There is no such thing as too much ventilation. The minimum Federal Housing Administration (FHA) -recommended area calls for one square foot of vent area for each 150 square feet of attic floor space. The FHA also recommends that at least 50% of the ventilation area be at the exhaust vents.
Attic ventilation
Almost all residential roofs receive significant solar exposure in the summer. Without proper ventilation, intense solar radiation can drive roof and attic temperatures above 140° F. These high temperatures make cooling systems work harder.
In the winter, good attic ventilation is needed to remove moisture from the air to minimize moisture build up and condensation damage to insulation. Condensation damage can significantly reduce the R-value – the ability to resist heat transfer – of the insulating material and increase heating and cooling costs.
Good air circulation requires a balanced ventilation system that utilizes the stack effect, which is caused by differences in air temperature between the lowest and highest vent openings.
The term stack effect comes from the way an old-fashioned stove works. Cold air is drawn into the hot stove by the temperature and pressure differences created by hot air in the stack. The cold air is heated and rises to the top, pulling more cold air into the stove.
The Stack Effect Wind Pressure
Air movement in the attic may be caused by the stack effect, wind pressure or both.
Balanced ventilation also uses the natural forces of wind pressure to create areas of high and low pressure around a house. High-pressure areas form where wind strikes the building and backs up. Low-pressure areas occur on the downwind side. Air moves into the building through openings in high-pressure areas and exits at low-pressure areas.
A Balanced Ventilation System
To achieve a balanced ventilation system, we recommend:
- Adequate free area around vent openings
- Approximately equal areas of intake and exhaust vent openings
- Exhaust vents near the highest parts of the ventilated space and intake vents near the lowest part
- Continuous vents – both high and low – that allow the system to be effective regardless of wind direction
- Design features that prevent rain from infiltrating the attic space or penetrating insulation or structural parts
Recommended ventilation system
The best choice in attic ventilation is ridge vents coupled with continuous soffits, because they provide an opening along the entire ridge line of the roof. The continuous ridge is located at the highest possible point of the attic space and lets the hottest air escape. It also vents the roof between each set of roof rafters.
Power ventilators
Power ventilators appear to be justified only in unusual cases where there is inadequate ceiling insulation or where conventional ventilators cannot be installed.
A study conducted in Houston by the National Bureau of Standards indicated that in most cases, power ventilators are not a cost-effective investment. Although the study noted some savings in air conditioning energy, the savings was smaller than the energy required to run the power ventilators. Also, unless a humdistat is used, thermostatically controlled power vents do not operate in cold weather – when attic ventilation of moisture-filled air is needed most.