Special Report: Reflective Radiant Barriers
by: Ron Simmons, P.E.; Eric Benstock, P.E.; Rick Bonyata, P.E. and Nestor Camara, C.F.I.
Installation of reflective radiant barriers in residential attics around the country is becoming increasingly popular. The driving force behind the trend may well be the advertised savings of 10 to 70 percent on previous energy costs for a given structure.
Unfortunately, the potentially serious, negative side effects involving the installation of the product need further technical consideration. In many cases, the installation turns the entire attic surface area into a conductive electrical "ground path." Additionally, it's installed near much of a home's electrical wiring.
As investigators with McDowell Owens Engineering, we have examined several residential fires recently and determined the installation and presence of this product played a key role in the causes. We suspect even more cases will be forthcoming.
Back to the basics: transfer of heat
Most solar heat gain in the summer enters homes through roofs and attics. Likewise, most heat loss in the winter also occurs through roofs and attics. The theory behind the use of reflective radiant barrier is based on how heat energy propagates through roofs and attics. Heat transfer involves the movement of heat from a warmer body to a cooler body, which if you recall in your high school physics class, occurs in three primary processes—conduction, convection and radiation.
Composition of radiant barrier material
The materials used in reflective radiant barrier foil products have characteristics specifically selected to provide optimum product performance in factors such as strength, durability and "reflectivity." From a practical standpoint, however, the product is essentially a thin sheet of flexible poly fabric (plastic), which is laminated between two layers of aluminum foil.
Radiant barrier installation methods
An installer uses one of two main approaches to install radiant barrier foil as a retrofit in an existing home. One approach involves stapling the barrier to the underside of the roof rafters, and the other involves applying the barrier onto the attic floor over the existing insulation. Both of these installation methods, however, introduce a disadvantage—the possibility of becoming electrically energized. If the barrier becomes energized, two potentially dangerous conditions are introduced—electrical shock and fire.
Potential electrical dangers involve shock, even electrocution
In warmer climate areas of the country, residential attics contain many of the utilities, including much of the branch circuit wiring and even HVAC air handlers, HVAC coolant lines, HVAC ducting, water pipes and water heaters. In colder climate areas, you are less likely to find HVAC equipment and water heaters in attics, but their metallic venting systems may pass through attics. Additionally, in all climate regions, many branch circuit conductors, as well as electrical junction boxes and recessed lighting fixtures, may protrude into attics. When installed, the barrier material most likely comes in contact with many of these electrically grounded utilities and/or appliances, and when it does the barrier material itself becomes a part of the grounding system.
Once the barrier is grounded, though, it becomes a very large electrical conduction path that covers the attic space. The reflective barrier now provides an excellent ground fault current path back to the electrical source if certain types of ground faults should occur.
Energizing the reflective barrier can also occur during installation of the barrier. Typically, installers use staples to secure the barrier to wood joists and can sometimes drive a staple into nonmetallic (NM) sheathed cabling. Often, the NM cables installed in the attic are concealed under their work areas. If a staple does penetrate the NM cable's insulation and contacts the energized conductor, the entire reflective barrier suddenly becomes energized since its aluminum coating is electrically conductive.
Energized barrier creates fire risk
An energized barrier can also create a significant fire hazard, which comes from the radiant barrier material itself. During a recent investigation, we discovered someone indeed drove a staple through the insulation of the NM cable. The driven staple caused a fault between a line (hot) conductor to the staple and subsequently to the radiant barrier.
When investigating an attic fire involving a radiant barrier, consider the role it may have played in the causation. Look for radiant barrier staples driven through NM cables in an origin area, ground fault failures in electrical equipment, and if you suspect a lightning strike, evidence of electrical tracking along the barrier (we have seen one case of this phenomenon). It's also important to note that our test results have indicated an unexpected but important phenomenon: Fires can begin virtually anywhere on the energized radiant barrier foil, not just where it becomes energized.
©September 2009. All rights reserved. The contents of this article have been prepared by fire and engineering professionals of McDowell Owens Engineering, Inc. Neither the whole article, nor any parts thereof, may be reproduced or distributed in any manner, without the expressed written consent of McDowell Owens Engineering, Inc.
For the full report please visit www.firefindings.com