Blown-in cellulose In attic above garage ceiling
I will be installing air conditioning in my garage, but first would like to insulate the attic above my garage ceiling, which is currently not . I would also like to install a plywood floor over some of the area, which will be used as storage space. It looks like blown-in cellulose is the best way to go. The insulation company has offered to add either 6.5″ for R19 or 10″ for R30. The problem is that I only have a cavity depth of 3.5″, as I have trusses with 2X4’s (24″ spacing).
My question is whether to ask them to blow in only 3.5″, or to overfill the cavity and lay the plywood on top, thus compressing the insulation. The best thing I can find is the attached chart. It seems to be saying that 10″ of loose fill will produce the equivalent of 3.5″ of dense pack insulation, which would result in R-13–about the same as 3.5″ of loose pack. So I should have them just blow it in to the top if the 2X4’s, and not overfill.
Is this correct, or am I missing something? Advice would be appreciated.
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You are right that filling to 10" and compressing to 3.5" will leave you with performance not much better than filling to 3.5" in the first place. But rather than accepting that, build some support for your plywood 10" above the drywall. Have them fill to 10" and then put the plywood on those supports.
You can build up a deeper floor cavity as Charlie suggests but depending on your temperature delta between garage and exterior, (climate zone?) the additional R value may not be worth the work. Make the ceiling is as airtight as possible. Pays dividends faster than the extra framing and fluff. If your soffits and ridge are vented, don't forget insulation baffles in each bay at the eaves.
Thanks for the replies. I'm in west-central Florida (Venice). I'm thinking I should just cut down the storage area somewhat, go with the 3.5 underneath the plywood, and blow 10" on the rest. I also have a well insulated garage door, so that should help.