What ventilation does
In summer, a vented attic lets hot air out so the roof deck and shingles do not bake from below. In winter, it keeps the deck cold so snow does not melt and refreeze at the eaves, and it carries off moisture that rises from the house before it condenses on the cold underside of the roof.
Central Pennsylvania gets both problems: humid summers and long freeze-thaw winters. An attic that is too warm in either season shortens a roof's life.
Signs of a poorly vented attic
- Frost on nail tips or the underside of the roof boards on winter mornings.
- Dark staining or mildew on the roof deck, often worst on the north side.
- Damp or matted insulation.
- Ice dams and long icicles along the eaves.
- Upstairs rooms that stay very hot on summer evenings.
The basic rule
The International Residential Code, which Pennsylvania's Uniform Construction Code is based on, sets a minimum amount of vent opening for an attic: generally 1 square foot for every 150 square feet of attic floor, or 1 for every 300 when the vents are balanced between low and high on the roof and certain other conditions are met. Intake vents at the soffits and exhaust vents at or near the ridge, working as a pair, are the usual approach.
Mixing different exhaust vents on one roof, such as a ridge vent alongside a powered fan or gable vents, can short-circuit the airflow. A roofer should look at the whole attic, not just add more vents.
What to ask before the new roof goes on
- 1Are the soffit vents actually open, or blocked by insulation or paint?
- 2Will the new roof have a ridge vent, and will the ridge be cut open for it?
- 3Will any old vents be removed so the system is balanced?
- 4Are bathroom and kitchen fans vented outside the house, not into the attic?
- 5Is there anything about the attic, such as mold or wet insulation, that should be handled before the roof?
See also: Ice dams on Harrisburg-area homes · How long roofs last in central Pennsylvania
Rowhouses and flat roofs
Many Midtown and Allison Hill rowhouses have little or no attic, just a shallow space under a low-slope roof. Ventilation there is harder and sometimes not practical, and insulation above the deck becomes the main tool. See flat roofs on rowhouses.
Questions
+Can an attic have too much ventilation?
More exhaust without enough intake can pull conditioned air from the house. Balance between soffit intake and ridge exhaust matters more than the total number of vents.
+Do I need a powered attic fan?
Usually not with a working soffit-and-ridge system, and a fan can work against a ridge vent. Ask the roofer to explain the plan for your attic before adding one.
+Does insulation replace the need for ventilation?
No. They do different jobs. Insulation keeps heat in the house; ventilation keeps the attic close to outdoor temperature and dry.