What is the purpose of attic ventilation?It seems like a simple question, easy enough to answer. Unfortunately, all too often, that’s not the case. Most homeowners, and even some experienced builders and contractors, believe the only purpose of attic ventilation is to remove heat that builds up in the summer. That is accurate. But what that answer leaves out is just as important as what it includes. If you understand the principles of attic ventilation, you know an effective venting system provides year-round benefits.
- During warmer months, ventilation helps keep attics cool.
- During colder months, ventilation reduces moisture to help keep attics dry. It also helps prevent ice dams.
- Not all ridge vents are the same. If you look up and see a black rolled vent, that ridge vent is virtually useless. Install a baffled ridge vent. Do not install a new ridge vent before mold remediation in the attic.
- The soffit vents must equal the volume of the ridge vent. Typically, a properly installed 2? strip style soffit vent will yield 9 sq. inches per linear ft. and a 3/4? ridge vent will yield the same proving a balanced ventilation system.
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Ventilation During Warm Weather
Dealing with the effects of heat. Why, on a hot day, are the upper rooms of a home always warmer?
Part of the answer, of course, is simple physics: hot (lighter) air rises while cooler (denser) air falls. But in most homes- the vast majority of homes without adequate attic ventilation- a far more important factor comes into play: the downward migration of heat. Consider what happens in such a home on a typical summer day. Radiant heat from the sun hits the roof. The roof temperature increases, and heat travels into the attic. As heat builds up in the attic, it radiates to the attic floor, then into adjacent living areas, raising temperatures there. Eventually this accumulation of heat begins to have more practical and costly consequences.
The most obvious are the actions taken by homeowners to cool themselves. To reduce the effect of heat – not only the daytime heat gain but also the excess heat being stored in the attic – they turn on fans, window air conditioners or central air conditioning systems. As the hot weather continues, these appliances run longer and longer – a fact well documented by utility companies across the country. Homeowners pay for all this added energy consumption in higher utility bills.
Do not get talked into a solar powered exhaust fan for your roof. The soffit vents will not keep up with the volume of air being pulled from your attic. So guess where the makeup air comes from? Your air-conditioned house. So now you are air conditioning your attic. House gets warmer, air conditioner runs longer, etc. Not very green from an energy saving standpoint.
Over the gutter soffit vents are a bad idea in colder climates. In the summertime water in the gutters can evaporate directly into the soffit vents (commonly called hicks vents) and end up in the attic.
Another problem is exhausting bathroom fans into the soffit area. Soffit vents are intake vents. The moist bathroom air vented into the soffit will be pulled back into the attic.
Insulation: A 5% gap between the fiberglass insulation and ceiling joists can lead to a 20% loss of heat. Do not install insulation with a vapor barrier over insulation that has a vapor barrier already. Do not block the soffit vents and do not install the insulation upside down. Consider blown in insulation (14?) to help prevent heat loss and ice dams.
If you are upgrading your insulation to the new current standard of R-38, be sure to add additional ventilation to reduce the moisture in the attic that additional insulation will create.
Ventilation During Cold Weather
Improper ventilation can cause ice dam formation which is the result of continuous freezing and thawing of snow due to escaping heat from the house or from gutters being backed up with frozen slush. Ice dams occur when the following conditions exist: warm air accumulates near the peak of the attic, lower areas of the room remain cold and/or a heavy snow accumulates on the roof. When any of these occur, water may be driven under the roof which may cause ceiling, wall, insulation, and gutter damage.
Ice dams can be prevented from forming by:
- Installing a vapor barrier above the home’s warm space.
- Insulating the attic floor.
- Ventilating the attic properly.
- Damage from ice dams, if they do form, can be reduced by:
- Removing debris from gutters so that it does not build up over time.
- Making sure that the outer edges of the gutters are lower than the slope line. This will allow for snow and ice to slide clear.
- Installing eaves flashing, any brand of Ice & Water shield will do.
The amount of ventilation needed is determined by the size and design of the roof. For roof and attic spaces above an insulated ceiling, the vent ratio is one square foot of net free ventilating area/300 square feet. For low slope roofs or roofs with cathedral ceilings the vent ratio is one square foot/150 square feet.
Typically, an attic will have a ridge vent opening of 0.75? per linear ft. and a soffit vent opening of 2? per linear ft. (a net area of 9 sq. in.). You cannot have too much soffit ventilation, but you can have too much ridge ventilation. An undersized soffit vent will force the ridge vent to draw warm moist air from the living space.
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