Building codes require some type of system for attic ventilation to be built into every new house, but if you live in an older house that was built when the codes concerning roof ventilation were more lax, it's possible that your attic ventilation is not up to snuff. The house may have gable vents and/or roof vents, but those alone are not enough to create the airflow you need through the attic to keep it dry in the summer and prevent ice dams on the roof in the winter. Although you could add an attic fan to move the air, it's more effective to install soffit vents, and they won't bump up your energy bill.
The soffits are the undersides of the eaves, which are the sections of the roof that overhang the walls. Installing vents in soffits allows air to enter the attic freely and improves circulation. Far from being a major roof retrofit, the job of installing soffit vents is a DIY task for anyone with a few common power tools and the skill to use them safely. You can choose to install individual soffit vents between the roof rafters, but it's easier to install strip vents that run the length of the roof overhang.
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Why Do You Need Attic Ventilation?
In summer, attic ventilation allows cold air into the attic and hot air out, with the net effect being that the attic stays cooler. Besides reducing the cost of air conditioning in the living space, this also helps protect the roof shingles from overheating. In winter, this cooling effect helps reduce the incidence of ice dams, which form when the roof is warm enough during the day to melt ice and snow on the roof and cold enough at night to allow it to refreeze. These are important benefits, but there's another one that is even more important.
When the attic is full of moist air, condensation forms on the framing and the insulation, creating mold and mildew problems and rotting the wood. Homeowners who don't often go into the attic may not realize this is happening until the insulation has become black, and the wood has weakened to the point that some of it needs to be replaced. Air circulation created by an attic ventilation system keeps the moist air moving out and keeps the attic dry, which is the main reason up-to-code attic ventilation is required on all new construction.
Current Attic Ventilation Codes
A passive attic ventilation system that conforms to current building codes has both air intake and exhaust vents, and the soffits are the best place to install the intake vents for two reasons. The first is that the soffits, essentially being extensions of the attic floor, are the lowest point in the attic, and the second is that by virtue of being on the underside of the roof, the soffits are naturally protected from ice and snow. The basic idea is that outdoor air enters through the soffits and warms up inside the attic, and the warm air exits through the roof vents, creating a convection current that draws more air in through the soffits.
The 2018 version of the International Residential Code sets a minimum on the amount of open vent space in the attic based on the attic's floor space. The basic rule is that the ratio of vent opening space to floor space should be 1/150, but the code makes an exception for a balanced system in which the area of the intake vents is roughly equal to but never less than the area of the exhaust vents. In such a situation, the ratio of vent space to floor space can be reduced to 1/300. In climate zones 6, 7 and 8, the code further requires that the attic have a class I or II vapor barrier, which is basically a barrier of plastic or kraft paper placed between the attic insulation and the floor.
When you install soffit vents (intake vents), you have to make sure you have enough exhaust vents, and to do that, you need to do a little math. If you have a 900-square-foot attic floor, you need 900/300 = 3 total square feet of venting space to conform to the 1/300 rule, and that should be equally divided between intake and exhaust vents. If the existing roof vent area is less than half of the required venting area (3 sq. ft.), you can still install enough soffit vents to bring the total to the required number, but you can't do that if the roof vent area is less than 40 percent of the required venting area. In that case, you need to add more exhaust vents or install an active ventilation system, such as a plug-in or solar-powered attic fan or a turbine vent system.
Sizing Soffit Vents
When choosing a soffit vent product and determining how many you need (or how long it should be), pay attention to the vent's net free area (NFA), which is determined by the manufacturer and is listed on the vent or the product packaging. The NFA is the total area of openings in the vent, or all of the spaces where air can pass through. Soffit vents and other linear or strip vents (such as ridge vents) often have a stated NFA per foot. For example, if a vent has an NFA of 9 square inches per foot, and the vent is 8 feet long, the total net free area is 72 square inches. If you need to convert this to square feet, divide by 144: 72 divided by 144 = 0.5 square feet.
