Weight of Cellulose in Attic
I am planning to have a contractor out to blow cellulose in my flat roof attic. Currently I have plaster ceiling underneath the attic and I also have a furnace with ductwork in the attic. I’m not sure if the attic is strong enough to handle the weight of cellulose with r-38 requirements. I am in Zone 4a.
The contractor told me that he blows in 2.5 pounds per sq ft. I was looking online at cellulose fact sheet and it says the minimal weight per sq ft is 1.16 @ r-38. I think I would be okay with 1.16 psf but not 2.5 psf.
I am wondering why is there a difference of 1.16 to 2.5 for same r-38 value. I would think all the machines blow the same amount of cellulose in the attic.
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The structure itself, the framing, is unlikely to have any problems with the weight of the insulation. To put this in perspective, a 4x8 foot sheet of 1/2" ultralight drywall weighs about 44 pounds. that works out to about 1.375 pounds per square foot. A 4x8 foot sheet of 5/8" type X drywall is about 70 pounds, which is about 2.19 pounds per square foot. Either of these can be hung as a ceiling without much worry about structural issues.
The load on the ceiling material itself is where you need to be more concerned. 1/2" drywall is typically good for 1.3 pounds per square foot, but it depends on support spacing (usually 16 or 24 inches). I would expect wet plaster over wood lath to be able to safely support quite a bit more than that, but I don't have any specific numbers for you.
Weight for blown insulation is typically given in pounds per square foot for a given R value. For R38, Insulmax (as an example since I had their data sheet handy) gives 1.14 pounds per square foot as the weight of the material, and an installed depth of 12.33 inches. Your contractor should be using published data for the type of insulation you're having installed. the Insulmax datasheet I have maxes out at R60, and says that will be 1.87 pounds per square foot.
I'd recommend you have your contractor double check his numbers.
Thanks a lot Bill! You’re very knowledgeable!
Underneath the plaster and lath ceiling, I also have a 1/2 inch drywall ceiling connected by 2x4s to the outside walls (not to the plaster ceiling). I guess this was created because the plaster was cracking and/or may have lead in it.
The contractor said it would be easier to blow cellulose in the attic where the plaster and lath flooring is. I am just nervous about the structural issues. Adding more weight (~1,200 lbs) to the house.