Engineering One Cool Roof
There are many ways to maintain comfortable temperatures within a building other than turning up the air conditioner or starting up the furnace. The best way is to begin with the building itself. This week I look at the materials associated with roofing designs known as cool roofs. The materials used in the construction process can play a major role in how energy efficient the building will be. When it comes to controlling the thermal management aspects of a structure, one of the key areas to evaluate is the roof. Choosing the right materials, as well as having a well insulated attic, can make the biggest impact on just how efficient the roof is. With energy prices constantly on the rise, the long term savings from using energy efficient materials and designs must be taken into account when building or upgrading structures. Other main factors in choosing a roofing material are its durability and environmental impact. While roofing is currently a large contributor to solid waste generated in the United States, the increasing usage of recyclable and eco friendly materials is gaining traction and should help to reduce this problem.
Benefits found in metal roofing
One type of roofing, known as a cool roof, relies on two surface properties; a high solar reflectance and a high thermal emittance. The reflectance is the amount of solar energy that is reflected away from the surface, while the emittance is the percentage of thermal energy a material can radiate after it is absorbed.
[i] Some of these types of roofs use materials made from clay, metal and stone. You would typically find these types of roofing materials desirable if you reside in a warmer climate with more direct sunlight, and thus higher heat indexes. By keeping solar absorption low while maintaining a well insulated attic space, the less you would need to rely on some type of air conditioning to cool the structure. While material and installation costs of these materials is usually higher than that of the traditional wood or asphalt shingles, the environmental and durability aspects are better. The initial and maintenance costs for a metal roof, for example, generally are higher but will usually last 2 to 3 times longer than its wood counterpart. The energy savings associated with these roofs over time also help to reduce the costs over its lifespan. The metals used can also be recycled or made from recycled components which help from an environmental standpoint. Asphalt shingles are petroleum based products which require more energy to produce and have no real recyclable value. Asphalt does have some upsides when used in colder climates to assist in reducing heating costs, as it absorbs heat to help raise the temperatures within a structure.
A heat sink on the Roof?
Ballasted roofing is made from placing river-rock over a synthetic rubber (EPDM) membrane and rigid insulation on top of a metal roof. This combination of materials utilizes its mass to act like a heat sink and simulate the performance of a cool roof, as it has high solar reflectance and high thermal emittance. This is the type of roof you would find on larger commercial building due to the weight. A ballasted roof will have a minimum weight of 10 pounds per square foot once it is completed. Studies have confirmed that the roof system’s mass acts as a shield protection the building from solar radiation that reduces peak temperatures and also delays potential heat flow into the building so that more of the cooling load can be moved to off peak hours of the day.[ii]
So as I discussed n our previous blogs, the world’s population is continuing to grow at an astounding rate. With this growth, come more construction, more energy use, and more landfills. While the construction will be needed to keep up with the growth, the energy use and landfill problems can be confronted and dealt with by engineers utilizing these newer technologies and materials as we move forward.