We often talk about attic and roof ventilation when inspecting homes. Proper attic and roof ventilation is a delicate balance between air intake and air exhaust.
All to often we find attics with too little or short circuited ventilation.
During the winter the purpose of attic and roof ventilation is to prevent excessive moisture from some heating systems, and closed house conditions along with preventing ice dams from forming which are created by melting snow and ice. This is accomplished by helping keep the roof surface as close to exterior temperatures as possible. The melting snow is typically caused by heat loss due to in adequate insulation resulting in convection heat loss and/or air leeks from the conditioned (living) space.
During the summer air flow created from the intake and exhaust air vents along with the natural flow of air through the hot attic provides ventilation. The purpose of this ventilation is to remove moisture and reduce the temperature of the roof surface. Insufficient attic ventilation and high moisture levels is known to reduce the life expediency of roof shingles, cause roof sheeting rot and promote organic growth. Insulation is just as important during the summer months as it helps reduce heat gain thus reducing cooling costs.
Through the years attics have been vented by the use of a combination of some of the following ventilation methods including soffit or eve vents, gable vents, static vents, turbine vents, power vents and more recently ridge vents. The right amount and combination of attic/roof ventilation is best calculated by a design professional or a insulation and ventilation contractor.
When homeowners have new roofs installed roofing contractors will often install a ridge vent. The addition of a ridge vent may require some additional ventilation modifications to prevent short circuiting. Ventilation short circuiting can occur with the combination of soffit, gable and ridge vents. To maintain proper air flow and turn over in most, but not all cases the gable vents should be eliminated or covered to prevent short circuiting of the roof and attic ventilation. Figure 1 represents proper air flow. Short circuiting will leave areas of the attic with no air flow or air turnover. Figure 2
When homeowners add attic insulation, which we will discuss in more detail in a future blog article the most common error is the method of insulation. The proper method of installing insulation is imperative as to not to impair, restrict or block the flow of air from the soffit vents (air intake) to exhaust vents.
The U.S. FHA (Federal Housing Administration) recommends the attic ventilation ratio of 1:300. This means that for every 300 square feet of non-conditioned attic space there should be 1 square foot of combined intake and exhaust air.