[Greenbuilding] Weight of spray cellulose

[Sacie Lambertson]

I have a situation where cellulose will be sprayed under an existing attic
roof.  Thanks to a suggestion on this forum we are making ply gussets to
extend the existing roof rafter to allow for at a minimum of 11 " of dense
pack cellulose to be blown through netting against the spray foamed under
roof.

My question is how heavy is 11" of sprayed dense cellulose?  At the
perimeter of the space, where the extended rafters meet the floor joists
over the rooms below, will the cellulose sag so much it will create a
potential bulge problem on the drywall that will be covering the ceiling
below?

The floor joists of the attic are old full 2x4s 16" apart.  Where the
potential sag might occur therefore is a space that is about 14"x11".  The
suggestion has been to take attach thin 2" wide plywood across the space,
thus bridging it so the dense pack cellulose won't weigh too heavily and
protrude into the ceiling space below.

However if we do this, we will obviously have to cover the entire ceiling
this way before we put up drywall.

(The unseen dilemmas of rebuilding a very old house)

Thanks for all good opinions.

Sacie
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