Ventilation, for homeowners
Does your roof breathe?
A new roof on an attic that cannot breathe wears out early, grows ice dams, and can run into trouble with the shingle warranty. Most estimates say "ridge vent" and stop there. This page explains what to look for and the six questions that turn "ridge vent" into numbers.
The Attic Airflow Check is a free two-minute calculator: put in the vents on your roof and see whether intake and exhaust are in balance.
Signs an attic is not breathing
- Ice dams at the eaves most winters
- Frost or damp on the underside of the roof deck in cold weather
- A second-floor that stays hot long after sundown in summer
- Shingles that curled or lost granules well before their rated life
- Soffit vent covers with solid wood or blocked insulation behind them
The basics
Intake low, exhaust high, and the two must match
Intake
Air comes into the attic low, at the eaves, through soffit vents. If the soffit is painted shut, stuffed with insulation, or has no opening behind the vent cover, there is no intake no matter what the vent looks like from the driveway.
Exhaust
Air leaves the attic high, at or near the ridge, through a ridge vent, box vents, or a powered fan. Exhaust without intake pulls air from the house instead of the eaves, which is the worst of both.
Balance
The two need to roughly match. Extra exhaust does not make up for missing intake. Use one kind of exhaust: mixing types, like a ridge vent with box vents or gable vents left open, can short-circuit the airflow.
The rule of thumb
The code rule is one square foot of net free vent area for every 150 square feet of attic floor, split about half intake and half exhaust. In cold climates like Minnesota, the code allows one for every 300 only when both are true: a vapor barrier on the warm side of the ceiling, and vents balanced high and low. Not sure about the vapor barrier? Use 150.
Net free area is the open space air can actually pass through, after the screen and the louvers. It is printed on the vent product, and it is always smaller than the vent looks.
Why the shingle warranty cares
Manufacturers require adequate ventilation for their shingle warranties, and the requirement is spelled out on the product warranty sheet. Check your product's warranty sheet, and ask the bidder to show you the ventilation numbers that meet it. A roof that cannot breathe wears out early: heat cooks the shingles from below in summer, and in winter the warm attic melts snow that refreezes at the eaves as ice dams.
Ask every bidder
The six ventilation questions
Ask all six, in this order, of every contractor who bids the roof. A good roofer has the answers and will put them in writing. If the answer to the last one is no, that tells you something too.
The ventilation line is one of the seventeen in Compare my estimates, and the questions come from Guide 2.
- 1
What are you doing for intake, not just exhaust?
- 2
How many square inches of net free area does this roof need, and how many will it have when you are done?
- 3
Are you keeping or removing the gable vents? (Mixing gable vents with roof vents can short-circuit both.)
- 4
Are you using one kind of exhaust, or mixing ridge vent with box vents?
- 5
Are the soffits actually open behind the vent, or painted and insulated shut?
- 6
Will you put the ventilation numbers in writing on the estimate?
Print it
Take the six questions to the table
The six questions above, ready to print and take to each bidder. Ice dams and attic airflow are also covered in Guide 8: Storm Ready: Hail, Wind and Winter.