Things You'll Need
How to Install Soffit Vents
Soffit strip vents come in a variety of lengths and widths, and the easiest to install are overlapping ones that can run continuously from one end of the soffit to the other. One well-reviewed vinyl product comes in strips that are 2.98 inches wide and 8 feet long, providing 9 square inches of net free area per lineal foot. You install this and similar strip vents according to this procedure.
Step 1: Draw Cut Lines on the Soffit
Using a chalk line or a straightedge and pencil, draw two straight, parallel lines on the soffit from one end to the other, spacing them by the width of the vent opening. Stop the lines a few inches from each end of the soffit and then mark perpendicular lines to represent the ends of the cutout.
Note that strip vents usually come with an attachment flange on either side that isn't included in the width of the vent. The flanges rest on the soffit material outside of the cut lines, and you screw through the flanges to secure the vent. If the strip vent you're installing doesn't have flanges, adjust the spacing of the cut lines as needed to ensure the edges can be screwed to the soffit.
Step 2: Set the Cutting Depth of the Saw Blade
Bore a 1-inch hole somewhere between the cut lines using a drill and a spade bit. Use a tape measure to measure the thickness of the soffit material. Set the cutting depth of a circular saw to slightly more than this measurement (about 1/8 inch more). Because you'll be working on a ladder and cutting overhead, you'll want a small, lightweight saw. A battery-powered saw is easier to handle than a corded one, and a compact saw with a 4 1/2-inch blade is best.
Step 3: Cut Out the Vent Opening
Cut along both long lines with the saw, stopping at the end marks. Cut along the end marks with a hammer and chisel to complete the cutout. Pry out the soffit material with a flat pry bar, removing any nails (or screws) as needed.
Step 4: Install the Vents
Set the vents on a flat wooden surface and drill 1/8-inch holes in the flanges for the screws, spacing these holes 12 to 14 inches apart. Lift the vent into place over the vent opening and screw it to the soffit using 1/2-inch sheet metal screws. If any insulation is poking out of the vent opening, push it back into the attic before attaching the vent.
Step 5: Clear Insulation From the Vents
Go into the attic and pull back any insulation that is covering the vents. If the insulation is the loose, blown-in variety, use a length of 2x4 lumber to rake it back. The vent opening must be completely clear of attic insulation to provide proper venting.
Step 6: Install Baffles
Baffles are cardboard or plastic panels that fit between the rafters and are raised in the middle to provide an air passage under the roof decking. You should install one between each pair of rafters by screwing it to the underside of the roof deck with 1/2-inch screws. One end should be directly above the vent opening, and the other should extend toward the roof peak. Baffles prevent insulation buildup over the vent opening and direct air into the attic space.
Other Types of Intake Vents
Strip vents that extend the length of the soffits may not be the best option depending on the style of the house and the actual amount of venting space the attic requires. Individual soffit vent covers may provide more flexibility, given that you can install only the number you need, and you position them in places that make good design sense. Each one is basically a louvered metal cover that fits over a precut hole in the soffit (the hole is usually cut with a jigsaw) and gets screwed to the soffit. You can buy round soffit vent covers, rectangular ones and even oblong ones. Just be aware that these offer a small amount of net free vent area per vent.
If your house does not have conventional eaves that extend beyond the walls and you're installing a new roof, you can have the roofers use a special drip-edge material called a drip-edge vent. This aluminum material is like a regular drip edge, but it has a grid that extends under the roof decking and provides a vent opening at the very edge of the roof line. It has 9.2 inches of net free area per lineal foot, which is a little bit more than standard soffit strip vents.
- International Institute of Building Enclosure Consultants: Attic Ventilation 101
- Green Building Advisor: All About Climate Zones
- Benjamin Obdyke: What Is Net Free Area (NFA)?
- Roof Hub: 11 Best Types Of Roof Vents + Understanding Attic Ventilation
- HomeAdvisor: Increasing Attic Ventilation with Soffit Vents