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Replacing a Roof? Here’s How to Increase Its Energy Efficiency
Replacing a building’s existing roof with new roofing material provides an excellent opportunity to increase its energy efficiency and reduce energy costs.
Since building roofs usually are replaced only once every 20 years or more, most roofs needing replacement today were installed in 2000 or earlier – before building energy codes were adopted in many areas. Therefore, replacing roofs with high-efficiency and durable materials can generate significant energy and cost savings. And not only should you update your roofing material, but a re-roofing is an incredible opportunity to add insulation underneath your new roof, improving energy efficiency even more.
We’ve got some thoughts about materials and technologies as you think about your next reroofing project:
Some Factors to Consider
Energy absorption and reflectivity – The color of exterior roofing material contributes significantly to the amount of solar energy absorbed by the roof. This energy is transferred to the building itself, increasing the amount of energy required to cool or heat the building. High solar-energy reflection can lower a roof’s surface temperature by as much as 30 percent.
As expected, darker colors tend to absorb more solar energy while lighter colors reflect more energy. Buildings and homes in warmer climates benefit more from white or light-colored roofs, while cold-weather areas are better suited to dark-colored roofing.
Check out the U.S. Department of Energy’s Cool Roofs Page for more.
Insulation qualities – Adding insulation underneath roofing tiles or shingles can greatly enhance energy-efficiency, reducing energy costs and associated greenhouse-gas emissions. Plastics-based materials, such as spray foam and rigid-foam boards of polyiso and polystyrene, offer many advantages. These modern materials offer high insulating R-values to greatly reduce energy transfer between indoors and outdoors. They also are lightweight, durable, and long-lasting – most are reusable and may last well beyond your next re-roofing, further avoiding carbon emissions otherwise generated through production of new insulating material.
A library of fact sheets with information for state energy-code requirements applicable to roofing and re-roofing is available from the Polyisocyanurate Insulation Manufacturers Association.
Spray Polyurethane Foam
Spray polyurethane foam (SPF) is used widely as an effective way to insulate roofs, both externally and beneath other roofing materials. Applied as a spray that hardens to a tough finish, SPF does an excellent job of sealing cracks and providing resistance to heat, wind, rain, and water vapor. It also requires low maintenance – damaged areas often can be repaired with a fresh foam application without the need to remove the roof. SPF’s insulating R-value ranges between 3.6 and 6.6 per inch, depending on the type of foam used.[1]
SPF applied to the underside of a roof
SPF is applied externally to many flat or low-pitched roofs, often as a light-colored foam or covered with a light-colored sealant to increase solar reflectivity and enhance durability.
SPF can be applied to the underside of a roof deck, directly to shingle-supporting boards and structural members. The foam expands as it dries and hardens, helping seal cracks and bond with roofing materials to keep wind, heat, cold, and moisture from entering attic spaces.
In areas that frequently experience high winds from hurricanes or tornadoes, SPF applied to a roof’s underside can provide exceptional wind-uplift resistance by bonding roofing components together. This increases a roof’s strength and ability to repel wind-driven water and moisture.
Installing SPF insulation also helps contribute to sustainability by increasing energy efficiency and driving down greenhouse-gas (GHG) emissions associated with home heating and cooling, with an “environmental payback” period (i.e., amount of time needed for a product to offset its embodied carbon) as low as seven to eight years.
Rigid-Foam Insulation Boards
Rigid-foam boards made from dense sheets of plastic foam, including polyiso (polyisocyanurate), extruded polyurethane (XPS), and expanded polystyrene (EPS), are often used as roofing insulation, as well as in many other areas of homes and commercial building, including basements, living areas, and attics. Rigid-foam boards installed on roofs usually incorporate reflective plastic membranes to reflect sunlight and enhance weatherproofing against rain and wind. The boards are sold in large sheets that are easily installed and exhibit high strength and durability.
Rigid-foam boards are especially useful in re-roofing situations where an existing membrane and insulation are left intact and re-covered with new board insulation and topped by a new membrane. This lowers the installation cost while preserving the initial R-value.
Structural Insulated Panels
Growing in popularity due to their high energy efficiency and availability in a variety of shapes and sizes, structural insulated panels (SIPs) are composed of a rigid plastic-foam insulation core sandwiched between two structural skins (typically made of oriented strand board or OSB). Together this sandwiched structure provides high strength and insulating qualities. SIPs are used as roofing components with embedded insulation, as well as throughout new buildings. These panels do require a vapor barrier to protect them from moisture infiltration.
Structural Insulated Panel
Vinyl Roofing Membranes
Single-ply vinyl roofing membranes are light and flexible, and have been used extensively for decades on flat- and low-sloped industrial and commercial roofs. Made from polyvinyl chloride (PVC), vinyl membranes are extremely durable and water-repellent. They’re also highly wind-resistant thanks to their welded-seam construction that creates strong bonds at the joints, which resist water penetration. PVC membranes are also resistant to fire and chemicals, making them well-suited for industrial applications.
Vinyl roofing membranes are typically light-colored to maximize their ability to reflect solar energy away from the building. Their R-value is low, however, so they must rely on quality insulation in the supporting structure to meet energy-efficiency code requirements.
As with rigid-foam boards, vinyl membranes can be used to re-cover an existing roof, installed together with a new layer of insulation. They can also be used in “green roofs” that incorporate soil, vegetation, and irrigation into an environmentally friendly system.
Highly Efficient Choices
These modern, high-performance roofing materials offer multiple benefits for both commercial and home construction: enhanced energy efficiency, light weight, durability, low maintenance, and more. These advances help make plastic roofing insulation materials a frequent choice for designers and homeowners alike when considering a roof replacement.
[1] The higher the R-value, the greater the insulating power. Ask your seller for the fact sheet on R-values.
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