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Community and Q&A

Is my plan to insulate scissor trusses correct?

Joshua Costello | Posted in Energy Efficiency and Durability on

I’ve made the mistake of not planning my insulation details before building, but because of this GBA (and specifically Martin Holladay) I’ll be much more prepared next time. In the mean time, here’s where I’m at.
I’ve come up with, what I believe to be, an adequate plan to insulate my addition. I’m open to hearing suggestions, pitfalls or critiques. Thanks in advance.  

I’m currently building a 30’x33’ addition on my house in Maine (Zone 6a). We used scissor trusses with a 14” energy heel. The top chord is 8/12 and the bottom chord is 4/12. The trusses are spaced 24”oc, and it will be vented from soffit to ridge vent. We are planning on using 1” polyiso  to build the rafter vents with a 1 1/2” space for the air flow. I’m planning on using 24” of blown cellulose. The cellulose will be sitting on the air barrier which will be either 1/2” osb taped with 3m all weather flashing, 1” xps foam (taped) or, my least favorite option, 5/8” gypsum taped and mudded.  Next, I plan on installing flat 2×4’s to create a small service cavity. I like the idea of the service cavity for the ease of installing our lights and running electrical without having to seal around them since the layer above is providing the air seal. The lights I’m going to use are recessed, but only require 2” of installation depth instead of the usual 7 1/2”+ recessed can lights require. Finally we plan to install 3/4” tongue and groove as the exposed layer.

I’m doing all the work myself. My issue with 5/8” gypsum is that it’s heavy, more expensive than the osb option, and provides next to nothing for insulation value. I’m leaning towards the osb option, but I’m open towards other options with a reasonable explanation.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Joshua,

    I'd favour using OSB too. If you did go with drywall use 1/2" ceiling boards. They are lighter but just as strong as 5/8".

    There is no advantage to extending the baffles beyond the point where the 24" of cellulose stops touching the top chord of the trusses.

    1. Joshua Costello | | #2

      Thanks for the information on the baffles. That will save some time and money.

      1. Stanfo3 | | #28

        There are advantages...1)foil faced polyiso with an air space acts as a radiant barrier reducing the temp in the roof when installed all the way up. 2) if you get a roof leak water will travel down polyiso to exterior of house and out to outside of wall so insulation wont get wet (if baffles are airsealed). Airsealing baffles reducing wind washing of insulation also.

        1. Expert Member
          Malcolm Taylor | | #29

          Stanfo3,

          1) If you want additional insulation, it makes more sense to add it to the attic floor.

          2) I'm not sure directing water into exterior walls and soffits, or hiding the location of leaks is a useful purpose of baffles.

          3) Cellulose insulation in attics isn't subject to any appreciable degradation in R-value due to wind-washing. It's unclear if baffles even reduce the R-value of unprotected batts in cathedral ceilings by any significant amount.

  2. Stephen Sheehy | | #3

    We applied an air barrier membrane to the underside of the bottom truss chords which were 24" o/ c. We then ran 1x4 strapping perpendicular at 16" o/c. Then we installed 1/2" drywall. Then we put about 20" of cellulose on top. If you do something similar, you could skip the drywall above your ceiling boards. You could strap with 2x4s for your service cavity.

    Instead of having sloped ceilings throughout, on the north side, where the entry, bathrooms, kitchen and mechanical room are, we put in a flat 8' ceiling. Above that, inside the conditioned space, is a sloped utility chase where we installed the hrv pipes, plumbing lines, wiring. Those ceilings have recessed cans. There are no penetrations through the cathedral ceilings, except for a single plumbing vent.

    We're in Maine, too.

    1. Joshua Costello | | #5

      Thanks for the information Stephen. I've been reading about your "pretty good house" for a week now. Super interesting.
      I hadn't really considered an air membrane just because I would have to double strap the ceiling to be able to install the t&g pine boards, but it's definitely something I'll look at. It would be a lot easier to install instead of the osb.

      We're up on Verona island (near Bucksport)

      1. Expert Member
        Michael Maines | | #20

        Joshua, you don't have to double strap--you can put a variable permeance membrane directly on the trusses, then strap with a single layer of 2x. Wiring needs to be at least 1 1/4" from the face of framing, so 2x strapping is a simple way to keep wiring on the inside of the air control layer (in this case, the membrane).

