Materials & Shingles · FAQ

How much soffit intake area does an Illinois attic need?

The IRC requires a 1:300 ratio of net free vent area to attic floor space, split roughly 50/50 between intake (soffit) and exhaust (ridge). For a typical 1,500 square foot Madison County attic, that’s 7.2 square feet total net free area — about 3.6 square feet of soffit vent.

Most Illinois homes built before 2000 have nowhere near that. Trill checks soffit intake on every install and adds vented soffit panels where needed; the upgrade adds about $4 to $7 per linear foot of soffit. Without adequate intake, even a perfect ridge vent will short-circuit, pulling outside air down the back of the ridge vent rather than up through the attic. That’s why proper attic ventilation has to be designed end-to-end.