how much attic Ventilation do you need?
Most american building codes require sealed-air-conditioned attics to be ventilated, and asphalt shingle manufacturers require minimal levels of attic ventilation (say, 1 square foot of ventilation per 175-300 square foot of attic surface).
But that's just broad references.
There has been - during the last decades - much controversy over vented vs. non-vented attics and roofs. And though many studies - involving cold and mixed climates - confirm the benefits of roof ventilation (see for instance: OSTI.gov Technical Document on Attic Ventilation), there is not a wide consensus over the issue or the best solution.
Attic ventilation is typically seen as a way of addressing attic moisture and overheating problems, but the issue is complex, and there are wrong claims that must be debunked.
Attic ventilation can be useful and needed, but we should not overrate its importance or its role.
Existing homes & attic ventilation
In existing homes, before increasing attic ventilation, consider the efficiency of your existing system.
Are the roof vents working properly?
Soffit-ridge vents systems are the best attic ventilation systems. Other ventilation systems often raise more problems than they can solve.
If you intend to increase or fix the roof ventilation, do it right: Your attic must be ventilated with air from the outside, not with air coming from the living space. And this requires a very airtight attic and very high levels of attic floor insulation, and a generous venting system at the perimeter of the roof (balanced by a proper top/ridge venting system). Attic air-tightness and very high levels of attic insulation are preconditions to a sucessful and effective attic ventilation system.
See also: Roof-Attic Ventilation Does roof ventilation cause heat loss during the winter?
There’s a widespread perception that roof ventilation is a cause of heat loss during cold weather. Which may be true, in many climates, in houses with leaky and poor insulated attics. But the root of the problem is not the ventilation system itself: it's the lack of insulation and attic air leakage paths.
Attic ventilation in new houses
Keep in mind the basics:
To prevent attic overheating or attic moisture you should keep the ceiling plane very airtight and install very high levels of insulation.
Do not locate ducts in the attic. Do not locate other stuff in it.
Just install very high levels of a good insulation material on the ceiling plane; it’s the most important factor to prevent problems in your attic.
If new homes with basement foundations, make sure that the basement is also extremely airtight, to avoid air exchange between the attic and the basement.
Do not use solar or electric fans. They are mostly useless in well insulated and airtight attics.
If you are going to ventilate the attic, consider a continuous and generous air entry all along the perimeter of the roof. In other words: the ridge and the soffit venting systems should be relatively balanced, but energy experts are now stressing the importance of putting more vents at the perimeter of the roof than at the top exit points (ridge).
That's better than a very strict balance between the ridge (top) and soffit (bottom) vents.
That's the most effective way of ensuring that attics are ventilated with air from the outside.
Attic ventilation can also be important in hot climates, in cooling strategies. But be aware: if the outside air is more humid than the inside air, attic ventilation can be harmful. It will increase attic moisture, instead of decreasing it.
Also be careful when considering (in hot tropical climates) operable ceiling vents. They can be great in cooling strategies, to generate stack ventilation – where operable ceiling vents allow hot air to escape… - but they often collide with an airtight attic-ceiling approach.
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