WRAP IT UP
- By Eric Schakel
- July 1st, 2004
Air duct systems may appear passive, but their performance plays an active role in the indoor environmental quality (IEQ) of schools. Is the sound of the HVAC equipment so distracting that students can’t hear the teacher properly? Sheet metal ducts transmit equipment noise as well as cross-talk. Add to that the fact that metal makes expansion and contraction noise of its own. Are there wet spots on the ceiling tiles, even though there is no leaky pipe in the space above? The culprit may be condensation on a bare sheet metal air duct. Are energy costs too high even while some areas of the school are too hot and others too cool? Uninsulated sheet metal air ducts strike again.
Modern HVAC systems in schools have one purpose: to create a comfortable, healthy environment for productive learning. But sometimes that purpose is compromised by the ductwork in the air distribution system. Fortunately, the shortcomings of bare sheet metal ducts are easily overcome by insulating them with fiberglass. Some improvement in thermal performance can be achieved by wrapping ducts with fiberglass insulation, but the greatest IEQ benefits are realized when metal ducts are lined with fiberglass or replaced altogether by ducts fabricated from fiberglass board.
Improved Classroom Acoustics
Perhaps the most widely appreciated feature of fiberglass is the material’s sound-control capability. The issue of classroom acoustics is becoming more critical as research reveals a clear link between unwanted noise and poor student performance. The most obvious victims are students with hearing disabilities, but noise impairs learning for students with other types of learning or behavioral disabilities as well. Even children with normal hearing and abilities suffer from unwanted noise. According to the Acoustical Society of America (ASA), many U.S. classrooms have so much background noise that the speech intelligibility rating is 75 percent or lower. This means that children with normal hearing hear clearly fewer than three out of four words spoken. This can be a significant problem for young children who are less able than older students and adults to put what they hear in context. Once younger students miss a few words, they are likely to have missed the whole message. The ASA report on classroom acoustics is available for free download at .
According to the Noise & Vibration Committee of the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE TC 2.6),fiberglass duct systems insulation continues to be the most cost-effective solution to noise control in most HVAC air duct systems. The consulting engineering firm of Gordon Gumeson & Associates located in Grand Junction, Colo., agrees. According to Jay Ferrare, who was the firm’s team leader for the Grand Valley High School project in Parachute, Colo.,We as a company have a standard specification for lining ducts with fiberglass liner to minimize noise transmission from HVAC equipment. This paid off at Grand Valley. Steve Brubacher, the school district’s owner’s representative says, We were interested in silent air handling. In the gyms and other areas with oversized ducts where noise is typically a problem, we have almost no background noise at all from the air handling system. It seems to be working quite well, and we are very happy with it.
Fiberglass air handling insulations also contribute to IEQ by delivering conditioned air at the desired temperature throughout the building. Take the example of a bare sheet metal duct running through plenum space that is 80 degrees F. Air cooled to 55 degrees can rise almost 10 degrees moving just 100 ft. under such conditions. In contrast, air carried in a duct fabricated of fiberglass rises less than two degrees. Not only does this deliver greater comfort, it is a proven opportunity for cost saving because it reduces energy consumption. Large amounts of energy are required to condition air to desired warm or cool temperatures. Any conditioned air that is lost through leaky ducts is equivalent to energy lost. The high R-values of fiberglass duct insulations prevent this expensive loss. Energy codes are rapidly changing from state to state and even county to county. By keeping current with local energy codes or following the guidance of the International Model Energy Code or ASHRAE Energy Standard 90.1, school facilities mana-gers can deliver the most conditioned air benefit for the lowest cost.
The thermal insulating properties of fiberglass also protect against damaging moisture. Condensation on the outside of sheet metal ducts can occur when there is no insulating barrier between the conditioned air inside the duct and the ambient air outside it.
This is one reason the ductwork in Redmond High School, in Redmond, Wash., is lined with fiberglass duct liner. Mechanical engineer Chris Caffee of BCE Engineering, Inc., in Seattle explains, If the duct runs through warm, humid conditioned space and the air inside the duct is cold enough, condensation forms on the bare duct. This can drip onto ceiling tiles and leave brown watermarks and spots. I think this is worse than a leaky pipe because it is so hard to diagnose. If temperatures change, the maintenance crew may not see condensation on the duct and could assume they have a roof leak or some other problem. Condensation is rare, but we always recommend lining ducts to avoid it.
Indoor Air Quality
The health benefits of preventing excessive noise and maintaining a constant, comfortable temperature are clear. In addition, school administrators and parents are anxious to keep irritants like mold out of classrooms. Preventing mold growth depends on the proper design, filtration, maintenance and operation of the HVAC system. A number of studies show that, contrary to what some believe, mold growth does not depend on substrate surface or relative humidity of the air in the system. Rather, mold and fungus can grow on any surface, including bare sheet metal, given the right combination of nutrients and water. Typically the water is present in systems where air is cooled to a point below the dew point, causing moisture to condense.
Insulating ductwork with fiberglass liners helps prevent this problematic condensation and avoid the mold growth it causes. Further, coated fiberglass substrates offer additional anti-microbial protection. Designers should specify liners or duct board with a coating that guards against the incursion of dust or dirt into substrate, minimizing potential for biological growth. For example, many Johns Manville duct liners and duct board products are coated with a tough acrylic polymer that contains an immobilized EPA-registered anti-microbial agent to further guard against potential growth of fungus and bacteria. An airstream surface with a quality coating can be cleaned using industry-recognized dry methods, should such maintenance be required. When properly installed and maintained, fiberglass ducts and liners will not support microbial growth and will help prevent mold growth on other surfaces like ceiling tiles by stopping condensation.
Unfounded concerns about safety of glass fibers were conclusively put to rest by the International Agency for Research on Cancer (IARC), an agency of the World Health Organization. Their studies of fiberglass over 15 years show no evidence of increased risks of lung cancer. Their conclusion is that fiberglass cannot be classified as a human carcinogen.
The HVAC system, including air distribution, plays an invisible but important role in the quality of the learning environment. Installing air ducts lined with fiberglass insulation is a non-intrusive, cost-effective way to improve classroom acoustics and comfort. Both contribute to a more productive learning environment.