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Community and Q&A

Furring for Rafters

cgmorgan | Posted in General Questions on

Hi all,

The rafters on my Cape Cod are only 5″. Above the knee wall and below the ceiling I’ll be furring it out with a 2×3 perpendicular to the rafters to add an extra 1.5″ and provide a bit of a thermal break. Below the knee wall, where my ducting is, I have a few more inches to play with. I was contemplating putting 2x4s perpendicular on edge, to provide an extra 3.5″ of depth.

I will be doing dense blown cellulose behind Intello and am wondering what the best method to secure the 2×4 on edge below the knee wall will be. Is there a particular Simpson’s strong tie which would work well for this, or should I be looking at 5″ screws directly through both members? Pocket screws?

I did look into adding rigid foam instead of furring it out, but opted to avoid it as I may be adding foam to my roof in the years to come.

As a side note, for the portion below a knee wall which is out of sight, is drywall required for a fire barrier or can I leave it as exposed Intello given the cellulose is fire retardant? It’s going to be a pain in the ass to get drywall behind those walls.

In case it matters, I’m in climate zone 6 Montreal QC.

Thanks for your advice,

Chris

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Chris,

    I would use plywood gussets every - say 24" . If you want a thermal-break you can use 2"x3"s and leave a gap between them and the existing framing. Dan Kolbert uses that technique. I guess you could call it an interior Larson Truss. https://www.jlconline.com/projects/energy-efficient/building-a-tight-house_o

    Edit: I'm not that familiar with the Quebec building code, but I don't think I've never seen a requirement that insulation (except foam) be protected in an attic. If you can blow as much as you want onto the floor, I don't see why the walls would need a barrier.

  2. DC_Contrarian_ | | #2

    I'm thinking a 7" block of 2x4. Tack the wide side to the side of the rafter with a nail gun or screw gun, then tack the narrow side to the side of the furring piece. If you have a nail gun you probably only need to put a block in every other cross, you can just toenail the other half. I don't think toenailing alone would be strong enough to hold the furring against the roof.

  3. cgmorgan | | #3

    Hi guys,

    Just wanted to thank you both for weighing in. That link is great and helps me with a lot of other details I was curious about as well.

    Chris

  4. cgmorgan | | #4

    Hi Malcolm,

    I was going over the the design by Dan Kolbert and have a question about the placement of the "2x3 ceiling build-out". I notice in the image it's in line with rafters, allowing for only a small amount of space between the two: is there any reason you couldn't put the 2x3 on the other side of the gusset, thus offsetting it from the rafter and providing for a larger thermal break?

    Thanks again,

    Chris

  5. Expert Member
    MALCOLM TAYLOR | | #5

    Chris,

    You could. Keeping them aligned and alternating which sides the gussets are on yields more structural rigidity, but I'm not sure it's that important. However, any energy gains from offsetting them are in the weeds.

  6. Expert Member
    BILL WICHERS | | #6

    You could probably press some hurricane ties into service here. I would probably try the H1 for this.
    https://www.strongtie.com/seismicandhurricaneties_strapsandties/category

    You would have to make sure you could handle the loads though, since this isn't the application these would normally be used for. The H1 would brace the horizontal 2x4 more securely than a strap on only one side. The think to keep in mind with these "connectors" is that the metal used to make them is pretty strong in tension, but that's about it -- it can flex and bend if you load it in ways it wasn't intended to be loaded. The H1 is something like a guesset plate that can handle two perpindicular surfaces at once, which might be a good fit for you here.

    Bill

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