Once again, the real world can interfere with this perfect application of theory. Typically, in vaulted ceilings, or the sloped portion of a semi-bungalow ceiling, the builder will pack the space with as much insulation as possible, to gain the most R value they can. And I have seen it packed! Packing fiberglass into areas actually decreases its insulating qualities. If you stuff R20 insulation, normally for a 2×6 wall or ceiling, into a 2×4 wall or ceiling, you’ll actually end up with less of an R value than you would have had you used the standard R-12 insulation. So often, in an attempt to handle a difficult to insulate area, the builder will actually make things worse, to say nothing about the fact that there is no way air can get to the roof deck to cool it off.
The real world tends to be about compromising, and coming up with the best solution, recognising all of the factors involved.
One of the mistakes I see contractors and homeowners make is assuming that because they put in a couple of vents, or a big Whirlybird on their home, their attic space is vented. The style and placement of those vents is critical to proper ventilation. The laws of physics do apply!
Warm air rises. Many people correctly figure that in order to let warm air to escape, they should place vents as high as they can. What they often forget is that in order for warm air to escape, cool air must get in. In our perfect example, that cool air comes from the soffit vents. In the real world, we try to get as close to that ideal as possible, by providing intake air as low as possible, and placing vents for exhausting air as high as possible in the space. Typically, you should have one square foot of open space ventilation, both intake and exhaust for every 300 square feet of floor space. For instance, if your home is 1800 sq. ft., you should have about 6 sq. ft. total of both intake and exhaust ventilation.
In semi-bungalows, I’ve had good success placing gable vents as low as possible, and as close to the eaves side of the knee wall space as possible. Combined with roof deck vents placed as high as possible and closer to the centre of the roof, this has been effective in reducing ice damming problems.
I really like the ventilation products produced by the company Maximum Ventilation:
They are solidly built, and, because they are produced by a company situated in Quebec, are engineered and tested for real world Canadian conditions. Their roof top vents are more efficient than anything else on the market at ventilating attic spaces, are designed to sit above snow pack, and are practically impervious to water infiltration, unlike whirlybird style ventilators. They have a line of vents designed specifically to address the issue of getting air intake ventilation low on the roof deck, as well as many other unique situations.