At first glance, it can seem counter-intuitive: You insulate your home to reduce temperature fluctuations and save on utility bills, but then you allow fresh air to flow through the attic no matter the time of year. The science behind attic ventilation, however, is sound. Sealed attics trap excessive heat and moisture, which can lead to reduced shingle life. And the extra heat is not just a summer concern-come winter, hot attic air can melt snow on the roof during the day only to refreeze when temperatures drop overnight, creating ice dams that lead to interior leaking and roof damage. Ensuring your home has the proper attic ventilation according to these guidelines, however, can save yourself the stress and hassle of an emergency roof repair.
COOL AIR IN, HOT AIR OUT
Attic ventilation works on the principle that heated air naturally rises, primarily utilizing two types of vents:
• Intake vents, located at the lowest part of the roof under the eaves, allow cool air to enter the attic.
• Hot air exhaust vents, located at the peak of the roof, allow hot air to escape.
Taking advantage of this natural process, referred to as passive ventilation, is the most common way to vent an attic. In order to facilitate this exchange of warm and cool air, the general rule of thumb suggests installing at least 1 sq. ft. of vent for every 300 sq. ft. of attic floor. Building codes vary, though, so do check with your local building authority for the specifics that pertain to your community.
An attic's intake vents are most commonly installed directly in the soffit, either as individual vents spaced every few feet or as one continuous perforated soffit running the entire length of the eave. While effective at pulling in cooler air, the biggest problem posed by this type of soffit vents is their positioning: Homeowners can too easily inadvertently block them when insulating the attic. Unfortunately, blocked soffit vents are as just bad as no soffit vents, because they prevent fresh air from freely flowing into the attic.
Houses with gable roofs may also have vents located on the side of the house as high as possible within the peak of the gable. Whether round, triangular, or rectangular, these gable vents can be painted to match either the siding or the trim work so that they add to rather than detract from the home's exterior. What's more, they're particularly valuable for their ability to function as both intake and exhaust vents, depending on the wind direction. Most of the time, their position near the peak of the roof allows heat to dissipate out through its cover. When there is wind flowing perpendicular to the roof and of sufficient speed, it can enter through the opening; however, winds that are too light or not flowing directly at the vent's entrance will do little work to cool down the space.
Releasing all of the heat that rises and gets trapped in the attic can be achieved with one or a combination of the three following vent models in addition to the multipurpose gable vents mentioned above.
Ridge vents-openings that run the entire length of your roof along the ridge-are often visible only to a trained eye. Hidden in plain sight and often camoflauged by specialty ridge shingles, these are a particularly popular means of ventilation because they create no disruption to the roofline. Installation of this type of attic ventilation involves leaving a gap in the sheathing along the ridge, and covering it with a perforated vent.
Static vents often protrude from roofline thanks to special covers intended to keep all precipitation-rain, sleet, hail, and snow-from entering the attic. Homeowners can choose from a variety of shapes and colors that closely match their shingles so that the vents won't appear too out of place on the roof. One static vent style is the turbine vent, which uses wind to power its enclosed fan-all it takes is a light breeze to rotate the blades and suck heat out of the attic. Again, whatever the type of static vent, it must be located as close to the ridge as possible; homeowners worried about how the addition might affect curb appeal can place them only along the roof's backside in order to minimize visibility from the street.
Finally, unlike the rest of these models that utilize passive ventilation, powered exhaust vents feature an electric- or solar-powered fan to create an effect similar to that of a turbine. A standard powered exhaust vent turns on when the temperature inside the attic reaches a pre-set limit and runs until the temperature drops. While these powered vents do effectively draw out the heat, they will pull more cool air from any air leaks in the ceiling of the house (read: your home's central air conditioning) than soffit vents simply because it's easier. Considering that they already require some amount of electricity to power, additional energy spent on air conditioner cooling the whole house may make this type of vent a less desirable option-especially if your attic is not well-sealed.
WHAT ABOUT FINISHED ATTICS?
With square-foot living space at a premium, many homeowners turn to their attics for a little extra room. When the attic becomes part of the home to be heated and cooled, open-wall gable vents and roof vents are no longer feasible, but the underside of the roof (the sheathing and rafters) can still get blazing hot without airflow.
The answer is rafter venting. Rafter vents, or insulation baffles, install in any rafter space to create narrow gaps that direct fresh air from the soffit vents to the peak of the roof. These specialty vents do not affect the finished look inside the remodeled attic. Instead, fresh air still flows in through the soffit vents and travels along the underside of the sheathing until it reaches a ridge vent or can be vented with another type of exhaust vent-allowing homeowners to keep cool without cutting into their aesthetics.