Let’s start with some basics about attic vents. The building codes require proper ventilation.
Proper ventilation operates primarily by air convection. The cold and warm temperature will drive air flow through the attic.
For thorough ventilation, you must have grilles at the soffits. This is where the air comes in (intake air).
The exhaust outlets are located near the roof ridge or along the roof ridge (continuous roof ridge vents). There might also be gable end roof vents, especially in older homes.
Not All Vents Work Well
My recommendation is to stay away from units that twirl from wind convection or use mechanical systems to operate.
The reason I say this is because what is either mechanical or has continuous movement is always at risk of breaking down.
I have so often seen the twirling units squeal because components are breaking down inside.
Vents that operate simply with air convection are the safest bet.
Attic Vents Need to Ventilate
Canadian code requirement for adequately sized vents is 1:300 (1 SF of vent for every 300 SF of attic floor area).
To ensure optimal ventilation, ensure that ventilation is evenly distributed.
All your grilles should be screened. Both the grilles in the soffit and the vents on the roof ridge line should be screened. This screening helps reduce the possibility of rodents and insects migrating into this space.
Soffit grilles require a space between the roof sheathing and the insulation. This space will ensure air will pass. This space can be achieved with a polystyrene board. It is readily available in all hardware stores in the roofing department.
There are a myriad of different types of roof vents. They come in all different sizes and may operate somewhere differently from one another.