VENTILATION

The ventilation of attic spaces is an important component of building technology. In steep slope roof construction it is recommended that the attic cavity be properly ventilated. The cost to ventilate this space is typically not a significant issue with the exception of cathedral ceiling type of systems where the space needed to insulate and ventilate is too small to satisfy both requirements.

Why is ventilation important?

Here are several reasons why it is important to ventilate an attic cavity:

· To reduce cooling costs in the summer.

The temperature in attic spaces can reach more than 165 degrees F. This heat radiates down into the living spaces forcing the homeowner to use air conditioning to cool these spaces. Proper ventilation would allow the hot air to escape, replacing it with cooler air from the outside and thereby keeping the living space cooler naturally.

• To reduce ice damming problems.

The worst ice damming problems usually occur when the heat that has escaped into the attic, melts the snow on the roof, while the temperature outside is below freezing. If the heat is removed through ventilation, then the attic is kept cool, resulting in less snow melting and thereby reducing the formation of ice dams significantly. Adequate insulation amounts must also be considered in the solution to this problem.

• To dry out an attic.

Humidity in the home commonly will escape into the attic and condense on cold attic surfaces. If the deck and other structural members are left exposed to this moisture for extended periods of time, it can cause deterioration of those members. Proper ventilation can remove the moisture and allow the system to dry out.

• Shingle warranties

Many shingle manufacturer warranties require that the attic cavity be ventilated and if it is not the warranties can be significantly reduced or eliminated altogether.

• Building codes

Building codes require that attic spaces be ventilated. There are two different formulas to determine the amount of ventilation that is required.

The first formula requires one square foot of net free ventilation for every 150 square feet of attic floor space. This formula is used if all the ventilation is at or near the ridge.

The other formula requires one square foot of net free ventilation for every 300 square feet of attic floor space. This is used if half of the ventilation is at the ridge and half at the lower edge. Note: This formula can also be used if there exists a vapor retarder with a perm rating of less than 1 and all the venting is at the ridge.

These formulas refer to the Net Free Ventilation Area, which is defined as the actual open space after the louvers and screen obstructions are removed. The net free area of a vent is typically approx. 50 to 60 percent of the actual opening. Vents are rated for their net free ventilation area.

It is important that air is able to move freely between lower and upper vents. This will sometime involve chutes which are placed in the cavities between the rafters at the point where the rafter sets on the wall and extending upslope to keep the air channels open from insulation blockage.

How can I ventilate my attic?

Ventilation of an attic can be accomplished through various types of vents.

The most common type of vent is a low profile roof louver. These vents are made of plastic, aluminum and steel and are available in a number of different colors. We recommend using a bird resistant type vent which prevents birds from nesting under the top and blocking the flow of air. The standard size of this type of vent has a net free ventilation area of 50 –60 square inches. These vents can be used for either air intake or exhaust.

Another type of vent is a turbine type vent. Turbine vents are made from steel or aluminum and are also available in several colors. These set on top of a metal cylinder and turn in the wind, which draws the air out of the attic. This is an exhaust vent only and requires approximately two square feet of net freevent area to provide the intake for each 12 “ diameter vent. The typical size of this vent is 12” but they are also available in other diameters.
 

Ridge vents are gaining popularity since the introduction of shingle over ridge vents. These types of vents are covered over with ridge shingles that both enhance the appearance of the ridge and hide the vents. The net free vent area of these types of vents is approximately 18 square inches per lineal foot of ridge vent. An important consideration of this type of vent is that it is intended to be an exhaust vent and it must have at least an equal amount of intake area provided at the lower edge of the roof, ideally through the soffit.

There are many homes that cannot use this type of vent because of the way they are constructed with little or no soffit. Therefore, they are unable to achieve a balanced amount of intake ventilation to support the ridge vent.

In certain applications wall vents can be used to ventilate attic spaces. These are typically louvered vents installed in gable end walls and there net free vent area can be included in the calculations required by code.

Electric powered fans are also available for venting attic spaces. These are exhaust vents and it is very important that the proper amount of intake vents be provided. Intake requirements typically are provided with the fan. Fans can be operated by thermostats and humidistats that will turn them on when the temperature or humidity reaches a set level. It is a good idea to have an override switch that can also be used to operate the fan.

These are not recommended if there is a possibility of drawing inside air and humidity up into the attic through bypasses because of insufficient air intake.

To summarize, you first need to calculate the square footage of the attic floor area. Then determine if the vents will be at the ridge or half at the ridge and half at the lower edge. Do not forget about the exception for the vapor retarder with a perm rating of 1 or less. Next calculate the amount of ventilation you need using the appropriate formula. Lastly figure out which vents you want to use to achieve the net free vent area that is required.

Ventilating attic cavities is an important consideration of a healthy home. Vents are often added when the roofing work is done but can be installed at any time. Please let us know if we could provide any service in this area.