VENTILATION BASICS

• Proper attic ventilation is an important part of a healthy home–both for the structure and its occupants. This document explains how attic ventilation protects a home from moisture and how to install vents that will keep your home in good condition.
  
  
• There are a wide variety of sources of moisture in a home, from the building materials themselves to normal everyday activities. Cooking, bathing and washing clothes all release gallons of water vapor into the air, for example.
  
  
• That vapor isn't a problem inside the average home because the temperature inside the home is warmer than outside for much of the year. Warm air holds more moisture–in the form of water vapor–than cool air.
  
  
• The problem is that vapor gradually works its way out of the living area and into the structure. As warm, moist air cools, the vapor begins to condense into water droplets. If that happens inside an unfinished attic, for example, it can get insulation and framing materials wet. That not only reduces the value of your insulation but can cause mold, mildew and rot.
  
  
• During the summer, when the outside temperature is typically much higher than the inside temperature, attic ventilation serves a different purpose. An unfinished attic builds up a tremendous amount of heat, and if that heated air has no place to escape, it can make the inside of the house much warmer or cause an air conditioning system to work much harder to cool the house.
  
  
• Building codes specify the minimum amount of attic ventilation needed in a new home to prevent winter moisture buildup, but your summer needs are much greater. Also, older homes were often built with inadequate attic ventilation–at least by today's standards–and may need to be retrofitted with proper attic ventilation.
  
  
• A good attic ventilation system is designed for summer needs. It includes two types of vents: intake vents are placed along the soffit to allow fresh air into the attic, and exhaust vents are installed in the upper third of the roof to allow attic air to escape. The object is to create a continuous "wash" of air along the underside of the roof sheathing. The rule of thumb in the summer is that you should provide enough ventilation to completely change the air in your attic every six minutes.
  
  
• There are three common types of intake vents:
  
  
• Gable vents (Fig. 1) are triangular vents installed in the gable wall just below the peak of the roof. As a rule, gable vents are the least effective type of vent, because air circulates only near the gables and does not wash the entire roof.
  
  
• Static vents, also known as roof line or eyebrow vents, consist of a sheet metal cylinder with a flashing collar and a metal hood to keep rain out. They are installed in rows along the face of the roof by cutting holes in the roof, nailing the flashing collars to the roof sheathing and shingling around the vents. Their effectiveness depends on how many are installed; probably their greatest disadvantage is that like any roof penetration, they may leak.
  
  
• Soffit vents are made usually with a screen to keep insects out and of an aluminum panel with louvers punched into the face to allow air flow. They may be 4" or 8" wide and 14" or 22" long, so they'll fit between 16" and 24" on center rafters. They are installed simply by cutting rectangular holes in the soffit and screwing the vent over the hole.
  
  
• A continuous soffit vent is of similar construction, 4" wide and 96" long. It is installed by cutting a long slot in the soffit and screwing the vent over the hole.
  
  
• Circular vents range from 1" to 8" in diameter. They are installed by drilling holes in the soffit and pressing the vent into the hole.
  
  
• Exhaust vents fall into two basic categories. Static vents simply allow air to escape while power ventilators actively suck air out of the attic. Within each category there are a number of types:
  
  
• Ridge vents are installed along the peak of the roof and replace the ridge singles.
  
  
• Power Ventilators are turbine vents that consist of a turbine mounted on a sheet metal cylinder. They are installed like roof line vents along the face of the roof. When the wind blows, it spins the turbine, which in turn draws air up out of the attic. Their effectiveness, naturally, depends on whether the wind is blowing or not.
  
  
• Fan-driven ventilators are powered by electricity and usually controlled by a thermostat in the attic. They are very effective, but since they are motor-driven, the extra cost of running them partially offsets the energy they conserve.
  
  
• Most builders agree that a ridge vent system is the most effective as well as the most cost-effective.
  
  
• The number of vents you'll need depends on the type and size of the vents. Vents are rated according to their square inches of "free vent area" (FVA)–in other words, the amount of open space in the vent. You can't just measure the size of the vent to find the FVA because the open space is reduced by louvers and by the screen mesh that covers the opening.
  
