I will initially concede that living on the border or in Canada will have different building practices.
I am from New England (just a few miles from Peter) and I will be the first one to say that regardless of standards or codes, different parts of the country require different practices whether they are adopted by the AHJ .
Considering the laws of psychometrics and fluid dynamics, proper attic ventilation requires upper and lower openings to allow convection to occur.
The IRC and UBC both recognize that less ventilation is required per square foot of the attic space if more ventilation is located on the upper level of the roof than the lower eve.
If you suck air out, you must allow it to come back in somewhere.
The attic produces a significant stack effect and if there are not soffit ventilators, this air must be drawn in from ceiling openings on the upper floor of the building. This causes a major efficiency issue.
I realize that increasing airflow and making a cold roof causes ice damning and water damage issues so I am not arguing the point.
However, people that have a ridge vent and a gable vent are providing a greater source of air ventilation to the exterior at the upper portions of the attic space as required to reduce the necessary ventilation of the attic.
So where have we evolved into saying that if you have a ridge vent and a gable vent that you should close one of them up?
The more ventilation on the upper part of the roof the greater convection that occurs and except for our friends in the North that have to deal with ice damning, the more up high the better.
In the South, we like to stick HVAC equipment in the attic without increasing the attic ventilation. This is stupid on top of stupid.
So for anybody in the South that takes on the substantiated practices of the North, you are probably wrong.
I am open to suggestions and clarification and I specifically request clarification of perspective from my friend Peter Russell who obviously has a real good reason for controlling ventilation at the soffit.
My point is, just because Peter does it, does not mean you need to worry about it if you’re in Houston Texas.