The problem is overheating. Anyone who has been in an attic knows that attics get hot!  If the heat in the attic is allowed to sit there, it will conduct heat into the house, or, at the very least, prevent the heat in the house from leaving.

The most economical answer to this problem is to ventilate the attic, moving air through the attic to reduce the temperature.  Most homes have passive attic ventilation in the form of a ridge vent at the peak of the roof, gable vents at the ends of the roof, soffit vents in the eaves or some combination of these vents.  Turbine and or roof vents(passive vents that penetrate the roof) are often used as a fix for older roofs with inadequate ventilation.  The problem with passive vents is that they require some driving force - wind or temperature differential (hot air rising)- to move air.  Usually the hottest days of the year are the stillest, with little or no wind.  Temperature differential doesn't have much energy, so it is slow.  Just when you need venting the most, the vents work least effectively.  There is also the installation problem.  If your roof was designed without sufficient attic ventilation, adding more passive vents may be impractical.

Some homeowners take a step further and add powered ventilation by using either an attic fan or a whole house fan.

Attic fans are usually roof mounted or gable mounted fans that draw air out of the attic, relying on existing passive vents to supply cooler, outside air to replenish the air being exhausted.  Unfortunately, an attic fan does not exhaust the heated air from the living space.  A gable end mounting system will only work with gable vents and many homes don't have them.  A roof mounted attic fan is difficult to install because of the location and may detract from the home's appearance.  There is also the possibility of costly leaks or roof damage if the installation is not done correctly.

Whole house fans are different in that they are mounted in the attic floor, pushing hot air out through the passive vents and bringing cooler air in through the house.  This gives the additional advantage of directly cooling the inside of the home, enhancing the comfort level for the occupants.  Whole house fans are traditionally sized by taking the square footage of the house and multiplying by 3.  By this method, a 2000 square foot, two story house would require 6000 cubic feet per minute (cfm)of air flow!  Fans of that size are notoriously loud and power hungry.  You must also have sufficient passive attic vents to handle that much airflow.  At 6000 cfm, an attic will require at least 8 square feet of unobstructed attic vent openings.  If this home is built to code requirements, there will be 3.33 square feet of unobstructed attic vent openings.   Additional vents will need to be added - about five 12" turbine vents or 14 roof vents.

So why is this method used for sizing whole house fans?  The theory has been that you need such a large fan to create a breeze through the home to cool the people as well as the building.  The breeze moves air over the skin which makes the occupant cooler.  Unfortunately, the fan required to move that much air is loud and people don't like the noise and so they turn the fan off.

And there is a maintenance issue with these large fans.  Many are belt drive, which requires belt maintenance, and they all require some type of home-made cover that is used, at most, twice a year.  The homeowner has to climb into the attic to cover or uncover the fan or live with the heating or cooling losses.

So what's the answer? Lets compare an HV to a gable fan and a standard whole house fan:

About the Author:
Tamarack Technologies, Inc.

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