  
• Most manufacturers provide both FVA ratings and ventilation recommendations for their products. In order to estimate, you'll need to know the total square footage of your attic and possibly the slope of your roof. To find the square footage of your attic, multiply the width of your house by the length.
  
  
• Roof slope is expressed as a ratio–for example, a 5:12 slope means that the roof rises 5" vertically for every 12" of horizontal distance. To find the approximate slope of your roof, go into the attic and measure the vertical distance from the peak of the attic ceiling to the ceiling joists in feet (e.g., a 75" measurement would be 6-1/4').
  
  
• Multiply that measurement by 24, then divide the result by the width of your house (also in feet). The answer is the first half of your slope ratio. For example, say your house is 30' wide, and the peak-to-ceiling-joist measurement is 75" (6-1/4'):

  6-1/4 x 24 = 150

  150 divided by 30 = 5

  Your slope is approximately 5:12

INSTALLING ATTIC VENTS

• Installing attic vents in an existing roof is a relatively simple job that most do-it-yourselfers can handle. Remember to follow basic safety procedures when working on the roof:
  
  
• Wear loose clothing and rubber-soled shoes with good ankle support.
  
  
• Only work on the roof in dry, calm weather.
  
  
• Be alert for slippery or loose shingles or rotten decking that you might put a foot through.
  
  
• Avoid power lines and TV antennas.
  
  
• Keep children and pets away from the area so they aren't hurt if something falls off the roof.
  
  
• Your extension ladder should be angled so the base is away from the wall a distance equal to 1/4 of the ladder's length plus the width of the soffit.
  
  
• Intake Vents (Fig. 7)–To install intake vents, set your circular saw blade to a depth about 1/8" greater than the thickness of the soffit (soffit materials are usually 1/4" thick). Lay out the location of the vent between the rafters, then cut the hole with the circular saw. Screw the vent to the soffit, covering the hole.
  
  
• If you have fiberglass blanket insulation in your attic, make sure the blankets are positioned so they cover the top of the exterior wall but still allow at least a 1" space between the top of the insulation and the underside of the roof sheathing. Otherwise, the insulation will block the air flow and your soffit vents will be useless.
  
  
• If you have loose fill insulation in your attic, you'll need to install baffles in each rafter cavity that contains a soffit vent to keep the air space clear.
  
  
• Roof Line/Turbine Vents–To install roof line or turbine vents, first locate the vent between two rafters. Use a utility knife to cut away the shingles and felt paper, then use a saber saw to cut a hole in the roof the same size as the throat of the vent.
  
  
• Butter the inside of the vent base with plastic roof cement, then slip the base into position over the hole. The top of the flashing should be slipped under the shingles above the hole and lap over them below the hole. Nail the base in place with 1-1/2" galvanized roofing nails and cover the nail heads with roof cement.
  
  
• If you're installing a turbine vent, slip the turbine onto the base and level it. Fasten the turbine in place with sheet metal screws.
  
  
• Gable Vent–To install a gable vent, cut away the siding and sheathing with a circular saw. Be careful not to cut too deeply into the gable studs. Caulk the rim of the gable vent, then set it over the hole and fasten it in place with screws.
  
  
• Ridge Vent–To install a ridge vent, first remove the ridge shingles as specified by the vent manufacturer–usually to within 6" of the end of the ridge or a foot from a chimney or roof intersection. Cut away the felt paper with a utility knife and pull out all staples and roofing nails (Fig. 8).
  
  
• Snap a chalk line along the roof sheathing on either side of the ridge; the manufacturer's instructions will tell you how far from the peak the line should be. Set your circular saw blade to a depth slightly thicker than the sheathing, then cut away the sheathing along the line. Remove the cut pieces of sheathing and any nails that remain.
  
  
• Install the ridge vent over the peak (Fig. 9). You can start the vent at the end of the roof or the beginning of the slot, whichever the manufacturer recommends. Different ridge vent systems use different methods of making the vent weathertight; follow the manufacturer's instructions.