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A-Frame House Roof Insulation

pottsypotts | Posted in General Questions on

Hi everyone, I’m building a 1000 sq/ft A-frame house in the Catskills NY, climate zone 5. 

Would really appreciate some advise on the roof assembly. I’d like to avoid spray foam unless convinced otherwise. I’d like to get to or above R50 roof.

The Roof Rafters are 12” thick. I would prefer to use mineral wool in the cavities but haven’t worked out how to get 12” of it in there sensibly. Think I will likely settle on 12” thick R38 fiberglass bats for the low cost and ease of install but interested in the blown-in options too. Currently planning on using Intello Plus interior membrane and on the outside Ply sheathing with Mento 1000 WRB membrane. 

My framing/roofing contractor wants to install horizontal 2”x4” purlins directly on the Ply/WRB at 16”oc with 1.5” foam-board or rockwool in between the purlins with standing seam metal on top. I’d prefer to install the purlins vertically to allow for any drainage and venting to the top but my contractor says it would be too hard and not worth it.

Im concerned this assembly is too low R value and I’m curious if we in fact need to be venting too. My contractor was trying to minimize the already large facia size from 12” framing hence the insulation between purlins and he thinks considering the steep pitch of the roof that venting is not necessary. 

Looking for recommendations and advise. Thank you!

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Replies

  1. user-6310848 | | #1

    I renovated a vintage 1960 A-Frame in central Vermont, Zone 6, and applied 5.5" of EPS as a continuous layer over the original sheathing followed by 1" of foil faced EPS, horizontal strapping, ZIP sheathing and finally standing seam. Consensus among my building science group is that the horizontal strapping was a mistake (although no known problems after 4 years) for the very reason you point out so press your contractor to view the roof as if it is really a wall and hopefully this will open their mind. Yes, more challenging to fix a vertical strap but it creates the desired stack effect for drying. Also, consider a thermal break layer above the 12" rafters. Rockwool Comfortboard should be dense enough to be applied as a continuous layer and can be sized to bring your R from 36 up to the desired 50. There are fixation devices available also to more rigidly attach this layer to the underlying rafters. Then vertical strapping, sheathing, metal.

  2. pottsypotts | | #2

    Thanks Philip, appreciate your input on this! Curious what kind of fixation devices, also you mention strapping then sheathing on top under the standing seam? Does this mean there would be 2 layers of sheathing? 1x .75” on top of the rafters w/ WRB then 3” rockwool (would need at least 3” to get near r50) then strapping purlins 2” or 1”? then another 0.5”? ply under standing seam? Guessing the second layer ply is for rigid/flat support under the metal to avoid any waves. This assembly would require something like 18.5” facia boards how would you recommend doing those?

    What are your thoughts on using an R6 ZipR as the first layer of sheathing to get the thermal break and cut the WRB membrane then id only need 1.5” rockwool? It would be .75” thinner total with roughly the same r value (maybe more with the thermal break) and maybe we could cut the second layer of ply too?

    Thank you so much for your input!

  3. Expert Member
    Akos | | #3

    In zone 5 you need 40% of your insulation to be exterior rigid for condensation control in a unvented roof:

    https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies

    1.5" of foam board over 11.25" of fluffy is nowhere near enough. Usually it is about 3" of polyiso over with R24 batts for an R38 roof.

    A simpler option is to build a vented roof. You can do it with 2x12 and HD batts or go with I-joists and dense pack. I-joist generally works better as it has much narrower cross section, thus significantly less thermal bridging but it is harder to insulate with batts (thus dense packing). The top flange on the I-joist also makes for a convenient built in vent spacer. Just staple housewrap/insul mesh/fiberboard to the bottom of the top flange for a 1.5" vent gap. The most important detail for a vented roof is making sure your warm side air barrier is very tight. With an A frame construction you'll probably have some floor joist and collar ties poking through the air barrier, make sure to detail these well. Air leaks there cost a lot of energy and create issues with condensation near the ridge.

    If you don't want a vented roof, I would see if the rafters can be reduced to 2x6 or 2x8 and install enough exterior rigid on the outside for condensation control. This rigid can than be either strapped out with 1x4 or install another layer of sheathing for the metal roof. The horizontal strapping doesn't matter much, most metal roof profiles have enough grooves to allow airflow. An unvented assembly is easier to air seal, your main air barrier is your roof sheathing, so no floor joists poke through it. Make sure this is well taped with a quality tape and is continuous down to the foundation.

  4. gusfhb | | #4

    There is no need to have a huge fascia. End the rafters at the wall and extend whatever foam is appropriate for the overhang. Worst case you have 2x4 rake extensions and stub rafters. 4 inches of foam and 1/2 plywood would most likely support itself forever without support for a foot but builder may be skeptical. They do make flat head foam screws to screw through the plywood, foam and into the roof deck/rafters

    1. pottsypotts | | #5

      Hi Keith, thanks I'll have to do some research on rake extensions and stub rafters above my pay grade that part. At the moment I'm thinking this set up

      12" Fiberglass Fill - R38
      ZipR 1.5"- R 6.6 taped at seams. (I can then remove my WRB membrane from materials)
      Rockwool 2" - R8
      Strapping over Top instead of in between- Vertical/Horizontal or grid/both? 1.5" or 2.5"?
      Standing Seam
      Total RValue 52.6 less the rafter studs is probably actually in the mid to high R40s I haven't worked out the R Value calculators yet. I'm thinking the zip r will be a good thermal break and also good bang for buck in the R and thickness dept and saves me buying anexternal WRB membrane.

