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Community and Q&A

Strapping for dense-packed cellulose

415irwin | Posted in GBA Pro Help on

Looking for recommendations on the correct strapping spacing (12″ or 16″) to help prevent bulging with a dense pack cellulose installation on walls. We will be using Insulweb over 8″ cavities, 16″ on center framing.

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  1. GBA Editor
    Martin Holladay | | #1

    Most people who install dense-packed cellulose in a wall cavity don't bother to install strapping; they just staple the heck out of the InsulWeb.

    For more information, see How to Install Cellulose Insulation.

    That article is an interview with Bill Hulstrunk. Here is an excerpt:

    Q. How should air-permeable netting — for example, InsulWeb — be attached to the studs?

    Hulstrunk: The best technique is to staple it with a pneumatic stapler right to the face of the stud. By ‘face,’ I mean the 1½-inch-thick edge of the stud facing the room. The staples have to be less than 1 inch apart, so you’ll go through a good number of staples.

    Some installers prefer inset stapling or “lip stitching” — they put the staples at the corner of the studs, wrapping the InsulWeb a little bit around the corner as they staple. But that really isn’t necessary. I don’t like the lip-stitch method, because I want the insulation to be close to the sheetrock. Installers who use lip-stitching are usually worried about bulges, but we’ve found that an aluminum roller works well to eliminate any bulges.

    There’s another method of installing the InsulWeb: using glue. First, you tack the InsulWeb up quickly. Then you apply watered-down Elmer’s glue to the studs through the InsulWeb, using a trim roller — you roll through the InsulWeb to push in the glue. It gets the InsulWeb really tight and nice. By the time you get to the last wall in the house, the first wall you glued is usually dry enough to start blowing cellulose. It’s best to wait at least 6 hours for the glue to dry.

    Q. What do you do about bulges?

    Hulstrunk: After you’re done blowing the walls, the material will be bulged out a little bit between the studs — maybe out an inch or so in the middle. So you take an aluminum roller — it’s about 1 foot long, maybe 3 inches in diameter, and sold specifically for this purpose — and you roll the bulge really quickly. If you do it right, it will leave the cellulose flat, and a long metal straightedge will touch the studs. We recommend that owners or builders include this language in their wall insulation specs: “The material will be rolled flat, ready for drywall.”

  2. davidmeiland | | #2

    Good advice. When done right, sheetrock installation is only a bit harder than normal. You don't need (or want) strapping.

  3. 415irwin | | #3

    Thanks for steering me to that article Martin. In our area it has been difficult to find a contractor that is familiar with blowing cellulose in at a density greater than 3 lbs/cubic foot and express concern with bulging if the cellulose is installed at a greater density. They have recommended strapping to deal with the bulging. But it sounds like it is totally unnecessary... Great!

  4. davidmeiland | | #4

    You can make things a bit easier by using 5/8" drywall, but it's by no means necessary.

  5. ChrisCT | | #5

    I just finished dense packing my new home about a month ago - 10" double stud walls. Definitely face staple to the 1-1/2" dimension of the stud and be sure staples are very close together and the netting is tight. I started by lip stitching the staples to the side of the studs and it was very fussy and ultimately not needed. I would not plan to strap walls as it provides a chase for air movement in the walls. You really want the back of the drywall to be in direct contact with the cellulose to minimize air movement. Instead, plan for 5/8" drywall on the exterior insulated walls to address the bulging. My drywall sub only charged me the material cost difference between 1/2" sheets and 5/8" sheets. For a 2,000sf house that came to be $250+/-. You will need the roller that Martin and Bill Hulstrunk mention. I would highly recommend working with National Fiber and Bill Hulstrunk if possible.

  6. squibt | | #6

    A services chase behind the drywall would be ideal, strapping is OK....don't ya think? Nothing wrong with air behind the drywall if your insulation and air sealing is done right. No penetrations makes for an airtight wall.

  7. rocketconstruction | | #7

    My intaller wants to use furrings to allow for some bulging and then suggests i put vapor barrier over furrings and then install drywall.Is it a bad idea to have the vapor barrier in direct contact with the drywall?

  8. GBA Editor
    Martin Holladay | | #8

    What's your location or climate zone?

    There are no code requirements for an interior vapor barrier in any U.S. location. Codes may require a vapor retarder (a less stringent barrier than a vapor barrier) in Climate Zones 5, 6, 7, and 8, and Marine Zone 4. In warmer areas of the country, there are no requirements for interior vapor retarders on walls.

    The code requirement for a vapor retarder in cold sections of the U.S. can be satisfied by using vapor-retarder paint.

    In most locations of the U.S., you don't want to install interior polyethylene, especially if your house will be air conditioned.

    For more information, see these two articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

  9. rocketconstruction | | #9

    Any ideas on where we can purchase the aluminum roller? We've searched high and low and can't find one.

  10. GBA Editor
    Martin Holladay | | #10

    Aluminum rollers are available from:

    National Fiber
    50 Depot St.
    Belchertown, MA 01007

    Here is a link to their catalog, which shows the rollers:

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