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Alternatives to this Pretty Good Roof Assembly

igrigos | Posted in General Questions on

Looking for opinions. We are working on the design for our pretty good house in central MA and am having some debate about the best way to construct the roof. The house will be a historic looking cape with the goals of R40 walls and R60 roof. Walls will be GWB> 2×6 24″OC w/ cellulose > OSB sheathing > self adhered WRB > ~4″ reclaimed rigid.

For the roof, I am considering 16″ TJI’s, with tyvek or 1/8″ plywood under the top chord to form a ventilation channel, and dense pack cellulose filling the cavity. This would give me close to R60, a 1.5″ ventilation channel and hopefully be cheapish/ easy to install. Exterior rigid seems like it would add labor cost and be a bit more expensive overall.

My question is, is there a better alternative here? As with every project, we are on a tight budget (especially with current material costs). Should we consider parallel chord trusses or something else entirely. (Preliminary exterior design attached)

Thanks!
-Isaiah

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Replies

  1. igrigos | | #1

    Additionally, I can't seem to find any details in the detail library (while full of info, the library is fairly difficult to navigate). Are there any I-joist details readily available that I can use a basis for our plans?

  2. Walter Ahlgrim | | #2

    I have to ask why shoot yourself in the foot with the first stoke of your pencil? The choice of a half story design makes your life more difficult in every way. Air sealing is difficult if not impossible. The design forces you to fit to much stuff in to little space.
    You have no room for ventilation no room for low costs insulation no head room with low sloped ceiling. Any hope of getting the code required R value forces you to use ungreen and expensive foam insulation.

    I understand the heart wants what the heart wants but think long and hard about a different style. If you must go this route consider putting used foam sheet insulation on the outside in several layers with staggered joints.

    Walta

  3. Expert Member
    Zephyr7 | | #3

    1/8" hardboard might be cheaper than 1/8" plywood (if you can even find that) for your vent channels.

    I agree w/ Walta about the air sealing issues with your proposed design. If you do go with the 1-1/2 story design, I would recommend insulating your entire roof over that area so that the kneewalls don't need to be insulated. This is the best way to deal with the air sealing/insulating issue in that type of home, and brings that "devil's triangle" space inside the building envelope. You could rough finish those triangular areas and use them as conditioned storage space this way too.

    Bill

  4. igrigos | | #4

    There was a lot that went into our decision to go with a cape style. We're aiming for 1500-1600 SF, with 3 bedrooms. We also don't need a huge 2nd floor/ bedrooms as we are very active people and spend most of our time outside or in the living space. My wife does not want a single story home and we both want a farm / historic looking building. I have yet to see a 1500SF colonial that doesn't look awkward. We tried a lot of different designs, but always coming back to a cape.

    My plan is to insulate the entire roof and use the triangles for storage. I was thinking of having the entire 2nd floor ceiling sheetrocked (peak to wall/ roof intersection) prior to installing any partition walls to achieve a solid air barrier. Knee walls, partition walls and the dropped/ flat portion of the ceiling framed and sheetrocked later. The knee walls can be for storage, while the area above the flat ceiling may be good for HVAC ductwork.

    1. Expert Member
      Malcolm Taylor | | #8

      Isaiah,

      Capes with knee-walls are only a problem if they aren't thought through first. Your plan makes sense.

    2. Expert Member
      Zephyr7 | | #9

      I agree with Malcolm and should clarify my earlier post:
      Retrofitting these triangular areas for proper insulation and air sealing is nasty work. Doing everything right from the beginning isn't so bad, since you have access to everything. If you insulate your roofline like you're planning, and carry that all the way out to the exterior walls (which is what I was recommending and what you say you're planning), it isn't a big deal to get things insulated and sealed properly. Just BE SURE this all gets done at the proper time as you're building so that you don't block access an make things hard later.

      You can hide ductwork in those triangular areas, but that will interfere with your ability to use the space for storage. I'd try to route things in interior walls and between joists as much as possible and try to limit any crossover ductwork running perpindicular to framing.

      Bill

  5. Joe Norm | | #5

    I ended up with parallel cord trusses for my vaulted ceiling. One section was 24" thick and another 18". I was able to get two staggered courses of R-3o (or maybe it was R-38) batts up there and still had a healthy air gap. I specified insulation for metal studs so it would fit snugly in the cavity. The I-joist technique is great for providing a nailer for your proposed vent shoot. I would have used I-joists but didn't want to deal with sistering on the overhangs. With the trusses they built in the overhangs into the top cord.

    1. igrigos | | #12

      Thanks Joe, there's a local truss builder here in town, so I'll keep that in mind. Did you have any issues with gaps/ voids using batts in the trusses? I do like the idea of not having to deal with building out the overhangs

  6. Doug McEvers | | #6

    I like your intended roof design, have used it may times. I would use something more rigid than 1/8" for the baffles. Dense pack cellulose could bow that considerably. I like the idea of addressing the entire bottom of the ceiling with an air barrier and drywall, then build the knee walls. You did not mention the ridge support beam for the TJI's, what is the plan for load bearing?

    1. igrigos | | #14

      Good question about support Doug, and something that I should probably start to think about. A structural ridge is probably in the cards. I'll start working on figuring that out using some of the comments here.

  7. David Argilla | | #7

    475 has free Ebooks that show details for I joist roofs. The details of course highlight products they sell, but I thought the details where useful even if you don't use the products.
    https://foursevenfive.com/smart-enclosure-downloads/

    1. igrigos | | #13

      Awesome David, thanks for the link. Somehow this never came up in my online searches

  8. Expert Member
    Akos | | #10

    You can simplify your build and your air sealing a fair bit if you can make the dormer a bit wider. The idea is to make it the same width as the floor bellow and build it as full 2nd story. The roof overhangs would bolt on to the side of this structure to give the gabled look. This would avoid air barrier transitions from roof to wall back to roof which are always troublesome unless you go for SPF.

    The placement of the windows on the gable end make it pretty easy to run a post to support a structural ridge. The post would run between the windows and sit above an oversized header supported by a couple of extra jack studs above the door. This would mean that you don't need any rafter ties and you won't need to have any of your floor joists or collar ties poking through your air barrier.

    I have never been in a place where I've felt the ceiling should be lower. If you are running the interior drywall up along the rafters, I would leave that as the finished ceiling and enjoy the extra sense of space.

    The roof above the porch doesn't look like it ties in nicely to the house, being low slope and a membrane roof, it will most likely always be troublesome. I would try to get that better integrated with the house roof and covered with the same standing seam panels.

    1. igrigos | | #11

      Thanks Akos, making the dormer full length does make a lot of sense and definitely simplifies things. Adds to the interior space, while making construction easier...a win-win.

      I will definitely consider keeping the finished drywall, and not "dropping" the ceiling at the peak. I was trying to get creative in running our ducts for the ducted split system and the ERV ductwork upstairs. Keeping the drywall at the rafters and using the interior walls or floor saves yet another step if i can make that work

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