Look for:
- Intake vents low on the roof and exhaust vents near the peak;
- Intake vents blocked by attic floor insulation
- Vents installed but underlayment/sheathing not cut to allow airflow;
Attic Ventilation Performance
The performance of specific attic ventilation systems can vary with different climates zones, homesite locations and conditions, and weather conditions.
The typical approach is to thermally isolate the attic space from the living space by installing some type of thermal insulation on the attic floor. Heat that is radiated into the attic from sunlight shining on the roof is then removed using devices that allow natural air movement to carry hot air to the home exterior. This reduces summer cooling costs and increases comfort levels, and can help prevent roof problems that can develop during the winter such as the forming of ice dams along the roof eves.
Natural air movement is introduced by providing air intake vents low in the attic space and exhaust vents high in the attic space. Convection causes cool air to flow into the attic to replace hot air pushed out the exhaust vents by thermal buoyancy (the tendency of hot air to rise). Conditions that block ventilation devices, or systems and devices that are poorly designed or installed can reduce the system performance.
Adequate Attic Ventilation Formula (You'll be estimating this, no calculator necessary):
12 square inches of net free ventilation for each:
- With no low-perm (plastic) vapor barrier: 150 square feet of attic space
- With low-perm (plastic) vapor barrier: 300 square feet of attic space
Twelve square inches might be an opening of 3 inches by 4 inches.
“Net free” means that louvers or other devices that reduce the actual size of the opening must be subtracted from the total opening size to find the net free size.
Whole House Fans
In colder climates, whole house fans should have an insulated cover that will reduce heat loss during the heating season.
Ventilation Of Vaulted Roof Structures
Roof structures over vaulted (cathedral) ceilings are ventilated using soffit and continuous ridge vents. Properly constructed, thermal insulation is installed in rafter bays in a manner that leaves a 2” space between the top of the insulation and the bottom of the sheathing. Air enters this space from the soffit vents, travels up the air channel and out the ridge vents. This system helps ventilate the underside of the sheathing and keep the roof cool, helping to prevent the development of ice dams in cold climates and reduce cooling costs in warm climates.