Attic Ventilation

By installing attic ventilation items such as these, hot and moist air can easily be expelled from the attic. A properly constructed ridge vent should traverse nearly the entire length of the roof peak. Likewise, properly constructed soffit vents should traverse nearly the entire length of the roof eaves. The standard formula for calculating the minimum ventilation in a continuous vent system is 1 square foot of vent opening for every 300 square feet of insulated attic ceiling with a vapor barrier.

So for example, a 26’ x 36’ colonial style home needs 3.12 square feet of total attic ventilation (Av = [26 x 36]/300 = 3.12 sqft). Typically between 50-60% of the calculated total vent area should be allocated to the soffit vent and the remainder to the ridge vent. If the attic insulation does not include a vapor barrier on the warm-in-winter side of the ceiling, the attic ventilation area required doubles to a ratio of 1:150.

An alternative to installing ridge vent along the roof peak is to use cap vents such as turbine vents, pot vents or mushroom vents.

These types of roof vents are passive attic ventilation devices that also expel heat and moisture from the attic. However, the formula for calculating the minimum amount of attic ventilation changes when using a non-continuous vent system. When using a non-continuous vent system you need 1 square foot of vent opening for every 150 square feet of insulated attic ceiling with a vapor barrier. Thus you need to double the amount of ridge and soffit vent area. As a result, you wind up with quite a large number of pot vents on your roof. Consequently this is why ridge vents are most commonly used on homes. Cap and gable vents do, however, offer ventilation value and should be considered to help complement a continuous ridge vent system.

There are also power attic vent systems that run off electricity and solar energy. Again, these types of vent systems can help supplement a continuous ridge/soffit vent system but should not be the total solution for attic ventilation. In addition, the electric ones consume energy.

Another area that needs to be considered when ventilating an attic is how the attic insulation is installed. In particular, it is important that the attic ceiling insulation is not shoved up tight against the soffit vents such that it restricts airflow. Particularly on shallow roof lines, this can be a common problem. Consequently rafter babbles should be used to prevent attic insulation from blocking soffit vents and restricting air flow.

So the next time you are inspecting your roof or attic, make sure to take a look at the attic ventilation system and to asses is if it is adequate in capacity and functioning properly.