Keeping the Heat Out

Keeping the Heat Out

The roof system may be the first line of defense in beating back high energy costs. By David Kozlowski.

There are days when, common wisdom will assert, one can fry an egg on a sidewalk. But runny, grilled street sandwiches in Los Angeles will attest to the fact that this isn’t so. A better test might be a low-slope, black commercial roof. On the roof surface temperatures can rise 100 degrees hotter than the ambient air temperature. On a sweltering, windless day, the surface of that roof inches closer to fry time.

Culinary reasons aside, however, the searing temperature of the roof’s surface is not a good thing. It can be a detriment to the comfort of a building’s occupants and can increase energy costs. If there’s minimal thermal resistance to the heat, the sun’s energy will be conducted to the underside of the roof and into the facility, adding an additional load to the HVAC system.

While there are other systems on a building envelope that must contend with the temperature difference between the outdoors and indoors — and contribute to the overall energy efficiency of a building — the roof may be ground zero for combating the sun’s energy. The key weapons in this battle are reflective roof membranes and coatings, and roof insulation.

A Design Problem

For most facility managers in the United States, a building’s roof is in a rather unfortunate position: lying flat facing directly into the sun when the sun is at its peak. The roof is almost in its most efficient position for collecting the sun’s energy. The only way to make it more efficient is to color the roof black, which most roofs are.

Of the sunlight that hits the membrane, most of it is in the infrared range. What makes the roof hot is that the typical black roof reflects only about 5% of the sun’s energy, so it absorbs more than it radiates or emits.

A roof that gets extremely hot is a problem for a number of reasons. When the roof surface is at 180 degrees, the temperature on the underside of the roof or plenum space can be nearly twice the indoor ambient temperature and three times the temperature of chilled water in pipes or air in ducts running through the plenum. This heat, however, not only affects energy use, it can stress the roofing system and, anecdotal information suggests, help degrade the membrane.

The most efficient way to reduce the surface temperature of a roof is to reflect as much light as possible, says Ted Michelson, executive direction, Roofing Industry Educational Institute.

Energy Star Helps

Finding a roof that does this without sacrificing quality and performance of the roofing system is made easier with Energy Star. A reflective roof is defined by the U.S. Environmental Protection Agency’s Energy Star Buildings program as a roof that reflects a minimum of 65 percent of sunlight when new and 50% when it’s three years old, says Rachel Schmeltz, program manager for Energy Star. A white roof will reflect 80% of sunlight when new.

“Manufacturers must test their products on three different buildings for three years before they can even apply for a label,” Schmeltz says.

Reflective roofs are tested this way because, as they age, they lose some of their reflective ability due to dirt accumulation and general degradation of the surface. These roof systems tend to lose a significant percentage of their reflectivity the first year and only a negligible amount after the third year. That’s assuming normal maintenance, Schmeltz says.

How effective are reflective roofs? According to Danny Parker, a research associate at the University of Central Florida’s Solar Energy Center, reflective roofs can make quite a substantial difference, especially in cooling-dominated climates.

His team tested envelope-based strategies for reducing energy use on a number of buildings in Florida. The strategies included increased wall insulation, double pane insulated windows and reflective roofing. What they found was increased wall insulation offered a 0 to 10% gain in efficiency. Double pane insulated windows provided about a 15% advantage over single pane. Reflective roofs did the best with 19% gain.

“But the cost of insulation and windows was much more, whereas the reflective roofing didn’t cost any more than conventional roofing,” he says.

Most experts agree that a white single-ply membrane would be applicable for large buildings that cool year round. Assuming a building owner was going to replace a roof anyway, the reflective membrane doesn’t cost any more, Parker says, so there is no first cost penalty to pay, and any energy benefit is a plus.

Finding a Fit

In the case of coatings, however, it is not quite so clear. Coatings cost from 50 cents to $1 per square foot. If a coating was going to be applied to extend the life of a roof or to a new metal roof, for instance, then a reflective coating could provide an energy bonus. But energy savings alone might not justify a coating. Most coatings typically last five to seven years, and it might take that long to be paid back, depending on energy costs, says Andre Desjarlais, senior researcher with Oak Ridge National Laboratory.

The energy gain in the South can provide a payback for coatings, says Curt Liscum, senior roofing consultant at Benchmark, a roofing consulting firm. But in the North, that’s not likely.

Even for white membranes, the size and location of the building will have an impact on any energy penalty, Desjarlais says.

“As the cooling load goes down and the heating load goes up, the question becomes what is the right balance,” he says.

A benefit to reflective roofing systems that is sometimes overlooked is connected to a roof’s unusual position. A reflective roof reflects most of the sun’s energy during the hottest time of the day, when energy costs are the highest. It in effect becomes a load shifting strategy.

“During the mid afternoon, when the rates are the highest, the reflective roof is doing the most work for you,” Desjarlais says. “If your rates are going to quadruple between 2 and 5 p.m., then that makes it even more cost effective.”

Reflective roofs can benefit more than an individual building. When a building’s roof absorbs rather than reflects sunlight, it heats the air above the roof. Add more buildings and lots of dark colored pavement, and the ambient temperature of this micro-environment can rise 6 to 8 degrees on average higher than a rural, green space. This is called the Urban Heat Island effect, and it can affect cooling costs for everyone and contribute to the formation of smog.

Warming and Cooling Buildings

A white roof that is poorly drained will become dirty fairly quickly, so thermal insulation still must be part of a facility’s roofing energy program, says Dick Fricklas, technical director emeritus of the Roofing Industry Educational Institute. Insulation conserves energy in buildings in the North and South.

According to Jared Blum, president of the Polyisocyanurate Manufacturers Association, insulation on commercial buildings saves two quadrillion BTU per year. That’s about 290 days of gasoline for the United States.

And the building envelope is going to be the place where more and more managers are going to look for savings. Blum cites a study done by the California Energy Commission that showed a surprisingly large percentage of Southern California buildings had no insulation in their roofs. Given the energy crunch there, the state is acting quickly to amend this, he says.

In fact, Blum says, California and a number of other states are addressing energy efficiency with tax credits and rebates for insulation and other technologies.

Insulation serves multiple purposes on a building’s roof. Besides conserving energy, it can provide moisture and fire resistance, and it can help keep in place and protect a roofing membrane.

It can be divided into two general classifications: traditional insulation, which includes wood fiber, perlite and fibrous glass insulation; and “new” insulation, which includes polyisocyanurate foams or isoboards, and cellular foams or polystyrene.

When it comes to energy efficiency and insulation, the key issue is R-value cost per square foot. But a decision based on energy efficiency also has to be compatible with other issues concerning the roofing system, such as the type of deck and membrane and labor costs.

“It’s the R-cost per square foot that drives the issue,” Michelson says. “Most people aren’t going to put six inches of perlite down because of the cost, but it’s not the cost of the material so much as the cost of labor for doing it. Some insulation types just work better with some systems.

“You try to maximize the strength of the insulation you need and minimize its weaknesses,” he says.


David Kozlowski is senior editor of Building Operating Management magazine. This article Copyright 2001 Building Operating Management magazine, All Rights Reserved. Building Operating Management is a GreenBiz News Affiliate.