        1. Expert Member
          Malcolm Taylor | | #21

          Michael,

          I think the second layer of strapping is to allow the T&G ceiling to run horizontally.

          1. Expert Member
            Michael Maines | | #22

            Malcolm, good point. The strapping can run along each truss, but of course the finished plane depends on how good the trusses are. Joshua is not far from me and will likely use the same truss plant, who I've found do a pretty good job, but there are always horror stories. 1x3 spruce is standard strapping here and very affordable; two layers installed in opposite directions would do what Joshua needs.

          2. Joshua Costello | | #25

            Malcom, yes, that's correct.

          3. Joshua Costello | | #26

            Michael, I used Mainely Trusses, which I used for my garage. They seem to do very good work. Are you suggesting that I could run one layer of 2x strapping along the same plane of each truss instead of double strapping?

  3. Expert Member
    Michael Maines | | #4

    Joshua, why not use a variable permeance membrane for your air control layer? Siga Majrex and Pro Clima Intello are airtight, durable and roughly $0.50/sf, about the same as the options you're considering, and much easier to install. XPS has no place in a green home, due to its extremely high carbon emissions, unless you're getting it used.

    With 24" of cellulose there is no reason to spend the money or carbon emissions of polyiso for your chutes; 1/4" plywood or Brentwood Industries' Accuvent (available in 1 1/2" depth) would work fine. But there's no performance issue with polyiso, and it is easy to do.

    I'm also in Maine.

  4. Joshua Costello | | #6

    Hi Michael, I actually wasn't aware of the XPS carbon emissions. I will have to check that out. I'm fairly new to the idea of green homes. I got interested watching some Matt Risinger videos on YouTube.

    I was planning on using the polyiso for two reasons. There's a local Amish guy down the road that sells the 1" sheets for $10 which is pretty cost comparable to the plywood, but I get the added benefit of a couple R-value.
    I have been staying away from the accuvent because I was afraid it could get compressed and lose air flow when I blow the insulation in. Maybe that's just ignorance on my part. I don't have any experience with it.

    The Siga Majrex and Pro Clima Intello options are being considered now due to the ease of install. I didn't want to double strap the ceiling, but installing the membrane would be a whole lot faster and easier than the osb.

    Thanks so much for the info.

    1. Expert Member
      Michael Maines | | #7

      Some people on GBA probably get tired of a few of us constantly repeating it, but yeah, XPS is bad. Here's a neat chart that Dana has shared recently: https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png. I wrote about the different rigid foams here: https://www.finehomebuilding.com/2018/01/08/get-right-rigid-foam.

      Accuvent won't compress, or at least it would take a lot of pressure to compress it, and I don't know many insulators in Maine who would blow it that tight. I first learned of it from cellulose installers.

      I've heard of a place in Searsmont, ME that sometimes sells used foam but I don't have experience with them. A place in Skowhegan, ME sells foam seconds. At least two places in MA sell used foam.

      24" of dense-packed cellulose will perform at about R-90, and there are greatly diminishing returns above R-60. A rough guess based on a lot of prior energy modeling is that polyiso would save you $2 per year in energy. That may be high, probably not low.

      If you're anywhere near Portland you can get Siga Majrex at Performance Building Supply. Or order from their web store, sigatapes.com. Or get Intello at foursevenfive.com.

  5. Jon R | | #8

    Do make sure that you have lower (preferably 5x) vapor permeability on the interior side than the exterior side (which is the baffle). You do not want condensation/sorption causing moisture accumulation.

    1. Expert Member
      Malcolm Taylor | | #9

      With the difference in sl0pe between the top and bottom chords of the trusses, and a 14" raised-heel, he should only need baffles 3 ft long or so.

    2. Deleted | | #11

      Deleted

    3. Joshua Costello | | #12

      John, would you explain that just a little more? Please forgive my ignorance.

      1. Expert Member
        Malcolm Taylor | | #13

        Joshua,

        Any building assembly, whether a roof, or wall, that is designed to dry to the outside needs the surfaces closest to the exterior to be more permeable than those next to the interior. That way less moisture enters the wall or roof than leaves it.

        If the baffles leading to the ventilated space were low-perm, and the ceiling was similar (or as Jon suggests less than 5x) you might get moisture accumulating on them.

