Friday, November 11, 2011

Attic Ventilator Fans: The good, the bad, and the ugly!

It is common practice for homebuilders, roof specialists, and HVAC companies to tout the importance and benefits of attic ventilator fans. The argument, which seems to make a great deal of sense, is that by creating airflow in the attic (powered) the heat gains will drop dramatically, therefore reducing electric consumption for A/C units. Is this really the case? As with most things, there is not a clear “black and white” answer. The reality lies in the grey area that I have decided to study a bit.

Savannah Renovations conducts its business in and around Savannah, Georgia. We have a hot, humid climate to deal with most of the year. It is common during the summer for attic spaces to have temperatures in the 120 to 140 degree range and I have read that in some cases they can reach 160 degrees! The hot sun beats on the shingles conductively heating the roof deck, and then through radiant transfer, the air inside the attic space is heated to incredible temperatures. Most homes (especially newer ones) have the air handler which contains the cold coil that cools air and also removes humidity hence the need for the drip pan and drain on your home located in the attic as well…you can imagine what these temperatures do to your cooling bills.

The stage is set and many companies sell powered and solar ventilation fans touting that purchasing their equipment will save you thousands. The science seems sound and all us contractors rally behind it because it seems to be almost common sense. Pulling in air from the outside at 90 degrees and replacing the 130-degree air seems to be a no brainer.

Here are the real facts:

A study done by Danny S. Parker and John R. Sherwin from the Florida Solar Energy Center (FSEC) (1) Sheds some light on the actual benefits of attic ventilator fans. Their study of a single family home in Florida showed the fans (2 installed) reduced attic temperatures over 20 degrees and reduced air conditioning consumption by 6%. The downside of this is that the cost of the fans and installation made the return of investment very low taking almost 20 years to pay for itself.

In a more expansive study, John Tooley (Natural Florida Retrofit), and Bruce Davis from Alternative Energy Corporation's Applied Building Science Center, looked at the unintended consequences of the use of powered ventilators in attic spaces (2). Their study of eight homes found that the attic ventilators caused depressurization inside all the homes. This depressurization caused safety concerns. Specifically, they noted that a gas powered hot water heater was actually experiencing back draft issues causing normally vented gasses to be pulled into the home. In addition, there was the issue of loss air conditioned air; an average of 231 cubic foot of air per minute!

Does this mean that forced ventilation of your attic is a terrible idea? No, but care should be taken and hiring a qualified and knowledgeable professional to help choose the proper product and conduct the installation is important! This is a perfect place for me to mention Savannah Renovations as such a company.
In conclusion, there are benefits and consequences of installing attic ventilator fans. Here is a summary by bullet point of the most important:

Negative points to consider:

  • An electrically powered 1500cfm attic fan usues about 325 watts of electricity. In the Parker/Sherwin study their test fan used 284watts and generated 170watts of savings from reduced air conditioner use. Therefore, my opinion is that electrically powered fans are out of the question unless there is little or no insulation in the home. Solar ventilation fans use no electricity…nothing more to say about that.
  • Care must be taken in choosing the proper fan. Oversized (which is the norm) fans can cause depressurization, pulling air-conditioned air into the attic. They can also cause health and safety issues. Especially if you have gas powered appliances and utilities in your home. The Tooley/Davis study noted a story of a Colorado family who died from carbon monoxide poisoning when their attic fan caused their furnace to operate in back draft.

Positive points:

  • A properly installed attic fan should reduce the summer temperatures by about 20 degrees.
  • Attic fans reduce humidity in the attic area more than passive ventilation alone.
  • The above points result in a reduction in cooling bills (approximately 4-7% in my opinion), and increased longevity of air-conditioning units due to the decreased stress (less heat and humidity around the coil, air handler, ducting, etc.).
  • Solar powered units are the only way to go! They pull no parasitic energy from the grid, paying for themselves over the long term. Not only saving you money, but saving our environment as well.

Finally, I will reiterate a pervious point(s). Attic ventilator fans can be a positive addition to your homes energy system. However, hiring a knowledgeable professional to help with fan selection, location, and proper installation is critical to see actual savings and keep your family safe. I believe that Savannah Renovations is such a company. Give is a call or email to see how we can help you!

Robert F. Foy