The ratio of the upper to soffit vent net opening needs to be around 1:1.
The soffit openings can be more than the upper vents but not the other way around.
I’ve been told many times that the general guidline is 1 roof vent per 300Sq Ft of attic “floorspace”. Does anyone know if there is a similar guideline for soffit venting?
Soffits are to allow the escaping air to be replaced. The airflow capactiy must be balanced between intake and exhaust vents. In general the NFA (net free area) of intake (soffit) venting should be equal to or greater than the net free area of exahust venting.
This ia a good graphic and it just about sums it up!
Patrick, as you know, warm air flows up as heat rises, in through weap holes joining air from soffits which grabs all humidity it can carry as it races to the exterior through the maximum which by itself creates a chimney effect.
All this is gravitational and non mechanical.
If lateral vents are not blocked, then the air goes through laterals and out through the maximum leaving walls and soffits non ventilated, therefore the conditions may deterriorate, the roof surface will not last as long and so on!