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Attic Ventilation & Ice Dams
Poorly ventilated or insulated attics contribute to ice damming
after snowfalls. The loss of heat from the living space into the attic causes
accelerated melting of the snow on the roof. The melted snow flows down the roof
until it reaches the eaves or soffit area. This area is beyond the living space
and not heated, which will allow freezing of the moisture. The build-up of ice
over this unheated area creates a dam. The water from melted snow pools behind
the ice dam and is forced up, under the shingles to the sheathing above the
heated space and drips into the attic or living space. It wets the sheathing,
insulation, interior wall and ceiling building materials and finished
components. When insulation gets wet, its effectiveness is compromised.
A properly vented and insulated attic keeps the roof closer to the outside
temperature in the winter, slowing the melting of snow and greatly reducing the
chance of an ice dam forming. Ice damming mostly occurs on low-sloped roofing
and on the north side. The south side gets the sun, which usually keeps it warm
enough to eliminate ice-damming problems.
One method to determine if existing ventilation is adequate is by placing a
thermometer in the attic on a warm, windless day to determine if the temperature
is more than 10 to 15 degrees F above the outside temperature. If it is, more
ventilation is recommended.
The minimum ventilation requirement is a total net ventilating area not less
than one square foot for each 150 square feet of attic floor area.
Cross ventilation such as vents on the gable end walls of the attic is not as
effective as high - low ventilation. High – low ventilation allows air into the
attic at the soffits and allows the air to escape at the ridge. Due to natural
thermal convection and the fact that the temperature in the attic is almost
always higher than the outside temperature, ridge and soffit venting is the most
effective type of ventilation.
Tips for Homeowners
The best attic ventilation system is a soffit and ridge configuration. Up to
half of the required clear air should be located under the eaves or lowest area
of the roof, and the balance of vent area located at the roof ridge or the top
of the roof. Since warm air rises, this type of system takes advantage of
thermal convection or a natural chimney effect, and air movement will be created
through the attic, even when there is no wind. The soffit and ridge
configuration is compromised by additional vents or openings because the
additional openings interrupt the natural convection, i.e. gable vents or roof
vents, in addition to the soffit and ridge vents.
It is important that air flows freely over the underside of the roof sheathing.
This is especially critical with cathedral ceilings. Insulation must not be
allowed to block this flow. The amount of insulation in a cathedral ceiling
should be approximately 1-2 inches less than the depth of the rafters and not
block the soffit vents.
If condensation is developing in a cathedral ceiling system and the building is
5 years old or older, there is probably evidence on the drywall. Assuming the
ceiling has not been painted recently, look for faint stains that appear to be
shadows along the rafters, mostly the lower area of the ceiling and on the north
side. Shine a flashlight on the ceiling to make sure they are not shadows. If
they are stains from moisture at the joists, it is most likely condensation.
These will not be visible in new construction or freshly painted ceilings, and
may not be visible in situations with a minimal or modest amount of
condensation.
In addition to the standard ventilators (e.g. gable, louvers, soffit vents, and
ridge vents) for attic areas, there are wind turbine exhaust vents and motorized
attic vent fans (not to be confused with a whole house fan). On a hot, still
day, the heat rising up in the attic will start the turbine spinning. The more
heat going out, the faster it will spin.
Moisture from exhaust fans or vents originating in the house, such as a
dishwasher or bathroom fan should terminate to the exterior of the house. They
should never terminate in the attic area due to potential for elevating the
relative humidity, creating mold, compromising the insulation and, in extreme
situations, causing moisture damage to the roof system and interior ceilings.
Moisture generated in a home will only cause condensation during the winter
months. This condition can be aggravated if a homeowner seals the attic vents
during the winter. Vents in an attic should never be closed during cold weather.
With proper insulation at the attic floor/ living space ceiling, the ventilation
will have little, if any, effect on heat loss.
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