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While you’re poking around in the attic sealing air leaks and adding insulation, be sure to assess the attic ventilation. Proper ventilation—the exchange of indoor air with outdoor air—will allow excess heat to escape in the summer so your cooling system won’t use more energy than needed. Attic ventilation also prevents moisture from condensing inside the attic, which can compromise the effectiveness of your insulation and roofing materials. Moisture in the attic is also an opportunity for mold growth, which is a health concern for allergic individuals.

How to ensure proper attic ventilation

Local building codes specify the minimum amount of attic ventilation for new homes. Older homes often do not have adequate ventilation and may need additional roof vents.[1] If you live in a warm, humid climate, insulation contractors may not recommend attic ventilation because it may actually pull more moist air from the outside into your attic than enters from your home.[2]

A house needs two kinds of vents: roof vents to exhaust moist and hot air, and soffit (roof overhang) vents to allow fresh air to come in. These two vents keep air moving through the attic space for optimal moisture and temperature control. (Don’t worry about any detrimental effects of letting outside air come into your attic. Very little hot or cold air will get into your cooled or heated living area if you’ve sealed the air leaks and installed insulation with the proper R-value.) There are several types of vents:

Roof vents to evacuate air:

• Turbine vents are located near the ridge line. They look like a fan. The outside air blows across the fins as they exhaust the heat from the attic.
• Ridge vents run the entire length of the roof peak, blending into the roof-line.
• Attic fans are specifically designed to cool hot attics in the summer by drawing cooler outside air in and pushing hot air out. Attic fans need a power source—either electrical or solar.
• Gable vents are installed at or near the peak of a roof gable.
• Box vents look like metal boxes with openings on the sides. They are installed near the ridge.

Intake vents to draw air in:

• Soffit vents are installed in the underside of the house’s soffit.
• Rafter vents or baffles lead from the soffit vent into the attic space. Used in conjunction with soffit vents to be sure insulation does not block the soffit vent. Also called insulation baffles.

In addition to vents, many homes need vapor retarders (also known as vapor barriers) to prevent moisture from entering either the interior living area or the attic and condensing in the cooler area's walls or rafters. Made of plastic, foil, or treated paper, vapor retarders are moisture resistant. Use retarders with "perm" (permeance) ratings of 1.0 or lower.[3] Vapor retarders are not necessary in all climates. If moisture moves both ways for significant parts of the year, vapor retarders may not be recommended.[4] See the US Department of Energy's (DOE) Energy Efficiency and Renewable Energy map above for recommendations for your area.[5]

How many vents do you need?

1. Determine your attic area by multiplying the length by the width. For example, a 30-foot by 40-foot attic has an area of 1,200 square feet.
2. Building codes will dictate the minimum amount of ventilation but a general rule is a ratio of 1:150, or 1 square foot of vent opening per 150 square feet of attic. If your home has a vapor barrier (see below), use a ratio of 1:300.[6]
3. The vent area should be divided equally between the roof and soffit vents. So, in our example, a 1,200 square foot attic would need a total of 8 square feet of ventilation: 4 square feet for roof vents and 4 square feet for intake vents. If your house doesn’t have soffits, you can install roof vents near the lower edge of the roof.
4. Check to be sure the insulation doesn’t plug the soffit vents. Install rafter vents (available at home improvement stores) or a barrier to keep the insulation in place.

Vent installation

Try these DIY websites for installation advice:

• Ridge Vents: Do It Yourself.com - Attic/Roof Ventilation: Where Are You? and Easy2 DIY - Install a Ridge Vent
• Solar Attic Fans: Do It Yourself.com - Installing an Attic Fan
• Electric Attic Fans: Easy2 DIY - Installing an Attic Fan
• Basic instructions for several roof vents: Do It Yourself.com - Installing Attic Vents
• Attic Vents: Reader’s Digest Living - Add an Attic Vent to Increase Ventilation
• Animated presentation of soffit vent installation: Easy2 DIY - Installing a Soffit Vent

Find it! Attic vents

Ensuring proper attic ventilation helps you go green because...

• In the summer, it allows excess heat in the attic to escape, reducing a homeowner’s use of energy for cooling.
• It allows moisture produced within the home to escape so it doesn’t condense and cause insulation and construction materials to deteriorate.

While properly installed insulation offers significant savings in heating and cooling efficiency for the average homeowner, improper installation can limit energy savings by as much as 25 percent.[7] Proper installation includes adequate ventilation. In some climates, attic temperatures in poorly ventilated attics can reach 160°F![8] Without proper ventilation, the heat can stay trapped in the attic and move into cooled living areas. An insulated attic will slow down this heat transfer, but invariably the home’s cooling system will use more energy to compensate. Proper ventilation will allow hot air to escape through roof vents so the attic will be cooler.

Proper ventilation also ensures moisture passing from the home into the attic won’t condense on cooler attic walls, which will assist in prolonging the life of insulation and roofing materials.[9] Besides the obvious roof leaks or plumbing disasters, moisture in a house comes from many sources, including cooking, bathing, watering houseplants, and even from just breathing. In fact, a typical family adds about 3 gallons of water per day to their indoor air just by breathing and perspiring![10] If moisture accumulates in the insulation, it will decrease the insulation's effective R-value while increasing the likelihood of mold growth, particularly in cellulose insulation. Fiberglass, cotton, and certain foam materials may also be vulnerable to moisture accumulation and mold growth within a building's envelope.[11] Molds produce allergens and irritants, as well as toxic substances called mycotoxins, all of which may contribute to hay-fever symptoms, rashes, and asthma attacks in individuals who breathe mold-contaminated interior air.[12]

In cold, snowy climates, proper ventilation allows the underside of the roof to remain cold so the snow on the roof top won’t melt and form ice dams, which could damage the roof.

Ventilation controversy

Attic ventilation has been a standard practice for more than 50 years, but experts with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) now say it should not be a regulated practice in all climates. "While attic ventilation can be beneficial under some circumstances and climates, it should not be seen as the principal strategy to eliminate moisture and other problems in attics and rooms," notes William Rose, a research architect at the Building Research Council, University of Illinois at Urbana-Champaign in an ASHARE press release. "Rather, attic ventilation should be part of a broader range of control strategies."[13] Other control measures include better indoor humidity control for cold and mixed climates, and minimizing heat sources in the attic, as well as preventing warm air leakage into the attic from below. Rose and others say attic ventilation should be a design option in cold, wet coastal climates, and in hot climates where the amount of moisture entering the attic from outside may be greater than the moister coming from in the home. However, he does say attic ventilation is necessary in snowy climates to prevent icing at eaves, and in homes with cathedral ceilings in cold and mixed climates.[13]

External Links

• US Department of Energy - Insulation Fact Sheet
• Reader’s Digest Living - Add an Attic Vent to Increase Ventilation
• Ask the Builder - Roof Ventilation with Turbine Vents
• US Environmental Protection Agency - A Brief Guide to Mold, Moisture, and Your Home
• Florida Solar Energy Center - Performance Assessment of Photovoltaic Attic Ventilator Fans
• Building Science Corporation - Understanding Attic Ventilation
• Rebuild America - Energy-Efficient Ventilation for Apartment Buildings