If you want your housing envelope to perform at its peak, proper attic ventilation is key. This is because the regulation of the temperature, pressure and moisture in the attic to more closely resemble a conditioned space will reduce thermal loss and moisture vapor migration. Effective attic ventilation practices also ensure the durability of your roofing components and eliminate the possibility of damage by mold and mildew.
Why Ventilating Attics is a Great Idea
The average family of four generates 2 to 4 gallons of water through daily activities and proper attic ventilation will help to effectively move this water vapor from the home rather than leaving it to collect in the attic where it has the potential to cause mold and mildew.
A properly ventilated attic will also reduce the pressure on the HVAC system which allows that system not only to work optimally, but also to last longer. It also enables designers to place HVAC ducting in semi-conditioned parts of the home which reduces thermal loss and improves energy efficiency.
Ventilated attics prevent snow melt in winters which, in turn, prevents the formation of ice dams and helps to extend the life of roofing components.
In summer, effectively ventilated attics help to reduce the discomfort and excessive cooling costs.
Ventilation Best Practices
Code requirements place the ratio of attic ventilation at one square foot of net-free ventilation for every 150 to 300 square feet of attic space (assuming a vapor barrier is in place). If no vapor barrier is in place, the ratio drops to 1:150. Consult your local code requirements, climate zone and roof design when designing your own attic ventilation.
One of the most popular designs is a soffit-to-ridge design which is effective and inexpensive for both new builds and retrofits. This entails installing louvered vents along the entire length of the soffits and continuous vents along the roof ridge (stopping at least 6 inches from the gable wall or ridge end. This allows air to move freely from soffit to eve throughout the roof system with no hot or cold spots. This is especially effective as a passive chimney effect in raised heel truss roof installations.
The vents must be properly sized to create the passive chimney effect, or air pressure at the top of the roof will prevent air from below from effectively ventilating. This leads to a pressurized attic which cancels out the effects of the vents. Ideally, the ventilation system should be balanced and you can ensure this by checking that the total net free area of the intake vents either matches or exceeds the total net free area of the exhaust vents.
If you are adding insulation or re-roofing be sure the contractor does not cover any of the vent system.