Attic Ventilation 101
Find out the 7 Simple Rules for a successful attic venting system.
Q: What is a vented attic space?
A: A vented attic works on the principle that warm/heated air will rise naturally and create an airflow that removes excess humidity from the attic space under a roof deck. A vented attic allows for air flow under the roof deck and above the insulation installed on an air-sealed attic floor. Vented attics will work in hot, cold, and mixed climates when executed properly. Venting an attic can reduce accelerated deterioration to roof shingles and prevent water damage to roof sheathing and other structural roof components.
In a cold climate, the primary purpose of ventilation is to maintain a cold roof temperature to avoid ice dams created by melting snow and to vent any moisture that moves from the conditioned living space into the attic area.
In a hot climate, the primary purpose of ventilation is to expel solar-heated hot air from the attic or roof to reduce the building’s cooling load and to relieve the strain on air-conditioning systems.
Here are some simple rules for a successful attic venting system:
- Seal the attic floor (top side of a finished ceiling) completely. Air leaks at ceiling penetrations can allow for conditioned air to pass into the attic space. This can cause for energy loss, condensation, and water damage to wood components, and contribute to ice damming in cold climates. Recessed lighting and other ceiling mounted penetrations are the most common passages for conditioned air to pass into the attic space.
- Slightly pressurize the attic by installing more intake vents than exhaust vents. A good percentage ratio is between 55/45 to 60/40.
- Bulk up the insulation above the top plate (eave) of the roof. This is the attic insulation above the top of an exterior wall. The R-value of this attic insulation should be more than the R-value of the exterior wall assembly. (Note: this insulation should still allow for a clear path of airflow from the soffit/eave intake vents)
- Vent the soffit continuously to provide enough intake ventilation at the soffit/eave area. This vent should be placed on the far outside edge of the soffit (especially buildings with deep soffit areas) to prevent ice damming conditions. Insulation baffles should be placed on top of the insulation at the eaves in between each rafter bay to allow for a clear path of airflow.
- Provide airspace path for air to flow unobstructed from the intake vents to the exhaust vents. The International Residential Code (IRC) calls for a minimum of 1-inch of airspace but here at J2 we recommend a 2-inch minimum airspace between the top of the insulation layer and the bottom of the roof decking.
- Do not mix intake and exhaust vent types to avoid “short-circuiting” of the airflow through the attic. (i.e. Mixing dormer and ridge exhaust vents)
Make sure that dryer and kitchen hood vents are “tight-lined” to the exterior. This means that vent ducts related to dryers, kitchen hoods, and other HVAC equipment that passes through the attic space is insulated, has completely sealed joints, and exhausts to the exterior of the building.
Diagram Credit: FineHomeBuilding.com