Occasionally, some home inspector will make a big deal about mixing up different types of roof vents. There’s studies that kind of sort of maybe indicate that it’s not the best idea, or some true blue home inspector straight out of Home Inspector University thinks it’s a problem and everybody gets confused for a while. This is a response that Jim Katen, a pretty smart home inspector out in Oregon, gave to the question on another forum.
Question by the OP:
“I have a house that has gable end louvers and ridge vents but no soffits. The ridge venting is new with the roof. Is it even worth mentioning that the two should not be mixed? Would you mention that the attic may be over-vented? I want to recommend the ridge vent be removed.”
Answer by Katen:
“I’ve never seen the slightest evidence that mixing gable and ridge vents is a problem. The air in Oregon attics doesn’t seem to have read the textbook that show it how to behave.
Vents can be described as holes in the roof at a given height.
Traditionally, we put some holes low and some holes high. Air is only going to move through these holes when there’s a difference in pressure between one side of the vent and the other. Setting aside mechanical ventilation, that difference can come from the buoyancy of hot air or from wind blowing outside.
In terms of buoyancy, hot air in an attic doesn’t know the difference between a vent near the ridge and a vent high on a gable wall. The air gets hot, it rises, building up pressure, and the air from the high-pressure zone flows out through whatever openings are there. When this air flow out, the attic develops a low-pressure zone near the bottom of the attic and outdoor air flow in there.
The thing that boogers-up this process is wind. When the wind blows – even a very gentle breeze – the pressure it exerts on the vent openings is far greater than the measly pressure developed by buoyant hot air. If the wind blows toward a vent opening, hot air isn’t going to go out; it’s just going to sit there. In fact, outdoors air is going to blow in. And when that happens, the overall pressure in the attic goes up and air is going to leave the attic through whatever opening has the lowest pressure – that might be vent openings low on the roof on the lee side of the building.
Different combinations of attic ventilation and wind direction create very complex scenarios that can be very hard to predict. Even Joe L doesn’t go there in most of his stuff. It’s chaotic and it’s the reason that you will find two houses with exactly the same design and venting systems behaving differently when they’re oriented in different directions to the prevailing wind.”
So, there you have it. Houses are different. Your house is different than your neighbors. When your roofing contractor is talking about vents (he IS talking about vents, isn’t he?!?), you can now carry on the discourse in a reasonably intelligent manner.