Attic Ventilation in Thornton, CO

A ridge vent is a slot cut along the peak and covered by a low vent that runs the full length of the ridge, then capped with ridge shingles so it barely shows from the street. HomeGuide prices it at $7 to $15 a linear foot, or $350 to $600 for a typical house. Foot for foot it moves a lot of air: an IIBEC article on attic ventilation shows 15 static box vents matching the exhaust of 42 feet of ridge vent. The catch is intake. The same article says a ridge vent on a house with no soffit or eave vents does next to nothing, and can even draw warm, damp air up out of the house. Hip roofs with a short ridge, or none at all, can use hip-specific ridge vent products or a powered fan instead.

Intake is the half people forget. The code's 1-in-300 option only applies when 40 to 50 percent of the vent area sits within 3 feet of the ridge and the rest sits in the bottom third of the attic, which in practice means the soffits. IIBEC lists the usual ways intake fails: vents buried by insulation where no baffles went in, vents clogged with debris, and vented soffit panels nailed over solid wood with no holes cut behind them. Baffles are the fix. They are cardboard or foam chutes stapled between rafters above the wall, and the code requires at least a 1-inch air space between the insulation and the roof sheathing at each vent. HomeGuide puts new soffit vents at $300 to $650. Damaged soffit panels themselves are covered on soffit and fascia repair.

Box vents, also called turtle or low-profile vents, are square hooded openings cut into the roof near the ridge. They are cheap at $40 to $200 each, per HomeGuide, and the right answer on roofs with little ridge to work with. They are also easy to under-count. IIBEC puts a typical box vent at 50 to 60 square inches of net free area, and gives an example where a gable roof needs 240 square inches of exhaust but has three vents providing 180. Never mix them with a ridge vent on the same attic: IIBEC calls it a short circuit, because the air short-cuts between the box vents and the ridge instead of sweeping up from the soffits. Dented vent hoods are one of the post-storm signs on the hail damage roof repair page.

A powered fan pulls air out instead of waiting for it to rise. IIBEC cites the roofing contractors' association rule that a fan moving 1 cubic foot per minute per square foot of attic floor equals the 1-in-150 ratio, and stresses that the fan still needs matching intake. HomeGuide puts attic fans at $300 to $900 installed and a solar roof fan at $300 to $1,400. It says most professionals don't recommend electric fans because running them uses up whatever the cooler attic saves on air conditioning, while a solar fan costs nothing to run. A humidistat switches the fan on when the attic gets damp, not just hot. InterNACHI gives attic fans 15 to 25 years, so the fan will likely need replacing before the shingles around it.

Gable vents are the louvered openings in the end walls under the peak. With soffit vents below, IIBEC says they work as exhaust. Without them they serve as both intake and exhaust, and how well depends on which way the wind is blowing. Many are simply too small for the attic behind them. The code doesn't specifically address gable vents, so whether one counts toward the required area is the building department's call. HomeGuide prices a gable vent at $100 to $500, and notes that roofers recommend sealing existing gable vents when a ridge vent goes in, for the same short-circuit reason as box vents.