Insulating and Venting unconditioned attic/roof from gable ends
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.
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In colder climates such as CZ6, R-39 in the roof is the bare minimum you would want. In the US, for new construction in CZ6 we need R-60 in the roof, which with loose-fill insulation is usually not a huge upcharge from R-39. There are diminishing returns the more insulation you add, but loose-fill is relatively affordable so even when we only had to do R-49 here, I always aim for at least R-60.
I don't know what Canadian codes say about required insulation or venting. In the US, we can vent only at the gable ends as long as the net free area is at least 1/150 of the floor area.
Thank you Michael. Planning for R60 does not significantly increase difficulty/budget of the project in the grand scheme of things. I'm all about optimizing that price/performance ratio and hitting that diminishing returns point.
BC Code re venting doesn't have much to say specifically regarding gable venting, other than it's allowable.
Vent Requirements:
1) Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area.
2) Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area.
3) Required vents may be roof type, eave type, gable-end type or any combination thereof, and shall be distributed
a) uniformly on opposite sides of the building,
b) with not less than 25% of the required openings located at the top of the space,
and
c) with not less than 25% of the required openings located at the bottom of the space.
4) Except where each joist space is separately vented, roof joist spaces shall be interconnected by installing purlins not less than 38 mm by 38 mm on the top of the roof joists.
5) Vents shall comply with CAN3-A93-M, "Natural Airflow Ventilators for Buildings."