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Community and Q&A

Roof Insulation Approach

jsrooney | Posted in General Questions on

I’m building a single story addition and researching roofing insulation options. I’m considering an option where I use 11.5″ ijoist rafters with a luan panel nailed to the bottom of the top ijoist flange in each rafter bay creating a vent channel from soffit to ridge. For insulation – total to be R49 – I would install 2″ foil-faced polyiso to the underside of the ijoists and foil tape it to create my air barrier. Finally, I would dense-pack cellulose the rafter bays through holes in the polyiso. So I believe I have my air barrier on all sides of the cellulose. My intent is to allow the roof to dry to the outside – through the luan panels – for any moisture that may accumulate in the cellulose. Does this approach sound good? Any issues?

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  1. GBA Editor
    Martin Holladay | | #1

    Sounds like a good plan to me.

  2. Expert Member
    Dana Dorsett | | #2

    From a moisture control point of view it works but mechanically maybe not. The I-joist spacing might determine whether this is going to work or not.

    Luan is pretty flexy stuff, and 2" polyiso isn't exactly structural, not as stiff or strong as half-inch gypsum.

    At dense packing pressures with 24" o.c. spacing with 1/4" or even 3/8" luan you're almost guaranteed to bow the luan well into the vent channel, maybe even contacting the roof deck unless there is a support (such as a stripe rigid polyiso) of the appropriate thickness placed mid-way between I-joists.

    It's also not clear to me that 2" polyiso would stand up to dense packing pressures with 24' o.c. rafter spacing, unless strapped to the bottom chords with 1x furring perpendicular to the joists through screwed or ring-shank nailed to the joists 16" o.c. (or ,maybe even 12" o.c.)

  3. user-1072251 | | #3

    adding strips of EPS foam (or pretty much anything else) to the center of each bay above the vent should prevent the luan from buckling when you dense pack.

  4. jsrooney | | #4

    Thanks. I am also considering 1/4" plywood for vent channel, but foam strips are a good idea. I also had read about using 1x furring strips to support the polyiso but just didn't mention that detail in my post. My Ijoists will be 2' o.c.. Easier/better to net/blow the cellulose first then install the polyiso?

  5. Expert Member
    Dana Dorsett | | #5

    Blowing in netting only adds a step to the process, and makes it more difficult. The netting would pillow out quite a bit and need to be rolled flat. Tacking the polyiso in place with cap nails and taping it for air tightness followed by installing the 1x furring should work. Cut the dense packing holes with a hole saw and save the plugs for ease of repair/re-seal later.

    Quarter inch ply is fine if supported by spacer strips to keep it from buckling under the pressure.

    It doesn't take 2" of foam to hit code min performance in this assembly on a U-factor basis. The R49 spec is based on a milled 2x rafters or joists thermally bridging through the insulation layers. I-joists have about 1/3 the thermal bridging of 2 x lumber.

    Code requires a U-factor less than U0.026, which is a "whole assembly R" of about R38.5, after calculating the thermal bridging of the rafters, the R-value of the roof deck & roofing, the R value of the interior & exterior air films, and the air films in the vent channels, etc. The I-rafter webs are only about 1/3 the thermal bridging of a 2x milled rafter, so with 9" of cellulose R33-ish, before thermal bridging, R30-ish after thermal bridging you really only have to come up with another R8 or so. The roof deck is good for R0.5, the interior & exterior air films another R0.5, the air films in the vent channel about R, so with the roofing material and any finish ceiling you can probably get there with 1" polyiso, and would almost certainly have margin with 1.5" polyiso. This would have to be done more rigorously to fly with the inspectors, but it's about right.

  6. GBA Editor
    Martin Holladay | | #6

    For more hints on blowing cellulose into a roof assembly behind a continuous layer of polyiso, see this article: How to Install Cellulose Insulation.

  7. user-723121 | | #7

    You could also use fiberboard sheathing for your vent channel, what is the rafter spacing? If the fiberboard bows a bit due to the densepack it should not matter. I have used a similar vent detail when coupled with vaulted parallel chord trusses. I hung all of the ceiling drywall starting from the low point in the ceiling. I left an 18" space on both sides near the ridge with dry wall off so the insulation installer could see in the the rafter bays to blow the insulation. When they got to the ridge area they netted it and completed the blow.

  8. jsrooney | | #8

    Thanks again guys - very helpful information! Even though my polyiso will be my air barrier, should I still request my dense-pack to be 3.5"pcf or for about the same R value could a lower density be used? In other words, would the job be less expensive using lesser amount of cellulose?

  9. Expert Member
    Dana Dorsett | | #9

    Even at 2.8-3lbs density the cellulose won't sag or settle when it's less than a foot thick. I'm not sure if going lower than 2.8lbs would be advisable, but maybe.

    The difference in total installed cost between 3.5lbs and 2.8lbs won't be very much. Even though it's 20% less material, it's pretty much the same amount of labor & equipment time.

  10. jsrooney | | #10

    As another alternative to insulating a vented cathedral ceiling, I'm thinking that a product like Roxul MW batts should work as effectively as cellulose, with one advantage being that I could install it myself (less cost maybe compared to contractor-installed dense pack cellulose). From a building science perspective, do you guys see any issues with installing the MW batts in rafter bays and installing polyiso rigid foil-faced on the underside of the rafters? Again, polyiso would be my air and vapor barrier. Any issues with having small air gaps between the polyiso and the Roxul in the rafter bays?

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