Technical Elements

A roof assembly can play a vital role in reducing a building's energy consumption. To assist roofing professionals with installing energy-efficient roof assemblies, in 2007, NRCA, the Polyisocyanurate Insulation Manufacturers Association (PIMA) and The Roofing Industry Alliance for Progress developed EnergyWise Roof Calculator Online. The calculator is a Web-based application that allows roofing professionals to construct virtual roof assemblies to evaluate thermal efficiency and estimate energy costs via comparison with other roof assemblies absent other building envelope components. The calculator also can be used to determine minimum thermal insulation requirements established by ASHRAE Standard 90.1, "Energy Standard for Buildings Except Low-Rise Residential Buildings."

However, EnergyWise Roof Calculator Online has some limitations. You should be aware of them so you don't overestimate a roof assembly's energy savings and subsequent cost savings for building owners.

Limitations

EnergyWise Roof Calculator Online is intended to compare an existing roof assembly with a proposed new roof assembly or to compare multiple proposed new roof assemblies. The calculator examines energy costs related to roof assemblies only; it does not include walls, windows, or internal building components and operations.

For example, a 10,000-square-foot roof area on a one-story building will have a large roof-to-wall ratio. On the other hand, a 10,000-square-foot roof area on a five-story building will have a much smaller roof-to-wall ratio. If you were to use EnergyWise Roof Calculator Online to analyze the roof assemblies on these buildings, the calculator's output would be the same. However, in reality, installing identical roof assemblies on the two buildings would provide less energy savings for the five-story building because of its smaller roof-to-wall ratio.

To estimate possible energy savings, a whole-building analysis can be performed; however, this likely will not be done when reroofing is not part of a larger building rehabilitation project. EnergyWise Roof Calculator Online can be a component of a whole-building energy analysis, or it can be used on its own.

Sample analysis

Suppose you're replacing an existing roof system on a two-story building with a 7,500-square-foot roof area over a steel deck. The existing roof system consists of 1-inch-thick polyisocyanurate insulation with an adhered EPDM membrane and has an approximate R-value of 6. A replacement roof system consisting of two layers of 2-inch-thick polyiso­cyanurate, 1/2-inch-thick gypsum cover board and an adhered PVC membrane and having an approximate R-value of 25 is proposed. How much energy savings should the building owner expect with the new roof assembly?

After you input the building's information, the existing and new roof assemblies' specific information, and energy costs into EnergyWise Roof Calculator Online, a report is generated that provides annual heating and cooling costs for each roof assembly. For this example, heating and cooling costs generated from the existing roof assembly would be about $3,850. Costs generated from the new roof assembly would be about $1,050.

Because EnergyWise Roof Calculator Online only addresses heating and cooling costs associated with roof assemblies, it is necessary to compare the ratio of the calculator's output with actual heating and cooling costs. Building owners should be able to tell you their buildings' approximate annual heating and cooling costs. For the example, $4,500 is the total annual heating and cooling cost.

From this you can begin to determine the approximate heating and cooling costs associated with a new roof assembly. For example, consider 1,050/3,850 = X/4,500. X represents the approximate heating and cooling costs associated with the new roof assembly. In this example, X is $1,227, which equals a savings of $3,273.

Finally, adjust the approximate savings to the building's roof-to-wall ratio.

Know the program

I encourage you to take the time to familiarize yourself with the parameters of EnergyWise Roof Calculator Online so you can approximate buildings' energy savings. You can access EnergyWise Roof Calculator Online at energywise.specright.net.

James R. Kirby, AIA, is NRCA's associate executive director of technical communications.