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Attaching Insulweb to wall penetrations and preventing cellulose blow-out

Oak Orchard | Posted in General Questions on


(1) My ceiling: 24″ o.c.12″ deep rafters/cathedral; I have already installed 1/2″ low-sag Sheetrock glued and screwed horizontally.

(2) My walls: 24″ o.c. Larsen trusses.

(3) No top plates; Larsen truss wall bays pass directly up into the rafter bays. It’s a cantilevered pole construction; building load hangs on 3/4″ rod attached to poles (i.e. 6 X 6 square timbers)

(4) No basement: plumbing and heating PEX, wiring, etc. are in the walls; Stelrad radiators used for heating.

Insulation to be Dense-packed cellulose blown behind ceiling drywall; while on walls, we will blow through Insulweb.

We have generic plastic and metal electrical along with some gasketed electrical boxes in ceilings and walls. The wire penetrations in the boxes are sealed by spot spray foam applied on outside of boxes. When blown, the the dense-packed cellulose will pack up tight to and around the boxes; as well as around the supply and return copper nipples feeding the radiators


1. How should the Insulweb be attached to/sealed to the electrical and pipe nipple penetrations where there are no surrounding framing members? (Otherwise it will be glued and stapled on stud faces.)

2. How do I prep and seal the drywall gaps and spaces so my dense-packed cellulose will not blow out during installation and so it will not leak out later?

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  1. GBA Editor
    Martin Holladay | | #1

    Oak Orchard,
    I just spoke to Bill Hulstrunk, the technical manager at National Fiber.

    Here's the story: you don't have to worry about holes or cracks -- even cracks up to 1 inch wide. (If you have a really big hole, you can staple up a new strip of InsulWeb, from stud to stud.)

    The insulation contractor will fill the stud bay from the bottom up to the middle, and from the top down to the middle, dense-packing as he goes. When the hose nozzle gets close to a big hole or crack in the InsulWeb, he might temporarily put his hand over the hole to minimize the amount of cellulose that blows out. But not much will blow out. Once the bay is full and dense-packed, the cellulose won't fall out.

    In any case, it all gets covered with drywall soon enough.

  2. Oak Orchard | | #2

    That's good news. Haven't read this in any documentation on DP cell installation. This is a relief to DIYers for sure. Sounds like it is ruled by fluid dynamic principles. Blowing cell would be like working with pyro plastic flows.

    Thank you

  3. GBA Editor
    Martin Holladay | | #3

    In case you missed it, there is more information from Bill Hulstrunk here: How to Install Cellulose Insulation.

    After Bill Hulstrunk answered your question, I edited my interview with him to include his latest answer.

  4. Oak Orchard | | #4

    Thanks, Martin

    I read your interview with Mr Hulstrunk's several times and checked it before I wrote my question on this bulletin board. I also used his and National Fibres' on-line articles to convince my building code officer that it's ok not to vent a cathedral ceiling. I like the idea that you are updating that interview and will down load the recent version. I will also post a few more questions on DP cell. Thanks, again.

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