A while back, in a blog that asked "What is Giovanni Venturi doing in my attic?" I mentioned that combinations of different methods of ventilation can result in poorer or even no ventilation.
For example for the best method of venting attics---Ridge vents in combination with soffit vents---to function properly there should not be any "easier" path of air flow to the ridge. For example if you have a ridge vent and gable vents, air can move more easily from the gable to ridge than from the soffits to ridge leaving areas of the attic under-ventilated.
Another poor combination is when there are roof vents in combination with the ridge vent as pictured below. (The ridge vent is installed in a continuous line and capped by the ridge shingles.) In this installation it is pretty easy to see how when the ridge vent is drawing air out of the attic the easiest path of air flow is going to be through the roof vents. Again, this leaves most of the attic under ventilated.
Where you usually see these systems compromised is when the original roof had one method and when the roof was replaced someone said "hey, lets put this newer/better method of venting in." But then what do you do with the old roof vent holes? Obviously you just put the roof vents back in
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WA State, Home Inspector Advisory Licensing Board