Someone wrote and asked me:
I had my roof replaced 18 months ago and the roofing company found many of the boards to be rotted and moldy (even though the house had a complete reboard done only 9 years earlier). They informed me it was because I did not have enough ventilation. I had 5-6 can vents near the top of the roof, and gable vents on both ends of the single story brick home. The roofing company added “vented drip edge” to provide ventilation at the bottom of the roof since the house has no overhangs. This created an ice dam problem in the winter (northern midwest climate), so I’ve had an insulation company out that found mold starting to grow again on the same boards that were just replaced. They claim that having the drip edge venting, can vents and gable vents altogether on a 1000 sq ft house is counterproductive and causes air only to swirl around the attic but not exit. They recommend blocking the gable vents and installing baffles for the drip edge venting besides increasing the insulation. Does blocking the gable vents make sense? Is it true that having all three types of vents causes the air not to escape the attic?
I would not so much say that it keeps air from escaping but what happens is that you get very poor air exchange — it just goes in certain patterns and some air in the attic stays stagnant.
I concur with blocking off the gable vents … and I would say that the baffles are necessary as well.
I hope this helps.