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How to insulate attic wall without stud bays

greenleopard | Posted in General Questions on

Hello, I’m in zone 2A, I need to insulate a wall that is the side of my cathedral room. The attic is vented and will be insulated with a blown in product (most likely a non fiberglass one). The wall is a truss so it doesn’t have any framing to put batts into, it is already sealed up air tight. I’m wondering how to insulate it though…

It is a 24×6′ triangle, so 72 sqft.

1) DIY spray foam ($450 for R13)
2) somehow attach rockwool batts ($100 for R15)
3) cobble foam insulation two layers of 2″ then fill gaps with can foam ($100 for R15)
4) fiberglass batts ($75 for R19)

Any other ideas? I would like to do the rockwool solution if I could find a way to attach them. HOw can attach batts? Should I use faced or unfaced batts?

The picture shows about half the truss wall, from end to peak.

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  1. user-2310254 | | #1


    I am just a home owner with an interest in energy conservation, but here are my two cents.

    Sealing to eliminate air movement is probably your biggest concern. I would go through this area with caulk and canned foam before insulating the floor.

    Is that the entire wall area? If so, it seems to offer pretty good access for a cut and cobble installation. DIY foam would be my second choice due to cost.

    Here is a link to a relevant article. Also check out the titles in the sidebar. (

    Just out of curiosity, what is going on with the PVC vent?

  2. Expert Member
    Dana Dorsett | | #2

    You're making life too hard for yourself. Cut'n' cobble some split & compressed R15 rock wool batts into the truss webbing spaces, and install a layer of 1.5-2" of continuous polyiso over that, cap-nailed to the truss webbing.

    The IRC 2015 code-min walls for zone 2A is 2x4/R13, or U0.84, which is is effectively a "whole-wall R" of (1/U=) R12, that includes the R-value of the air films, structural materials, and wallboard. A continuous layer of 1.5" foil faced polyiso would be R9, the air films on both sides of the wall add up to R1.36, the wallboard is good for R0.6. The 1.5" of web timber is good for R1.8, so the framing fraction of the wall comes to R12.76. Tape the seams of the polyiso with foil tape, can-foam the edges.

    Filling the web cavities with anything more than R1.1 would get you there. The foil facer facing the truss cavities and the other facing the open attic space is probably enough to get there on it's own, but 1.5" of compressed rock wool would be R6.5+. An 8" bread knife or a purpose made batt knife makes easy work out of sculpting and fitting rock wool batts. A 4-5" steel wallboard taping knife sharpened on the edge, and a straight edge makes cutting the sheet polyiso pretty easy too.

    The framing fraction of the truss webbing is very high, and the performance difference between R6.5 and R9 in the truss cavity fraction barely moved the needle- it's less than an R0.5 difference in whole-wall R, rendering the cut'n'cobbled foam or closed cell foam solution not worth the additional cost. Adding a half-inch of thickness to the continuous foam is cheaper than a cut'n'cobble fill, and much more effective.

  3. GBA Editor
    Martin Holladay | | #3

    You don't have to fill in the irregular spaces between the truss members with insulation if you don't want to. Just install a continuous layer of rigid foam on the attic side of the truss, leaving air between the rigid foam and the back of the drywall. Make sure the work is airtight, and add as many layers of rigid foam as you need to get to the desired R-value.

  4. greenleopard | | #4

    Great info! Thank you all. I think I'm going for 2 foam layers of 2" R-Tech, leaving the voids in the truss, and seal with tape and can foam, that will get me to R15.4+

    Steve, the PVC vent is for plumbing, it just looks strange because of the camera angle.

  5. Expert Member
    Dana Dorsett | | #5

    R-Tech sells EPS with either plastic or foil facers, or plastic one side, foil on the other. If using EPS and no web-cavity insulation, use a type with foil facers on BOTH sides. Since one side will be facing a hot attic and subject to radiation from a hot roof deck at a much higher temperature than the outdoor air, it's not exactly like any other wall.

    The foil facer on attic side will keep the surface temperature of the EPS at about the attic air temperature, whereas a more IR-absorptive facer (or bare EPS) would sometimes have a surface temperature higher than the attic air temperature when the roof deck is hot. The foil facer on the truss cavity side adds another R1+ of average performance to that space. Dual foil facers are not a big upcharge (if any). See the Effective R-Values table on p.2 of this document:

    Cutting low density EPS with facers can be done with a side-sharpened taping knife too, but it'll be messier than doing it with polyiso, but still way cleaner than going at it with a saw. In my area foil faced polyiso doesn't cost more than EPS per-R (it's more $ per inch, but 50% more R per inch.) If you haven't seen it, see this short demonstration vidi:

  6. greenleopard | | #6

    Thanks Dana, I'll definitely do the taping knife trick. I would get the double faced EPS, but my hardware store does not carry it, I would love to get a 3" thick polyiso panel, then I'd only have to cut and mount 1 layer, but they don't carry anything over 0.75" of polyiso. I only need 75 square feet so special ordering anything is out of the question. There are some local insulation suppliers, I'll try calling them but my guess is they only sell to contractors.

  7. Expert Member
    Dana Dorsett | | #7

    With foil one side only and two layers you can still have one foil facing the cavities, the other facing the attic where they'll do some good, and EPS to EPS in the middle of the sandwich. Taping the EPS side of the first layer with housewrap tape is good enough.

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