Information on Attic Fans

Information on Attic Fans

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There is a lot of information available on line on the pros and cons of attic fans. A lot of this information is tainted by commercial interests of suppliers of certain types of equipment such as soffit vents, various roof vents and different types of attic fans and by people offering consultancy services. This discussion is an attempt to summarize the key aspects to help Fearrington homeowners decide whether to install additional vents and, if so, which type.

The importance of venting the attic should not be overlooked. During the cold months of the year, improper venting can lead to moisture buildup, which can cause mold to grow, wood to rot in extreme cases, or even ice to form in the attic insulation (although the latter is unlikely in Fearrington). During the summer, stagnant, superheated attic air can reach 160oF, which can dry out the roof support trusses, increase air conditioning energy costs, and make the house uncomfortable.

The basic physical cause is that hot air rises, due to a differential in density (mass), which causes an upward movement of the fluid (air is a fluid). One would think that it would be easy for hot air to rise out of your attic. Driven by the density differential, the exhausting of the hot air should bring in fresh air where vents are located around the base of the roof (soffit) and leave through openings near the top of the roof. However, there is no “pressure” that drives hot air out of an attic. The atmospheric pressure inside your attic is the same as it is outside (based on a no wind condition; wind blown across a roof can create a positive pressure on the windward side and a lower pressure on the lee side). Hence, a little help may be required.

Recommended venting areas vary, depending on the source, from 6.6 to 10 square feet of venting per 1000 square feet of attic area minimum, divided over the peak and the soffits. Fearrington houses typically do not have that much vent area; therefore, alternative solutions should be considered.

Many Fearrington houses do have ridge venting, placed at the peak of the roof. It is attractive and easy to install, but most likely it does not allow sufficient air to exhaust as the ridge vent consists of an inverted “V.” This creates a thermal check valve, which does not allow the hot air to escape properly during the summer and, of course, if covered with snow, is rendered useless.

Whirlybirds (the aluminum spinning vents) can help a bit, but have less than one square foot of roof opening and do not induce air movement while spinning. Larger, less conspicuous static vents are more efficient at moving air, but not very attractive.

A logical alternative is to install attic fans. These simply replace the air in the attic with outside air. If this is done often enough, the temperature in the attic will get close to the outside temperature. A commonly recommended frequency is ten times per hour.

The first step is to calculate the volume of the attic, as this will determine the size of the attic fan(s). Although this is a simple geometry exercise, it may be laborious to get hold of the drawings and calculate the volume of the various parts of the attic. The short cut approach is to use one of the available on-line tools, such as an Attic Fan Ventilation Calculator. Because the assumptions upon which these calculations are based are published by companies with a vested interest in selling equipment, it is important to check those assumptions carefully.

The next step is to decide what kind of fan to use. Considering the wide variety of “expert” opinions on the web, there appears to be a lot of disagreement among the experts. A good approach would seem to be to first analyze what ventilation is already in place and then to form an impression of how much ventilation should be added. Click to see a helpful online calculator.

There are basically two options to create additional forced ventilation, namely electrically (grid) powered and solar powered fans. The former requires running electricity to the fan, which is inconvenient and comes with operational cost. The advantage is that the capital cost is relatively low and that both small and larger fans are available. The capital cost of solar fans is higher, but no hook up is required and they don’t use any power. Installation is very simple. For an example, click here.  A possible downside is that they reportedly do not always deliver the advertised movement, so it is probably a good idea to err on the high side when choosing the fan(s).

In both cases it is a good idea to choose a thermostatically controlled fan. Typically this means that the fan starts up when the temperature in the fan housing reaches 80oF and stops working when the temperature drops below 65oF .

The final step is to measure the temperature in the attic on a hot summer day to verify that the attic fan is actually doing its job.