Attic ventilation provides a means for airflow throughout your attic space. It is important that the entire space is considered ventilated because if only a portion of the attic space “breathes” then it is no benefit to you.
So why ventilate at all?
There are many reasons sited as reason to ventilate the attic space. The most important reason in my opinion for attic ventilation is moisture control. In fact, this is the reason the attic ventilation discussion began in the 1930’s because as homes became better insulated moisture issues surfaced. Moisture can form in the attic space as the moist air from the house condenses in the attic space. You can sometimes see this in the winter as water droplets form on the nails protruding through the roof deck. Excessive moisture can degrade the life of the roof deck and cause other moisture related problems such as mold.
Ice damming is another major issue that proper ventilation can help resolve. Ice damming is a condition that is created when snow melts at the upper roof, where heat build-up raises the roof temperature above freezing, and refreezes at the lower roof creating a dam of ice. This in turn prevents water from snow-melt to reach the gutter system and instead backup under the roof shingles. This can create leaks in the home and cause considerable damage. Attic ventilation creates an even temperature in the attic space to promote even snow-melt.
Proper attic ventilation can also reduce the overall temperature of an attic. This can help in maintaining cool conditions in the summer time in the upper levels of a home. This can lower the amount of air conditioning required to cool the same space. When talking energy efficiency, attic ventilation should be considered in conjunction with attic insulation.
Another reason often sited for attic ventilation is extended shingle life. It is my opinion that there is not enough scientific research to support this. Shingle color, roof orientation, and geographic location have more impact on shingle life and shingle temperature than attic ventilation. Nevertheless roofing manufactures and organizations continue to beat this drum of extended shingle life and as a result you will find some roof covering warranties require proper ventilation. It would be more accurate to state that proper attic ventilation is needed to maintain roof shingle warranties rather than life expectancy.
So how much ventilation is enough?
The predominate theory and the one prescribed by most code officials is 1 square foot of ventilation for every 150 square feet of attic space expressed as 1/150. Or, if there is a vapor barrier installed under the attic insulation 1/300 is required. This calculation originates from a 1940’s document published by the FHA (Federal Housing Administration). Even though it does not seem to have much basis in scientific research it has persisted to this day and is considered the final word in ventilation requirements.
Types of ventilation.
There are many different ways to provide ventilation and no one method works for every roof type. The types of ventilation can be broken into two major categories; passive and active.
Ventilation needs to address the whole space. This is resolved by the methods applied drawing in airflow from the lower sections of the roof, as in the eaves, up to the upper sections of the roof. Continuous soffit venting, shingle vents, or can-vents can accomplish the lower roof section venting. Ridge vents and can-vents might be used on the upper section. Gable vents are an older approach and not as effective as other means. They also should not be used in conjunction with other vent types.
The previous approaches are all examples of passive venting. Active vents are powered vents with a thermostat and possible a humidistat, wind turbine vents that spin when the wind blows, and fan assisted gable vents.
You should consult a reputable roof contractor or home improvement contractor to determine what will best work with your roof type.
Common problems.
One of the most common problems with attic ventilation, especially in retro-fit situations, is a lack of eave venting. It is common for roofers to sell homeowners a ridge vent but with a lack of eave ventilation it serves no purpose. Other problems with eave venting can include the vents rendered ineffective by painting them shut. In the attic often insulation is blocking airflow. Baffles installed in the attic space can correct blocked airflow by insulation.
Another common problem is an unbalanced system. The quantity of free airflow at the eaves must be balanced with the upper roof ventilation in order to meet the 1/150 or 1/300 design parameters.
In conclusion it is best practice to properly vent attic spaces to prevent problems with moisture, ice damming, and excessive heat build up. (Note there is a hot roof theory that is beyond the scope of this discussion) Most new homes will have some kind of ventilation but that is not to say it has been properly balanced or installed. Many older homes lack ventilation entirely. Always consult with a reputably contractor about your options and do your own research. Then ask a home inspector to verify installation before making your final payment. If you follow these principals you should end up with a good working system.
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