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Dense Pack Cellulose – Problems Using Insulweb?

user-6154877 | Posted in General Questions on

I’m preparing to spray dense pack cellulose between my roof rafters for a cathedral ceiling

The cellulose will be sprayed between the ceiling drywall and the roof sheathing from above, since the roof is being re-done.

Since the existing sheathing has already been removed to rewire and repair, I was looking at using insulweb for the spraying, instead of through holes in the sheathing. Seems to make more sense since it will allow visual inspection of the fill

I have a couple concerns

1) Assuming we fill the cellulose correctly, dense packing and bulging the insulweb, will the force later applied by the roller to make the insulation flat to the rafters create enough force to damage the drywall ceiling below

2) Code requires the cellulose to be in direct contact with the roof sheathing. This can’t really happen if insulweb is used since it will be the insulweb against the sheathing then instead of the cellulose

Are either of these valid concerns

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  1. user-6154877 | | #1

    I should add that there will be rigid foam above the roof deck to control the sheathing condensation, and climate zone is 5

  2. Expert Member
    ARMANDO COBO | | #2

    We used your same application several years a go until we found thru infrared pics that, with time and gravity, the cellulose will settle to the point of creating voids just bellow the sheathing. We also used 5/8 drywall on 16" o.c grid to help support the full cavity fill. Since then we started using open cell sprayed foam and haven't had any issues. It does cost more, but it became a risk management issue at that point.

  3. charlie_sullivan | | #3

    I don't think the insulweb between the cellulose and the sheathing is a problem, assuming the insulweb is stapled flat across the top of the rafters, not to the sides of them. The insulweb is just a very thin layer of insulation that uses polyproplyene fibers instead of cellulose fibers. But I would not want it stapled across the sides.

    Dense packing is important to avoid the settling problem that Armando notes, and the dense pack pressure combined with the weight of the drywall and the cellulose could be too much for the drywall. An expensive but solid solution would be a layer of OSB between the rafters and the drywall as reinforcement. I'm not sure how the cost of that compares to open-cell spray foam.

  4. user-6154877 | | #4

    If using Open Cell spray foam instead of cellulose, this would be 8" of open cell in the cathedral ceiling, the space between the dry wall ceiling and plywood sheathing

    Reading other research, Open Cell foam is air impermeable at 3-4 inches

    Wouldn't this be a problem? Since the solution needs to dry to inside

    Would moisture not get trapped in between the open cell foam and rigid foam layer

  5. Expert Member
    Dana Dorsett | | #5

    The meaning of the code is that there can't be an air gap between air-permeable goods and the roof deck. Insulweb is air permeable. If dense packed to 3.5lbs (as opposed to damp sprayed at a lesser density) there should be no settling. With insufficient exterior R to limit wintertime moisture accumulation there is a higher risk of settling and voids, since seasional moisture cycling is a primary driver of mechanical creepage in cellulose inusulation.

    In zone 5 a minimum of 40% of the total R needs to be on the exterior of the roof deck for safe use only Class-III vapor retarders on the interior.

    The high air retardency/low air permeance of open cell foam is the opposite of being a problem- it's a virtue. With air impermeable foam all convective moisture transport is completely blocked, whereas with fiber (particularly at lower densities) it is not. With open cell foam by far the biggest moisture transfer mechanism is vapor diffusion. While open cell foam is somewhat vapor retardent, at only 8" most half pound foam is still above Class-III vapor retardency, and would need a separate interior side Class-III vapor retarder such as standard latex paint on gypsum board.

    A layer of OSB would be a Class-III vapor retarder, in fact it's better than a Class-III vapor retarder. When the proximate cavity air & room air are relatively dry ( which would be the case in winter) OSB is usually a Class-II vapor retarder, but at moisture levels high enough to support mold it's in the middle of the Class-III range. It's a "smart" vapor retarder.

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