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Community and Q&A

Vented vs Unvented Roof Assembly

TRIPPN17 | Posted in General Questions on

Hi, I am in the process of designing a net zero home with continuous insulation in Zone 6, near Kingston, Ontario.

The home is 48’ across by 44’ deep and will have a 1.5:12 mono-sloped roof.  The roof trusses are 28” deep, leaving a lot of potential room for insulation.

The wall assembly will have cedar siding, furring strips, 2 x 2” of Rockwool comfortbatt exterior insulation, taped and sealed zip sheathing, 2×6 wall on 16 centers filled with Rockwool R24 insulation batts.  I have a high level of comfort with this assembly and believe it will give the desired R value and moisture management for both summer and winter (please feel free to comment on this if I have overlooked anything!).

The roof assembly however is where I am not certain on the best approach.  I am seeking an assembly that will be effective in managing moisture during warm summers and cold winters, protects against the elements and hard driving rain, which we often get in the summers and provides an appropriate amount of insulation to minimize heat loss in the winter and keep cool in the summer.  What is critical for me, is a roof that minimizes risk of mold/moisture intrusion and leaks.

I am targeting an R value of over 60 and closer to 70 if possible.  From what I have read, once you hit R70 it becomes a point of diminishing returns.

Additionally, the house faces south east and the plan is to have a 10 kw solar array installed on it and ideally with a metal roof.

My question is, given the specifications above, what would be a recommended roof assembly for this home that minimizes risks around moisture and provides the desired R value?

Given the depth of the trusses, I would be interested in being able to use that cavity space to the greatest extent possible.  I believe in an assembly where you have insulation on the exterior of the roof sheathing and insulation below, you have to keep the ratio to roughly 60/40 R value split between the two.  In this case, I wouldn’t use much of the truss cavity and would add several inches of rigid foam on the exterior, making a very thick roof.

I would appreciate any insight/recommendations on how best to create this roof assembly.

Thanks!

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Replies

  1. Jon R | | #1

    Some claim that 3:12 works well with venting and anything less than 3:12 doesn't work with venting at all. This is overly simplistic. With enough venting and air sealing, a 1.5:12 vented roof will be more robust than 3:12. In your case, physics suggest that 2x over-sized venting would be conservatively safe, providing more airflow and thus outperforming a 3:12 roof.

    There is no doubt that cellulose + adequate venting (whatever this is) works well and is cost effective.

  2. Expert Member
    Malcolm Taylor | | #2

    TRIPPN17,

    Jon is of course right. You can effectively vent a 1.5 / 12 low slope roof - the problem is how.

    The assembly will have to devote a large depth of the truss space to ventilation, and rely on level of air-sealing that both initially and over time needs to be maintained for the roof t0 work.

    This problem comes up here on GBA regularly and stems entirely from designing a roof that paints you into a corner by having an inadequate slope, and a difficult roof structure.

    There is no magic bullet to solve this. The choices are to include a deep ventilated cavity with good intakes and outlets, with impeccable air-sealing, or go with an unvented roof using spray foam, or rigid insulation above the roof deck. The perhaps unpalatable but much easier and forgiving solution is to redesign the roof to provide adequate slope.

    1. Jon R | | #3

      > a large depth of the truss space to ventilation

      IMO, about 4" (out of 28"). And even with the same air sealing, this will outperform the well-accepted 2" vent, 3:12 roof.

      I suggest that air sealing is even more critical in an un-vented roof.

  3. TRIPPN17 | | #4

    Yes, agree with the critical importance of a good seal.

    One suggested non-vented assembly (starting from the inside) was:
    - pine board T&G
    - drywall, taped
    - 2x strapping/furring to provide wiring cavity and LED pot light cavity
    - Intello Plus air barrier/dense pack reinforcement
    - thermafil insul-fil blown in mineral wool between joist, up to 20” to get R60 (leave 8” of space for more or an air void if necessary)
    - Zip roof sheathing (tie into zip wall sheathing for continuous insulation/air tightness)
    - Mento Plus weather membrane on top for extra protection
    - vertical strapping with horizontal strapping for air vent between the metal roof and the sheathing.
    - metal roofing

    This seems to be an easier assembly to manage than one with rigid insulation on the top, however I’m not sure if this would adequately manage moisture.

    1. Jon R | | #5

      > suggested non-vented assembly ... air vent

      You have described a vented assembly. For it to work, you need proper vent area/size, perm ratio and air sealing. The issue becomes "what is proper?" in your case.

      In addition to basic physics, there is some good support for low slope roofs plus larger vents working well here. 1.5:12 with an 8" vent is very conservative compared to the 0:12, 3.25" that it allows. Note that it also specifies perms (less than 1.6 interior, more than 10 into the vent). And 45' (you are just over, don't worry about it).

      Zip has too low perms for good drying out the vent. Use plywood if you have to put the vent above the sheathing.

  4. TRIPPN17 | | #6

    Ok, that explains a lot. Appreciate the perspective.

  5. Expert Member
    Akos | | #7

    Vented assembly with a mono slope roof is very simple to build and can work well. Keep in mind that OBC requires 2.5" of vent space for this but even a bit more doesn't hurt.

    One benefit of a low-ish slope roof you can loose fill for a low cost high R value roof assembly.

    Make sure to figure out how to tie your wall air barrier to your ceiling air barrier.

    My goto solution is to put a wide piece of flashing tape with half the backing removed over the top plate+sheathing with the side with the backing hanging to the inside. Once the roof structure is placed, staple the tape to blocking between the trusses, remove the backing to stick it to your air barrier. 6mil poly works well enough as an air barrier if you take a bit of care but it also doesn't hurt to detail your ceiling drywall as one as well.

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