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HOW TO


Attic Ventilation
A PROPERLY VENTED ATTIC allows your house to breathe, drawing in cool air through soffit vents and exhausting hot and/or humid air through ridge or gable vents. Attic ventilation also flushes out moisture that's carried up from the rooms below, and protects the roof from rotting, freezing, and ice dams. In page we present a range of ventilation options and show you how to install soffit and ridge vents to make your attic cycle fresh air in and out.


WHY VENTILATION IS NEEDED Proper attic ventilation is needed in both the summer and the winter.
Summer. In the summertime, the sun can cause the air inside an unvented attic to reach temperatures of 150° F.-or more. When this happens, the insulation in the attic becomes a "heat sink," gradually absorbing the heat and passing it on to the cooler living space below (Fig.1). In hot climates, this area is often cooled by air conditioning. Super-hot attic temperatures require an air conditioner to work overtime in order to cool the living area. The situation is nearly as bad as having no insulation at all. The solution is to exhaust the heated air before it can build up. By providing good attic ventilation, you can reduce the temperature in the attic and your cooling bills significantly.
Winter. Ventilation is just as important in the winter as it is in the summer, but for different reasons. In the winter, moisture-in the form of water vapor-can diffuse through the ceiling into the attic. If the attic is not well ventilated, the vapor can turn to frost on the rafters and underside of the roof sheathing. As the weather warms up, or the roof is heated to above freezing, the frost melts onto the insulation, which in turn loses its efficiency. Excess water can also stain or damage the ceiling and walls below. Over time, this moisture will reveal itself in the form of rot or other structural damage to your house. Vapor barriers are designed to protect framing and insulation from moisture damage, but many houses don't have them, or they are poorly installed.
An unvented attic also traps warm air, which can cause snow to melt on the upper parts of the roof. The melting snow freezes when it hits the colder eaves, causing ice dams which can damage the roof (Fig. 1).


VENTILATION BASICS
Ventilating a roof requires making a path for air to move between the roof deck and the insulation. Natural (or passive) ventilation occurs when cool air enters the lower vents, is warmed, then rises and moves out the upper vents, exhausting heat and humidity. Some typical vents are shown in Fig. 3. Incoming Air. The most effective way for air to enter the roof is through vents that are placed in the soffits. Continuous soffit strip vents provide the most reliable port for intake air, while rectangular vents are next on the list (Fig. 2). Round ventilator plugs are easy to install, but usually are too small to provide adequate airflow. The important thing is to maintain at least 2 inches of clearance between the underside of the roof deck and all insulation. Insulation baffles ensure a clear pathway for air to travel between the rafters (Fig. 7).
Outgoing Air. Stale air escapes through ridge vents at the peak of the roof, box-type vents near the top of the roof, or gable vents on the end walls (Fig. 2). Continuous ridge


vents are the preferred type for pitched roofs, while box-type gravity vents are best installed where ridge vents are not feasible, such as in a hip roof. For houses that have open attics and insulated attic floors, vents located in the gable ends may suffice if the openings are large enough. Gable vents also can be used in a finished attic above an insulated flat ceiling.

VENTILATION REQUIREMENTS
The amount of area needed to provide for roof ventilation depends on whether or not the ceiling has a vapor barrier. If there is a vapor barrier, allow one square foot of free ventilating area for every 150 square feet of attic floor area. If there is no vapor barrier, double this amount. The total free ventilating area must be divided equally between the intake and exhaust ports.
Net Free Ventilating Area. Because the thickness of vanes or wires in a venting device reduces the airspace, use oversized vents to compensate. For example, if the vent has a screen with 1/8-in.-square holes, divide the area of the screen by 1.25 to get the net free ventilating area. If the vents used consist of louvers backed by screens, divide by 2.25 (Fig. 3).
Vent Calculations. The following example shows how to size soffit and ridge vents for a 25 x 40-foot house that has a vapor barrier in the ceiling:
• Figure the area of the roof: 25 multiplied by 40 equals 1000 sq. ft.


• The free ventilating area equals 1000 divided by 150, or 6.66 sq. ft. To convert square feet to square inches, multiply 6.66 by 144 to get 959
square inches. You need half of this amount (480 square inches) for the soffit vents and the other half for the ridge vents.
• Each 8 x 12-inch louvered soffit vent has an area of 96 square inches. Divide this number by 2.25 to get 43 square inches (rounded off) of net free ventilating area. The number of vents needed is 480 square inches divided by 43, which is 11. Use five or six on each side of the roof.


• The product literature that comes with the ridge vent provides information concerning the necessary free ventilating area per lineal foot. One rolled ridge vent product yields 17 inches, so the house in this example needs a strip equal to the free area required at the ridge: 480 square inches divided by 17 equals 28 feet.

INSTALLING SOFFIT VENTS - 1 The most effective way for air to enter an attic is through vents placed in the soffits of the roof, located at the lowest points of the roof. Continuous soffit strip vents provide the most reliable port for intake air, while rectangular vents are next on the list.

See the section on "Vent Calculations"' to determine the size and quantity of vents needed at each soffit. For every 150 square feet of attic space, install 1 square foot of net free vent area.
Cutting the Opening:
• Using the vent as a template, mark the location of the vents on the soffit.
• Using a 3/4-inch drill bit, drill a starter hole at one corner of the marked box.
• Insert the blade of a saber saw into the starter hole and cut the opening (Fig. 4a).
Attaching the Vent:
• Secure the vent to the soffit using the screws supplied with the vent. Use an electric drill equipped with a Phillips bit to screw the vent to the soffit (Fig. 4b).

INSTALLING RIDGE VENTS
For houses with pitched roofs, continuous ridge vents are the preferred choice for venting outgoing air. For houses with flat roofs, gable vents may suffice if the openings are large enough. Ridge vents are made of various materials and may be packaged in rigid sections or rolls. A well-designed vent allows air to escape, but keeps out rain and snow. Ridge vents are highly visible as they sit right on top of the roof. As a result, many homeowners choose to install shingles on top of the vents (Fig. 5).

Cutting the Opening: Use a circular saw with a carbide-tipped blade to cut a 2-inch slot along each side of the ridge. Start the cut 6 inches from one end of the ridge, and end it 6 inches from the other end. Set the blade depth to cut through the roofing and sheathing only, leaving the rafters uncut (Fig. 6a).
Safety. Wear goggles and make sure you have solid footing when cutting with a saw. Install roofing cleats if necessary to support yourself on the roof.
Lay out the extension cord so that there is no chance of tripping on it.
Attaching the Vent:
Secure a rigid vent to the ridge or uncoil the vent (if packaged in rolls).
• Extend the vent to cover the uncut 6-inch portions of roof at each end. If using a

metal vent that is not intended to be topped with shingles, nail the flanges to the roof with aluminum roofing nails. Topping the Vent:
For ridge vents that are designed to be covered with shingles, cut three ridge shingles from one three-tab composition shingle.
• Place the first shingle over one end of the ridge vent, aligning it with the edge of the shingles along the roof rake. Nail the shingle through the part to be covered by the next shingle.
• Using roofing nails that are long enough to penetrate into the sheathing, install one nail at each side of the ridge. Put the next shingle in place over the nails and continue. Be sure not to compress the vent material while nailing (Fig. 6b).

Safety.
When working on a roof, be sure to tie off ladders, use a safety rope attached to the peak of the roof, and wear rubber-soled shoes. Stay away from electric lines and other hazards. If the roof is steep, have professionals install the vents for you-they have special equipment that allows them to work safely.

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Updated: 11/2017   copyright 2012 U-Repair.com