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Furring out rafters

raul4817 | Posted in Green Building Techniques on

I have 2×10 rafters(9.25″) and an 1.5″ air gap for .5″ foam vent baffle.

So im left with 7.25″ of rafter bay. I need 11″ for my double r23 roxul batts. My plan is to use a .5″ foam board strips cut to 1.5″ wide and then attach a 2×4  with pocket screws. This would give me the 3.75″ needed for batts.  Is this plan reasonable or would using plywood gussets be an easier way of furring out the rafters? The foam strip is primarily a thermal break and to get the extra .25″ needed for my batt install. Any creative ideas or other methods anyone ever use? My roof is a very simple gable with a 5/12 pitch. 
Raul

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"My plan is to use a .5″ foam board strips cut to 1.5″ wide and then attach a 2×4 with pocket screws. This would give me the 3.75″ needed for batts. "

    Not really- that would give you 4.0" (not 3.75") , and you'd need to "fluff" the batts.

    The 2x4s are also quite a thermal bridge, and 0.5" foam isn't much of a thermal break. Depending on the framing fraction you might get better thermal performance out of 1" polyiso edge strips with 3/4" furring (1.75" additional depth), and swapping one of the R23 layers for R15s.

    With 7.25" of space you have exactly the prescribed depth for R30 rock wool batts. Installing 2x4s perpendicular to the rafters 24" on center and installing R15s in the 2x4s would DEFINITELY be higher performance than the proposes half-inch foam + 2x4 solution- easier to implement too. The center-cavity R would be R45 instead of R46, but with an R15 of thermal break over more than 90% of the rafter edge area the heat transfer through the framing fraction is much reduced.

  2. raul4817 | | #2

    Running the 2x4s perpendicular was my original plan, with r30 then r15. When I did some mock ups I had reservations and issues with that plan due to the exterior end side and collar ties. There may have been something I missed for that detail. So I settled for my current plan. my plan is not perfect but should work. The idea for the furring strips came from the bonfiglioli approach. What r value is the bare minimum for a thermal break? What r value is a decent to good thermal break? I still have time to play with the size of my foam sandwich. My Roxul has however already been delivered so I’m kinda married to the double r23 unless i want the pay a restocking fee.(special order).

    1. Expert Member
      Dana Dorsett | | #10

      >"What r value is a decent to good thermal break?"

      There is no hard set rule, but something that is at least comparable to the combined R value of the insulated portion of the rafter (7.75 " / R9.3) plus the new framing (3.5" / R4.2), which is about R13.5. With half inch polyiso that becomes R16.5, about an 18% reduction in heat transfer (1D modeling)

      If instead a couple inches of polyiso held in place with a 2x2 rail (1.5" / R1.8, for R 11.1 total rail + rafter) would be 3.5" deep, and would need 5" screws. The total cavity depth would be 10.75", so it would mean installing the 11" thick nominal pair of R23s into a space 0.25" shallower, an EASY compression fit. With the compression the net R-value at center cavity would be about R45 instead of R46 (a 2% increase in heat transfer) but the framing fraction goes from about R16.5 to about R26, about a 48% reduction in heat transfer.

      1. Expert Member
        Peter Engle | | #11

        If you rip the 2x2's from 2x4's, you get the extra 1/4" back, or 3/16" anyhow.

      2. raul4817 | | #12

        Dana, i f i were to go with the 2x2 plus foam, I could go with a 1.5" thick sheet and rip to 2.25" strips. Then place the 1.5" face of foam to the face of the 2X2, giving me 1.5"+2.25"=(3.75" rail). The 1.5" poly is an R9.3 but im sure the foil facer attributes some rvalue. If installed in 2.25" depths Im guessing the r value would stay right around R10. Plywood straps to hold in place get tricky and maybe excessive. a 5" screw screwed the face could work, I will try on a test rail first. Im actually liking this idea better. More of a thermal break and less weight from the framing member bearing down on the rafters. If I could furr out with only 1.5" foam ripped to depth that would be even better. I could screw that in place as you would attach exterior foam board( screw+washer). How could i attached the air barrier? I cant think of any easy ways to accomplish that part.

      3. raul4817 | | #13

        Dana, taking it a step further. Instead of 2x2's what if i use 1x2 furring strips and rip foam to 3" strips. I found some 5" fine thread screws that could work. I may have to pre drill the 1x2's to avoid splitting the wood. does this seem like a viable solution. The money saved in lumber would go towards the cost of thicker foam board.

