Figure 4. Climate Zones taken from Figure C301.1 of the 2012 IECC
One benefit of a vented rather than unvented attic
is that interior-sourced moisture will be vented out of the attic space before causing damage to the roof sheathing. Another advantage of vented attics in cold climates is that they help to reduce the chances of ice dam formation
on the roof. Ice dams occur when heat leaks from the conditioned space (through holes in the ceiling plane, insufficient insulation, or heat loss from ductwork) and melts the snow on the roof. This melted snow travels down to the edges of the roof, where it refreezes, creating icicles and ice dams. Attic ventilation helps to “flush away” this heat before it can melt the snow.
Figure 5. Ice dam
The vented attic approach requires that there be sufficient height at the attic eaves for code-level required amounts of insulation. In mixed- and cold climates (Climate Zones 4 and above), inadequate insulation raises the risk of wintertime condensation forming at the top plate due to cold surfaces. If the roof is too low at the eaves to install adequate amounts of blown insulation, then alternative solutions include filling the space over the top plate up to the baffle with closed-cell spray foam, or covering the top plate area with rigid foam board. (Spray foam and rigid foam have higher R-values
per inch than blown insulation. Spray foam has the added advantage of air sealing the top plate-to-drywall seams and the baffle-to-top plate seam.
) Another option is to convert the attic to a sealed insulated space by sealing the soffit vents and insulating along the underside of the ceiling deck with closed-cell spray foam or above the roof deck with rigid foam. In new home construction, a common solution to increase the roof height for insulation above the top plate is to use raised heel trusses. Older existing roofs are unlikely to have raised heel trusses, and they're are not likely to be installed as a retrofit measure unless a complete reconstruction of the roof is required, such as when, for example a second or third story is being added.
The vented attic approach requires that there be sufficient height at the attic eaves to maintain an air gap above the insulation for ventilation air traveling from the soffit vents to the ridge vents. The vent space should be at least 2 inches high as well as throughout the width of the framing bay at each soffit vent. Baffles should be installed at each soffit vent to provide this pathway for ventilation air to move up past the insulation along the underside of the roof deck to the ridge vents or mushroom cap vents located near the ridge. The lower edge of the baffle can be sealed to the attic floor at the outside edge of the top plate with spray foam, which also air-seals the top plate-to-drywall seams. The baffles also serve to prevent insulation from covering the soffit vents, and they can also prevent “wind washing,” i.e., when wind blows in the soffit vents and pushes the insulation back from the eaves.
If the attic does not have soffit vents, attic ventilation
can be provided by installing other types of vents, such as gable or eyebrow vents, down low near the attic floor or along with ridge vents, or by installing mushroom cap vents up near the roof ridge.
Using powered attic ventilators (which are thermostatically controlled fans that exhaust attic air during hot weather) can result in increased infiltration, and associated higher cooling loads. Studies have shown that powered ventilators consume more electricity than their associated savings through air conditioner use.
Installing HVAC ducts
and air handlers in vented attics is not recommended. Locating ductwork and/or air-handling equipment in a vented attic can contribute to energy losses, performance issues, and ice dam formation in snowy climates, especially if the ducts and air handlers are leaky or poorly insulated. One exception is if the ductwork can be encapsulated in spray foam and buried beneath the attic floor's blown insulation. The ducts must be tightly air-sealed and covered with a sufficient amount of spray-foam insulation to minimize the risk of condensation forming on the outside of the ducts. However, because mechanical equipment, such as air handlers, cannot be adequately air-sealed, locating them in a vented attic is not recommended. If the air handler must be located in the attic, then converting the attic to a sealed insulated space should be considered.
Figure 6. Vented attic with loose-fill, fibrous insulation at the attic floor
Recommended Method for Insulating an Attic Floor with Blown Insulation
- Before any retrofit work is done, the work area should be inspected for active knob-and-tube wiring, bathroom fans venting into the attic, roof leaks, and other hazardous conditions. These conditions should be repaired prior to adding insulation. Hazardous materials, such as asbestos-containing vermiculite insulation, should be inspected for and removed or remediated.
- Any existing insulation, debris and dust should be removed, and the work area should be cleaned to allow for an adequate adhesion of sealant.