In an effort to explain to a client why turbine roof vents are not a good thing I produced the attached graph.
It shows graphically that there is an inverse relationship between when the attic space needs the most venting and when the wind is blowing the most. So the turbine is;
- working its hardest when its needed the least
- working the least when its needed the most
The second point then brings up the question - When its working the least is it still working enough? I would suppose that would vary greatly on individual instalations.
The velocity of a spinning turbine can be attributed mainly to the force of exterior wind. The spinning from the force of air rising from the attic through the turbine is minimal in comparison.
Certainly there are also other issue regarding turbines such as noise and depressurization which in themselves may be reason enough not to use turnbinbes but I mainly wanted to illustrate the basic venting effects.
The actual data used to plot the graph is from weather data from my local area but I suspect most northern non-coastal locations are similar.