  3. raul4817 | | #3

    I could also use a 2x3 but then my foam would need to be 1.5". Ther value on the polyiso at this size is 9.7 or so. This would most likely rule out the use of the pocket holes and limit me to plywood strips. I dont think the screw would make a good connection through the foam board. But this is a minor adjustment to the plan that I can still make.

    1. Expert Member
      MALCOLM TAYLOR | | #4

      Raul,

      The problem is probably mine, but why are you thinking of using pocket screws over just... well... screws?

      1. mackstann | | #5

        He's joining the two 1.5" faces of a 2x10 and a 2x4 to each other... To use a normal screw you'd need the screw to either be about 6" long or you'd need to drill deep holes in the 2x4 so the head is recessed far into the board.

        I think what's more commonly done is using plywood webs on the sides of the boards to connect them to each other.

  4. raul4817 | | #6

    Walter,
    Attempting to make things a little less complex. Maybe more? I would need a pretty long screw to screw through a 4" sammie so I though this would be easier. Here is a pic of what it may look like.

    1. Expert Member
      MALCOLM TAYLOR | | #7

      Of course! Got it now. Thanks to you both.

  5. Expert Member
    Akos | | #8

    Raul,

    The other option is putting rigid insulation on the inside and putting your drywall directly over it. With 7.25 mineral wool (r30) plus 2.5" of polyiso you are at r45 with much lower bridging than framed. You would need some long drywall screws but those are common at most commercial drywall suppliers.

    I think Dana's solution with 2x4 at 90deg is much simpler/cheaper though.

    1. raul4817 | | #9

      Dana's idea was the original plan. Essentially a Mooney wall on the ceiling. Some of the framing members dissuaded me from doing this. This is for a conditioned attic so no drywall being hung. I was going to foam board over the rafters after batts but as Dana pointed out that there would potentially be a fire code issue with no drywall. Dana recommended using a fabric type foil for an air barrier. I really only need very little lumber for stapling up the the air barrier. I have pretty much settled on using a variation of the bonfiglioli method for furring out. I could in theory use a 2x2 with the remaining 2.5" being foam. This approach may not be the easiest or most effective but I feel comfortable that I can do it in a very precise way so I'm content with it. Going with more foam versus lumber would require some strips of ply to attach unless there is something I'm missing. There are some Simpson strong ties I could sub for ply but that can get pricey.

      1. Expert Member
        Akos | | #14

        In that case, we can look at your options.

        Assuming roughly a 11" cavity R46 insulation.

        The framing R value is
        -all wood R13.75
        -with 1/2" foam R15.5
        -with 2" foam R21.25

        Assuming 20% framing factor, your assembly R values:
        -All wood 1/( 1/46 *80% + 1/13.75 * 20%)= R31
        -1/2" foam 1/( 1/46 *80% + 1/15.5 * 20%)= R33
        -2" foam 1/( 1/46 *80% + 1/21.5 * 20%)= R37

        In terms of overall energy use, I don't think the foam strips or even the extra depth is really worth it.

        Since you are making it a conditioned attic, those extra framing members will make air sealing difficult, I would check with your structural engineer whether all the ties are actually needed especially with 2x10 rafters. Sometimes, the engineers spec in the ties out of reflex.

        EDIT:

        To address thermal bridging the R value of the foam needs to be somewhere around 1/3 to 1/2 of the cavity insulation R value, less then that the foam is not worth it.

  6. Expert Member
    Peter Yost | | #15

    I'm surprised no one has asked this in this string, but what climate are you in? That of course makes a big difference in terms of r-value needs/targets.

    Peter

  7. raul4817 | | #16

    I am in zone 5, I've come up with a solution that seems to me will work well. I did a test peice with some scrap foam board of 2" thick cut in a 3.5" strip. The 2x4 in the photo represents the rafter then 3.5" of foam then furring strip all held together with 5" screw. Instead of 2" foam I'm going to purchase 1.5" thick so my strips will match my rafters on the 1.5" side. This gives me 3.5" of foam for a pretty decent thermal break. Thanks to everyone that has responded much appreciated.
    Raul

    1. Expert Member
      Akos | | #17

      Looks great.

      I find 1x2 splits very easily, 3/4" plywood strips might be more solid.

  8. j_prescott | | #18

    I've been evaluating ways to extend rafter cavities in an old house where the edges of the rooms are under rafters. In my case, collar ties and kneewalls might make a convenient place to attach 2x4s between them, floating in front of the rafters for a thermal break while providing the ability to make the overall depth whatever we need.

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