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Community and Q&A

Loose-fill cellulose in a small attic

etting | Posted in Green Building Techniques on

It seems that the best way to insulate the vented attic on my 20×48 new house with a simple 4:12 gable roof will be to blow in loose-fill cellulose, but I wonder about maneuvering within the tiny spaces of the attic. The roof will be framed with standard trusses, with a flat drywall ceiling attached to their underside. The attic will not be used for storage or anything else. The house is in Climate Zone 2B in the Southwest. With a 4:12 pitch and a span of 20′, the roof will be only 40″ higher at its peak than where it meets the plane of the exterior walls; therefore, I’ll be crawling around between trusses with effective headroom of less than 3′. I could gain a little more height by using raised-heel trusses, but other design considerations preclude that choice, and I can get most of the other benefits of raised heels by installing strips of polyiso to maximize the R-value per inch along the edges where there isn’t enough height to use sufficient cellulose.

With too little room to walk on the bottom chords of the trusses, I will have to crawl, and it seems it may be difficult to stay on the trusses crawling, so perhaps I could crawl around on 2′-wide strips of 3/4″ plywood and then remove them as I work my way back from the far ends of the attic. Nonetheless, I wonder:

1. Will the bottom chord of a standard residential truss of this size hold a 165-pound person?

2.a. Is my attic just too small for crawling around to blow in loose-fill cellulose with a hose the way it’s usually done?
b. If so, what would be a good alternative?

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  1. davidmeiland | | #1

    You can definitely blow cellulose in your attic. Before you start, install a walkway down the center of the attic, on top of the truss bottom chord. Use 2x10 or 2x12 or rips of 3/4" plywood. Build a dam around the access opening. Start at the ends of the attic and work your way towards the exit. Finish the job standing on a ladder. Be sure to fill under the walkway. In the future, anyone who has to go in there will find the walkway and will use it.

    You could also omit the walkway and just stand on the trusses. They can take it.

  2. GBA Editor
    Martin Holladay | | #2

    David is right. If you don't want to do the work, any experienced cellulose contractor can handle the job.

    That said, it's always distressing to hear about a new-construction job in which (a) the owner or builder deliberately decides against the use of raised-heel trusses, and (b) attic access is cramped. You mentioned "design considerations," but I certainly hope that these design considerations are compelling. A good house deserves a better attic than the one you describe.

  3. etting | | #3

    Thank you, David and Martin.

    From what I've read, a standard truss heel is 4" high. Am I correct that I would need to raise the heel another 10" to be able to allow a 2" venting gap and blow in loose-fill cellulose at 12" to have it settle to the 11" I need for R-38?

    If so, the bottom of the truss at the end of my 24" overhang will project back horizontally to the wall 2.5" above the top of the top plate. To avoid needing extended sheathing/siding, would it make sense to install a wider fascia so that the vented soffit can project horizontally onto the standard wall height?

    What extra blocking, if any, would you recommend for a heel raised an extra 10"?


    With the heel raised, what other improvements would your recommend to make my attic suitable?

  4. GBA Editor
    Martin Holladay | | #4

    A raised heel truss is a good idea, because it provides enough room for insulation at the crucial area above the top plates of the exterior walls.

    Your original comments noted that the attic is cramped, with little head room. This problem can be alleviated by increasing the roof slope. In general, steep roofs do a better job of shedding water than roofs with a shallow pitch.

    Remember to make sure that your attic access hatch is generously sized, so that workers can enter the attic when necessary. One way to reduce the energy penalty associated with an attic access hatch is to create an access door in the gable wall. While this means that maintenance workers will need an extension ladder to access the attic, this approach limits air leakage through the insulated floor.

    It should go without saying that you need to pay attention to sealing air leaks at your attic floor before installing any insulation.

  5. Expert Member

    If you install raised heel trusses you inevitably have to extend your wall sheathing over the heels. You need it to provide lateral stability for the trusses, to create a dam for the cellulose at the wall, and to prevent air washing of the insulation. Lowering the soffit and hanging it off the fascia is something I've never seen done.

  6. etting | | #6

    I've been hoping a gable-wall access to the attic made sense, Martin. Thank you for confirming that it does. The expense of a steeper roof won't make much sense for me here in the desert, but I'm sure it will be well worth considering for other climates.

    Thank you listing the benefits of extended sheathing, Malcolm. I was planning on a baffle to dam and protect against wind-washing, but I hadn't thought the whole thing through. The 2012 IRC is a little puzzling about blocking with raised heel trusses. Essentially, it seems to say that solid blocking is required where the heel height is between 9.25 and 15.25 inches, but a structural panel that essentially extends the exterior wall surface can be used for heel heights greater than 15.25 inches. I generally understand "solid blocking" to mean 2x, although the diagram the IRC provides shows what looks to be a thinner board. What's puzzling is that the panel option doesn't seem to be allowed for the shorter height, which I would think would be more easily satisfied by blocking of lesser thickness, even though it's bracing in a somewhat different way.

    Instead of buying more expensive extended sheathing/siding panels or extra ones to cut up to cover the front of the raised heels, is there any reason I shouldn't just use equally structural plywood and then drop my fascia enough that the soffit will hide the plywood?

  7. Expert Member

    "any reason I shouldn't just use equally structural plywood and then drop my fascia enough that the soffit will hide the plywood?"

    None I can think of.

    For what it's worth, my experience with truss pricing has been that changing the pitch of the roof from say 3 in 12 to 6 makes very little difference, but unfortunately there is quite a premium to going with raised heel trusses.

  8. etting | | #8

    Thank you, Malcolm. I just got a couple of quotes, and the raised heels increase the price by roughly 20%. Do you--or anyone else experienced using raised trusses--know whether, in my previous post above, I'm reading the 2012 IRC correctly regarding using extended sheathing as the required blocking on a heel height less than 15.25"?

  9. Expert Member

    No sorry, I live in Canada. That's probably a question your local building inspector could answer through.

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