 5/8 but need to check.

Currently there is on average 12" blown in fiberglass, down to less than 4 in some places - specifically under a current catwalk nailed to the joists running the length of the attic (about 33 feet). The access hole is dead center with the catwalks running in both directions to the gables from there.

My plan is to use a rake and leaf vac to move the insulation from the one side of the attic out of the way. Then replace carboard baffles with accuvent and seal up the attic floor. Then repeat on the other side, blowing the insulation to the side I sealed up.
After I'm done with the "other side" I will leave all the insulation on one side and blow in cellulose on the empty side. I hate the idea of blowing cellulose over the fb. I have spent all the time I can take working in the attic inbetween fb. If I blow cellulose over fb, then I'm just mixing the 2. By keeping it to one side I can keep it isolated and potentially in future suck it out for replacement. Currently it is in decent shape.

I did some testing with a leaf vac and it works great but the mulcher "fluff" it up a bit. Should I be worried about the insulation-ability afterwards?

I will have to raise the catwalk and access hole to about 18". Not sure yet how I will tackle this problem. I need to go see what reserve lumber I have and then take a trip to the lumber yard and see what they have.
The attic will not be used as storage. The catwalk will just be so I can get closer the place I need to be when doing anything up there.

I'm also a teeny bit worried about ceiling sagging under the cellulose. I plan to be @ R60 which is about 18" for cellulose and 24" for fb.

3 Recessed light are IC & air tight, but I will build boxes in any case to seal them up for good. Gables will be blocked off with plywood.

What do I need to worry about and what else should I do?

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Old 11-10-2012, 08:01 AM   #2
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Attic Insulation & Sealing planning


You are certainly on the right track.

I personally like a cellulose cap over the blown in fiberglass as it makes that stuff work better. Stops most of the convective air movement.

If you combine the two, you can mitigate the weight of the installed cellulose and still get relatively the same R-Value.

R-60 is probably overkill but you could get there with about 6" of additional cellulose on top of the FG.

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Old 11-10-2012, 11:30 PM   #3
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Attic Insulation & Sealing planning


"I'm also a teeny bit worried about ceiling sagging under the cellulose. I plan to be @ R60 which is about 18" for cellulose and 24" for fb."--------

Cellulose Attic Insulation Question.

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Old 11-13-2012, 10:31 AM   #4
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Originally Posted by Windows on Wash View Post

R-60 is probably overkill but you could get there with about 6" of additional cellulose on top of the FG.
My calculation is as follows: FB = R2.5/" and cellulose = R3.7/"
So 12 FB = R30 and compressed would be somewhat less. That would require another R30+ of cellulose to get to R60.
30/3.7 = ~8+ inches of cellulose.
How do you get 6?
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Old 11-13-2012, 10:42 AM   #5
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Originally Posted by GBR in WA View Post
"I'm also a teeny bit worried about ceiling sagging under the cellulose. I plan to be @ R60 which is about 18" for cellulose and 24" for fb."--------

Cellulose Attic Insulation Question.

Gary
Thanks for the link - The document on the gypsum was new for me. I posted a question in that thread as my interpretation of table 2 is different in that 1.6# applies to wallboard and 2.2# applies to ceiling board.

Regardless my calculation is as follows (assuming I blow the cellulose over the FB):
12" FB = ~0.5# per sq ft (per Atticat spec sheet) and at R2.5/inch = ~R30
To get to R60 I need Cellulose of R30-38.
R38 weighs 1.16# per sq ft

1.16 + 0.5 = 1.66.
So I can safely blow 8-10" cellulose over the FB
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Old 11-17-2012, 01:53 PM   #6
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3" minimum of cellulose will stop any convective loops. 1# of weight is similar to a small water bottle every square foot = compressing the f.g., losing R-value.

Gary
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Old 11-19-2012, 10:12 AM   #7
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Just to close the loop - I finished my project.
First I sealed the attic over the Veterans day long weekend. This was a misrable project! Crawling through fb with a respirator and coveralls (get Tyvek 2sizes too large - others dont last). I installed accuvents in the eaves. My eaves are really small (due to very minimal soffit overhang) and I ended with only about 3-4" at the narrowest over the top plate. I also sealed the outside top plate as best I could prior to - and after installing the accuvent. The accuvent only got me up to ~16" above the ceiling, so I cut addition accuvents in 1/3rds and added the 1/3rds above to get to ~22". Additionally I systematically worked my way through the attic, moving fb out of my way as I go, and sealed all interior top plates and other penetrations.
Next up was the build walls around the access hole and a new catwalk 20" up.
This past weekend I blew in cellulose. I raked the fb away from the eaves. The logic was that seeing as the eaves are down to 3-4" I would rather put a material with R3.7/inch into that space than to go with 2.5/inch. Then I blew in the cellulose over the fb to a total of 18".
Keeping in mind that the fb was not quite evenly distributed (raked away from the eaves), and based on greenfiber numbers, I blew in an average of 12" of cellulose. A rough estimate puts me at about 2" compressed out of the fb and about R50 overall with combined weight of about 1.5# /sq ft.

Lessons learned:
-crawling in an attic sucks
-fb itch sucks ( I knew both these already)
-cellulose makes a LOT of dust
-accuvent is a great product except it is too short in shallow locations
-Home Depot and the blowers they rent is the weakest link in an otherwise easy job of blowing cellulose.

Thanks for all the valuable responses!
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Old 11-19-2012, 11:54 AM   #8
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Attic Insulation & Sealing planning


Quote:
Originally Posted by fstr View Post
Lessons learned:
-crawling in an attic sucks
-fb itch sucks ( I knew both these already)
-cellulose makes a LOT of dust
-accuvent is a great product except it is too short in shallow locations
-Home Depot and the blowers they rent is the weakest link in an otherwise easy job of blowing cellulose.

Thanks for all the valuable responses!
Good job and good feedback.

This is why most folks say to not skimp on the blowers.

Congrats on getting it done though!!


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