        1. Jon R | | #17

          > designed to dry to the outside needs the surfaces closest to the exterior to be less permeable than those next to the interior.

          I assume this is a typo - in a cold climate, the exterior should be MORE permeable.

          1. Expert Member
            Malcolm Taylor | | #18

            Jon,

            Yes good catch. I've edited my response.

    4. Expert Member
      Michael Maines | | #15

      Jon, I agree with you when it comes to walls, but vented roofs are a different story. The minute amount of moisture that will make its way to the baffle will continue to the vent space without causing problems, regardless of the baffle permeance. I used to worry about this but several experts, including Martin, have convinced me otherwise.

      1. Jon R | | #16

        We know that moisture entering a ceiling/roof is typically MORE than a wall, where there is plenty of data showing that there can be enough to cause problems, even when it is well air sealed. So even more moisture needs to exit and a baffle that is vapor impermeable certainly impedes this. The issue needs good measurement data, but everything I've seen is anecdotal - and goes both ways.

        There is also the question of why anyone in a cold climate would build vapor impermeable exteriors in cases where it's easy to be conservative and improve drying with more exterior side permeability.

        Edit: see here for "Vapor permeable baffles are recommended ... 10 perms".

  6. Joshua Costello | | #10

    Michael, I think I might back it down to 20" of cellulose. R90 seems a little much now that I've read that Stephen's "pretty good house" doesn't even have 24". I may have been a little overzealous.

    Accuvent does seem like a good cost effective option, as well as a fast install. However, do I need to be concerned about the nails coming through the roof decking and puncturing the accuvent when I install it? Or is this a non-issue?

    I'm about 2 1/2 hours northeast of Portland. Just 20 miles south of Bangor. That's not too bad of a trip for some building supplies, but it wouldn't surprise me if one of the local Viking Lumbers couldn't get it in.

    1. Expert Member
      Michael Maines | | #14

      Joshua, I've done up to R-140 for Passive House projects, and cellulose is relatively cheap, so there's nothing wrong with R-90. But I usually spec R-60, which is 16.5-17" of dense-pack or 15.5-16" of loose-blown.

      Roofing nails are short and will only puncture the Accuvent where the baffle touches the sheathing. Although in a perfect world the baffles would installed in an airtight manner, with your interior air barrier and with cellulose being resistant to wind washing degrading its performance, a few holes aren't going to matter a bit. In fact I wouldn't even seal the baffle edges to the rafters, though others might.

      I'm in Palermo, between Augusta and Belfast. Sounds like you're not far away. Let me know if you need local recommendations. Viking, Hammond and Hancock can all get the 1 1/2" Accuvent but it will be a special order, as they stock the 1". None of them will order Intello or Majrex but they both have simple online ordering.

  7. Joel Cheely | | #19

    If you strap ceiling and are getting electrical work inspected, you may have a problem with wiring in the strapping space. You'll have to figure out how to protect the wiring from screws, etc. to please electrical inspector. See E3802.1 of 2015 IRC.

    1. Expert Member
      Michael Maines | | #23

      Joel, in Maine I have had no problems getting inspections with 1 1/2" of strapping, either single 2x members or two layers of 1x. The code just requires 1 1/4" from face of framing to the cable.

    2. Joshua Costello | | #24

      Joel, there's no electrical inspections on Verona. For that matter, the ONLY inspections we have are for plumbing, and the inspector spent all of 3 minutes looking over my work after my first rough plumbing job. I try to keep things well within code, but, in some ways, it would be nice to have an inspector to bounce things off of.

  8. Expert Member
    Michael Maines | | #27

    Reply to Joshua #26: yes, exactly--if the truss chords are in plane (or nearly so), you don't have to run strapping across them. Although it's standard practice in New England, the rest of the country doesn't strap ceilings, as a rule. You can install the membrane directly against the trusses, then run a 2x in line with each bottom chord.

    Leave gaps where cables will run so the electrician doesn't drill into the membrane, or plan for them to install protective metal plates where cable is too close to the surface.

    I would probably cross-strap with 1x3s; I was just offering an option that could save some labor.

    I've had good luck with Mainly Trusses but I believe local lumberyards are now using Aroostook Trusses. Or maybe they are using both companies.

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