Some Comments Regarding Soffit (or Overhang) Vents
Soffit (or overhang) vents provide free vent area for cooler or drier air to enter into the attic and replace the hotter and/or damper air that vents out through the upper vents comprised commonly of a power vent, ridge vents, or stationary (can, turtle, roof) vents. The openings in the soffit vents must be large enough and the vents be spaced often enough to provide to allow air to freely flow into the attic. Here are some comments on the more common type of soffit vents.
It has been our experience that the soffit vents with larger diameter round holes function relatively well (Figure SV-1 & SV-2). The larger holes take much longer to fill with dust accumulations. Continuously perforated (with holes) panels are preferred. The spacing in Figure SV-1 might be inadequate depending on the attic size.
The soffit vents with the lanced openings (Figures SV-3 and SV-4) do not function as well. The small slit openings fill quickly with dust accumulation.
The older louvered vents also function well as long they are properly spaced and there is adequate upper ventilation (Figure SV-5).
Narrow width soffits provide little surface area for the openings. Narrow width soffits often provide inadequate free vent area unless they are perforated along the full length (Figure SV-6).