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Old 08-07-2020, 12:41 PM   #1
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cellulose above plaster ceiling


I'm planning to blow ~20" of cellulose into my attic this fall. My attic currently has wooden floorboards - about 3" wide - over 2x4 joists. Between the joists there's about 2" of rock wool. not enough for Minnesota! 3 questions:
1) Should I pull up the floor boards? I can see 2 benefits of doing so: a) I can do a more thorough air sealing job. I'm concerned that various top plates will have holes in them that I will only see if I remove the flooring. and b) without the flooring there will be fewer air pockets (clearly I could blow cellulose between the joists, but it would be harder to see if any spots got missed).
2) *If* I do pull up the flooring, then should I worry that the cellulose will push through the plaster ceiling? I'm 90% sure the answer will be "if the plaster is in half decent shape I'm fine", but just wanted to check.
3) should I dispose of the rock wool or just blow over it?
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Old 08-08-2020, 06:10 AM   #2
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Do it right, do it once. Imagine having to wade through 20 inches of pure dust because your house is still cold and drafty in the winter. It will cost more in time, money, and frustration to air seal after the cellulose is blown.

Now is the time to pull back the boards and batts to airseal the attic floor. If youre going to eventually cover it all with insulation, the boards serve no purpose so i would remove them. Otherwise like you said they will just create gaps between the existing batts and new insulation.

No need to remove the existing batts as they can be blown right over (unless wet, moldy, or filled with feces). Just be sure to pull them back temporarily as you air seal. Keep in mind youll likely find lots of holes in top plates for wiring and the vent stack. Its also not uncommon to find large chases either. I found a 3 foot by 3 foot hole in mine. That was like having a window open 24/7/365.

Only other thought is to thoroughly evaluate your existing attic ventilation and assess what improvements need to be made before blowing cellulose.
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Old 08-08-2020, 07:11 AM   #3
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Re: cellulose above plaster ceiling


Prepare to crack some ceiling joints in the plaster or wallboard if you are prying them up. You could, honestly, remove every other two boards and leave the rest. Move the insulation around, air seal, and then blow new cellulose over the top.
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Old 08-08-2020, 10:22 AM   #4
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Re: cellulose above plaster ceiling


So, sounds like no concern that cellulose will push through the plaster ceiling?
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Old 08-08-2020, 01:21 PM   #5
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Honestly have no idea and suppose it is possible. If i were you and had that concern, I would get the R30 fiberglass rolls, and roll them out perpendicular to the joists. It will be much easier to to move them and reinstall in the future if you ever need to access the ceiling to run cables, wiring, bathroom exhaust fans, etc. On the other hand, doing that with cellulose will be a mess.
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Old 08-10-2020, 02:18 PM   #6
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Re: cellulose above plaster ceiling


Quote:
Originally Posted by Steve in MD View Post
Honestly have no idea and suppose it is possible. If i were you and had that concern, I would get the R30 fiberglass rolls, and roll them out perpendicular to the joists. It will be much easier to to move them and reinstall in the future if you ever need to access the ceiling to run cables, wiring, bathroom exhaust fans, etc. On the other hand, doing that with cellulose will be a mess.

Do you have any sense about how common it is to blow cellulose over wiring? I would think it's quite common? I just re-wired my entire home, so I don't expect to need to touch it for quite a while.



What do people think about r30 bats vs an equivalent cellulose? Cellulose is roughly 3.5/inch, so I guess that comparison would be 8 inches of cellulose vs r30 bats?
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