GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Roof insulation and ventilation for a 1 1/2 story house in Zone 4A

Jake Apple | Posted in Energy Efficiency and Durability on

We’re building a 1 1/2 story on the northern edge of climate zone 4a. We’re going for a minimum of R50 for the roof, and we’re sitting at about R25-30 for the walls (JM Spider in 2×6 cavities and Zip R6 for sheathing).

We’ve decided to go with an attic truss design as opposed to roof rafters. But I’m not sure the best way to insulate and ventilate.

Attached photo shows a picture of the truss design, with a proposal for where we would put foam & cellulose. Does this make sense?

Other attached photo shows the plan for how we are planning on doing the exposed rafter tails. Although, if we spray foam the heel, I’m starting to think it might be easier to have the rafter tails come as part of the truss design.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Expert Member
    Peter Yost | | #1

    Hi Jake -

    Best way to air seal the transition from the eave wall to the roof truss is to design the truss with NO overhang; flush vertical leg in plane with exterior wall. That way you can easily make direct connection between the top of the wall and the heel of the roof framing. Then you build ladder overhangs and install them later.

    Also, see this GBA resource: https://www.greenbuildingadvisor.com/article/saving-sustainably-framing-roof.

  2. Expert Member
    Michael Maines | | #2

    Jake, the best approach from an environmental standpoint would be to skip the foam and insulate it all with cellulose. You can have the truss company pad down the angled sections to any dimension you want, leaving 1" or 1 1/2" of space for vent channels. Or you can install a heavy-duty, vapor-permeable membrane such as Pro Clima Solitex Mento above the trusses, run 1 1/2" furring above the membrane aligned with each truss, and install your roof sheathing above that.

    At the eaves the best location for vent intake is directly behind the fascia. Attaching the eaves as you show minimizes thermal bridging, but if you are venting the entire roof you would need to hold the ledger down 1" or 1 1/2".

    From a building science/durability point of view, the best approach would be to seal the wall sheathing to the roof sheathing, add rafter tails later, and insulate above the roof sheathing with enough foam or other rigid insulation to provide dewpoint control. But that's an expensive approach.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |