Insulating and Venting unconditioned attic/roof from gable ends – GreenBuildingAdvisor

Building a house in BC, Climate Zone 6.

Planning on a high heel truss and loose fill insulation above air barrier.

Walls will be 2×6 construction with Rockwool R22 (or R24 if available) Comfortbatt in the cavities and 3″ Comfortboard 80 (R12.6) outside the sheathing (total R36 minus framing factor).

With loose fill cellulose having an R-Value of 3.2-3.8 per inch, would 12 inches be sufficient in the attic (R38.4-45.6 minus framing factor)?

Our lot is fairly small at 34′ wide. We are trying to maximise the space by building to the City Bylaw minimum setbacks of 1.5m (24′ wide house). Eaves are allowed to project 1.0m into the setback, we are looking at a 2′ (0.6m) eave.

At 5/12 pitch, NFVA= 1425/150 = 9.5sq ft.  A 60/40 split for eave/ridge venting is often recommended to slightly pressurize the attic so 3.8sq ft in the ridge and 5.7sq ft somewhere else – usually in the eaves.

BC code does not allow venting any soffit less than 1.2m from property line, ours would extend to 0.9 from the property line in this case.

BC building code 2024 9.10.15.5. (11)

11) Where roof soffits project to less than 1.2 m from the property line, the centre line of a public way, or an imaginary line between two buildings or fire compartments on the same property, they shall:

    (a) have no openings, and

    (b) be protected by…

Is it feasible to achieve this venting effect/requirement on gable-end type vents? For example, two 12×18″ gable vents on each end of the house would provide 6sq ft venting.

One downside to this of course is that in soffit venting, it is recommended to vent as close to the outside as possible, to limit warmer air next to the wall from rising through the vent, however I don’t see any way around that.