The amount of ventilation necessary and the issue of attic ventilation are controversial. For residences where snow accumulations persist on roofs for long periods of time and that located in cold climates, the strongest case for ventilating asphalt shingle roof assemblies with outside air exists based on research conducted to date.
For removing excess moisture from attics ventilation has been shown to be a valuable strategy under those sets of conditions, thus preventing ice dam formation and preventing condensation that can damage roof sheathing. Mould prevention is essential.
It has also been observed that in summer average attic air temperatures is reduced by ventilation with outside air. On the average roof surface temperature a significant effect of attic ventilation has not been verified by researches.
In fact, research results indicate on average roof surface temperature ventilation has less effect than the roofing material color or facing direction of a roof surface.
Attic ventilation is necessary with asphalt shingle roof systems as suggested by some sources available to homeowners.
For example, on meeting their minimum ventilation requirements Asphalt shingle manufacturers' warranties may make coverage conditional. When selecting an asphalt shingle product for specific roof ventilation systems requirements homeowners should consult manufacturers. By using one of two methods ventilation of attic spaces can be accommodated:
• Passive ventilation • Powered ventilation
Powered ventilation
Forced or powered ventilation as an alternative to a passive ventilation system may be appropriate. To be equivalent to a 1:150 ventilation ratio the air displacement in the amount of 1 cubic foot per minute per square foot of attic floor area is considered generally.
To offset the amount of air that will be exhausted by the powered ventilation and provide for adequate amounts of intake air into the attic space to account for it is important when using powered ventilation Similar to passive attic ventilation.
As largely cancelling the benefits of balanced ventilation forced air flow may bypass the intake vents in combination with other vents placed high on a roof powered vents should not be used.
Also, through other exhaust vent openings in some instances, water infiltration may be resulted from the air suction created by powered exhaust vents.
Passive ventilation
For asphalt shingle roof assembly nonpowered, passive ventilation is the most common way to provide attic ventilation. On natural air convection this method primarily relies— because of its lower density on the upward movement of heated air—but advantage of wind-generated pressure differences may also be taken.
For initiating the upward flow of air through an attic natural convection is responsible. If to replace the heated air exhausted through vents intake vents in the attic placed low make colder air available placed high in the attic then to aid in continuous circulation of air through the attic this air current can be maintained.
When approximately equal amounts of ventilation opening areas are placed at or near the top of the attic space or eave and at the soffits convection-assisted ventilation is most effective. Your house must also have enough underfloor ventilation.
With wind-assisted exhaust vents Passive ventilation can be configured. To induce pressure drops across the vents these products employ wind speed so that the air is pulled or forced out of the attic. If adequate intake vents are also provided they are capable of significantly maximising the volume of air exhausted from an attic.