      Total roof thickness is a little more than before at around 18" but it's at the R value id like to get to without spray foam. I don't understand the implications on the Facia and overhang yet.

      1. Expert Member
        Akos | | #6

        Zip R is not rated for roofs. Even if you use it outside of manufactuers specs, it is not ideal as the sheathing is on the wrong side of the foam. This is fine in walls, but can cause lot of issues in roofs, google SIP roof rot.

        With R38 batts, in your climate, you need R25 rigid insulation. Your stack up is still not enough for condensation control. Without proper condensation control in unvented assemblies, you run the risk of moisture buildup in your roof and rot and/or dripping ceiling in the winter time. Increase the rigid insulation or decrease the amount of fluffy insulation in the rafters.

        1. pottsypotts | | #7

          Thanks Akos. Ive decided to use spray foam in the cavity. The ratio of Open to Closed Cell is still something Im trying to workout and would appreciate your input.

          If I use 2" closed and 9.5" of open cell I will be at R50 in the cavity.

          The reason I thought the Zip R6 sheathing would be ok is because the "roof" of the A-frame is kindof a wall.. At 57 degrees slope (12 on 18.5)

          On top of the ZIP R6 we plan on a vertical and horizontal strapping system underneath the standing seam. Im guessing this takes care of the venting and allows the zip plenty of drying air?

          We are putting the ZIP R sheathing on in 2 days so let me know if you think its still a bad idea!?

  5. user-7927072 | | #8

    I am doing something similar but limited to fiberglass batt inside 11-7/8" joist thickness. That would compress the R38 insulation to put baffles in so it is vented. The option here would be to pad the top of the rafter with 1x to get some extra room for venting under the sheathing.

    But, the budget would allow for OPEN-cell spray foam and get R38. (CLOSED-cell is too expensive. Question is, in a NON-vented application is open-cell spray foam ok to use? When looking at the hybrid roof assemblies, it called for closed-cell I recall. Given this is an A-frame with 64° (24.6:12) slope, I don't know how this may change things.

    Any advice would be appreciated.

  6. jollygreenshortguy | | #9

    This sounds to me like a perfect occasion for using SIPS. There are some people on this forum who have what seems to me a knee jerk reaction against SIPS because of problems related to air infiltration. Of course if you install them wrong you will have problems. But done right they could be a very efficient solution to this problem.
    The panels need to be taped on the inside at all panel joints, which requires some planning ahead where panels crossing framing. But the solutions are straightforward. I would also tape the outside panel joints as well for good measure.
    If you want to add a belt to your suspenders, put 2x4s on the flat on top of your SIPS, a further outer layer of sheathing, so that you have a 1 1/2" vented airspace above your panels. Apply your roof underlayment to the outer layer and your roofing on top of that.
    You will have ZERO thermal bridges.
    R38 batt (Owens-Corning Eco-Touch)
    R15 SIP (Murus EPS 4 5/8" thick panel)
    That's R53 even without taking into account the reduced thermal bridging. That's a bomber roof. Compare that to an "R60" roof with thermal bridging and you'll be on par or maybe even beating it.

    1. user-7927072 | | #10

      I started this project with SIPS in mind and after getting the cost for that and the engineering, it was significantly higher. I think it would be worth it, I am also in a remote mountainous area where they would have a hard time delivering and having the site accessible for crane and all that.

      1. jollygreenshortguy | | #12

        Logistics and budget are obviously very valid considerations. I totally understand that.
        If you're not using them for structure, but simply as a way to support insulation, then I don't see that there should be any special engineering involved. It's true that they could replace the rafters. But I wasn't suggesting that. I assumed the rafters were being kept and the rafter bays were being used for batt insulation. I was only suggesting the panels be used as a sheathing, rather like the Zip-R panels by Huber. In fact the Zip-R panels might work as well if they don't have the same budget/logistics issues. Again, make sure everything is well taped.
        As far as engineering, the only thing that might concern a building official is how the outer roof sheathing layer and roofing are secured to or through the SIP.

    2. Expert Member
      MALCOLM TAYLOR | | #11

      JGSG,

      This may well be a really good situation to use SIPs.

      I'm probably one of those you see as having 'knee jerk reaction against SIPs" and I'm not sure that's a fair characterization. I think it's a fair comment to make that along with spray-foam it is one of the few materials or technologies that has a fairly high rate of failure, and that the consequences of those failures can be irremediable.

  7. user-7927072 | | #13

    Any thoughts on if open-cell spray foam would be ok? Or is this a situation that calls for closed-cell?

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