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Dense packing cellulose – height of cavity

gf2011 | Posted in General Questions on

What are the thoughts out there on how tall a cavity in this double stud wall can be before there are concerns?

Here’s the wall:

There’s an outer 2×6 wall, 16″ O.C., and an inner 2×4 wall also 16″ o.c. There’s a 6″ space between the inner and outer studs, for a cavity depth of 15″.

The first floor walls are 9′ tall, with an 18″ deep floor truss above. In other words, there will be a 10.5′ height of cellulose. The second floor is about 8’4″.

Should these cavities be divided with horizontal partitions to shorten the height of each cavity? My plan is to dense pack to about 4 lb per cubic foot, using mesh to connect the inner and outer walls every 32″ (every other bay).

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  1. wjrobinson | | #1

    Put in one partition.

  2. LucyF | | #2

    The single most important thing is how good are the people who are doing the dense packing. It's a somewhat unusual insulation strategy here in SC so the guys aren't great at it. The owner of the insulation company wanted to dense pack at 2.5lbs/ft3 rather than at 3.5lbs/ft3. I had to insist on the higher number.

    I've included a link on Thorsten Chlupp's home in Alaska. He used dense pack cellulose in huge bays, but he is a builder in a cold area and knew he could get the density he wanted.

  3. STEPHEN SHEEHY | | #3

    My double stud walls were insulated a few weeks ago. The outer wall was 2x4, 24"o/c. Then 5 1/2" space, then inner 2x4 wall, also 24" o/c. The air barrier membrane was stapled and taped to the out s use face of the inner wall.

    Some of the walls are 14' high. The cellulose pushed in a few inner studs a good inch out of plumb. We drew them back to plumb by using long screws to connect the inner and outer studs, but it was a bother.

    If your two walls are not separated by a membrane, you'll probably be fine because I think our problem was caused by pressure on the membrane. You could tie the two walls together with screws or gussets, giving up a bit of thermal break. In our normal height walls(9') we had no such problem.

    The other issue was the need to get the insulation sub back because it was obvious that the pack wasn't dense enough. The only way to be sure is to put two holes per bay, at about three and six feet above the floor.

    We had about two feet of cellulose blown on top of the ceilings.

    Once we were done, though, the house was warm and toasty for the workers with just the heat from a single electric 17kbtuh heater and -15°.

  4. gf2011 | | #4

    I appreciate everyone's feedback - thanks.

    Would have been nice to have some insulation this past month (very cold here in Ontario) - but hopefully it will serve the house well for years to come.

  5. GBA Editor
    Martin Holladay | | #5

    Like everyone else who has dense-packed double-stud walls, Thorsten Chlupp has had a few problems. When he spoke last month in Burlington, Vermont, he mentioned that he ended up with voids under the windows on some of his early dense-packing jobs.

    He now leaves enough of a gap between the two stud walls that a person can stand between the walls, and he sends a worker into the gap to walk on the cellulose as it is being blown in. The idea is to walk around and jump up and down to help pack it down.

    Thorsten said, "It's not a fun job, but someone has to do it."

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