Attic and Roof Ventilation in Kansas City

An attic vents by convection. Warm air rises and leaves through openings near the peak, and that only happens if replacement air can enter down at the eaves. Exhaust without intake is a sealed jar with a hole in the lid.

What we find on inspections here, over and over, is a continuous ridge vent on a house whose soffit venting is either painted shut, screened over with solid aluminum, or buried under blown insulation that drifted down the slope and packed the eave cavity solid. From the street the roof looks correctly vented. In the attic nothing is moving.

In the worst version, the ridge vent still pulls, so it draws its makeup air from inside the house through recessed lights, bath fan housings and the attic hatch. Now you are paying to condition air and sending it out the roof, and you are carrying household humidity into the attic with it.

Code and manufacturer specs both work off net free vent area, meaning the actual open square inches after you subtract the louver and the insect screen. A vent stamped 50 square inches might deliver 9 of net free area, and the two numbers are not interchangeable.

The standard is one square foot of net free area for every 150 square feet of attic floor, and it drops to one in 300 when there is a proper vapor retarder and the venting is split within a few percent of half low and half high. Balanced is the operative word. Extra exhaust beyond the intake you have does not help, and it is not neutral either.

We measure the attic footprint, count the soffit openings, measure the ridge run, look up the actual net free values for what is installed, and put the arithmetic in front of you. It takes about fifteen minutes and it settles the argument.

A ridge vent and a set of powered or box vents on the same attic plane will short-circuit each other. The ridge vent is the highest opening, so it becomes the exhaust; the box vents lower on the slope become the easiest intake, and outside air gets pulled in there and pushed right back out the ridge a few feet away.

The far end of the attic never sees air movement. Worse, if a wind-driven rain hits that slope, the vent that has turned into an intake can pull water in with it.

Powered attic fans have the same conflict on a larger scale, and in a house with any real ceiling leakage they pull conditioned air out of the living space. If you have a mix up there now, the fix is usually to pick one exhaust strategy, close off the others properly, and open up the intake at the eaves. Baffles at each rafter bay keep the insulation from re-packing the opening.

In July an unvented attic here runs far hotter than the outdoor air, and asphalt shingles sit on top of that heat all day. Heat drives out the volatiles in the asphalt, the mat gets brittle, and granules loosen. A shingle rated for a long life can come up years short and every bit of that damage happened from below, where nobody looks.

Winter is the sneakier one. Cooking, showers and laundry put a lot of moisture into a house. If that vapor gets into a cold attic and cannot leave, it condenses on the underside of the sheathing. People find it as frost on the nail points, then as dark staining, then as mold on the north slope, and often mistake it for a roof leak. It is not. It is a ventilation and air-sealing problem.

Uneven attic temperature also feeds ice damming at the eaves, and it makes the upstairs of a story-and-a-half house impossible to cool. Fixing intake will not solve everything, but it is the cheapest thing on the list and it belongs first.