How do I know if my attic ventilation is wrong?
Common signs of poor attic ventilation include an attic that is extremely hot on a mild day, upstairs rooms that are hard to keep cool, rusty nail tips on the underside of the roof decking, and dark stains on the north side of the attic. The most common cause is too little intake at the soffits, often because insulation has covered the vents. Adding more exhaust vents without fixing the intake can make the problem worse.
How attic ventilation works
Air enters the attic through intake vents at the soffits, warms up, rises and leaves through exhaust vents near the ridge. A working system needs both intake and exhaust, in balance.
When the soffit vents are blocked, a ridge vent will pull air from wherever it can, including from inside the house through recessed lights, attic hatches and other gaps in the ceiling. That means cooled air from your living space is drawn into the attic and out through the roof.
A common guideline is one square foot of net free vent area for every 150 square feet of attic floor, or one per 300 square feet where there is a proper vapor barrier, split roughly evenly between intake and exhaust. Net free area is the open area after screens and louvers, which is smaller than the size of the vent itself.
What poor ventilation does to a Texas home
In summer, a poorly ventilated North Texas attic can run forty to sixty degrees hotter than the outside air. That heat ages the roof deck and shingles from below and warms the air conditioning ducts that run through the attic.
In winter, poor airflow traps moisture. Warm air from the house carries water vapor into the attic, where it condenses on the cold underside of the roof decking. This leads to damp decking, rusted nails and mold stains, usually on the north side of the roof, which gets the least sun.
Bathroom fans and dryer vents that empty into the attic are a common cause of moisture problems and are worth checking first.
Use one type of exhaust vent on each roof section
Combining a ridge vent with powered fans or turbines on the same section of roof can cause the vents to work against each other. The stronger vent pulls air from the nearest opening, which is often the other vent, and air stops moving through the rest of the attic.
The standard practice is one type of exhaust vent per roof section, sized to match the available intake. Mixed vent types are common on roofs that have been worked on by several different contractors over the years.
Gable vents combined with a ridge vent can cause the same problem. The gable vents act as intake for the ridge vent, and the lower part of the attic gets little airflow.
The best time to fix attic ventilation
A roof replacement is the most cost-effective time to correct ventilation. The decking is exposed and the ridge is already being opened, so adding or moving exhaust vents costs much less than on an existing roof.
Soffit intake can be improved at any time, and it is often the less expensive part of the fix. Baffles can hold insulation away from the vents, or additional soffit vents can be installed, without any work on the roof itself.
For an attic with no ventilation at all, a roofing contractor can walk you through the options for adding it.
Related Questions
Is a powered attic fan a good idea?
It can help in an attic with enough soffit intake. In an attic without enough intake, it will pull cooled air out of the house through gaps in the ceiling, which can cost more than it saves.
Can ventilation be fixed without replacing the roof?
Yes. Adding exhaust vents to an existing roof costs more per vent because each one has to be cut in and flashed. Improving soffit intake does not require any roof work.
How much will fixing it lower my energy bill?
It depends on the home. Ventilation works together with insulation and duct sealing, so savings vary. Better ventilation lowers attic temperature, which reduces the load on your air conditioning.
