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Q. I created a project for a warehouse located in Keene, NH. The total roof area is 20,000 square feet. The roof assembly consists of a metal deck, 2 inches of polyisocyanurate, ½ inch-thick wood fiberboard and a polymer-modified bitumen roof membrane. The building is cooled and heated with a gas-designed unit. I also used the default energy cost values. The total annual energy cost was calculated to be about $4600. Why is this number so low?

A. EnergyWise Roof Calculator Online examines energy costs only related to the roof assembly; it does not include walls, windows, or internal building components and operations. The calculator is not intended to be used as an accurate energy cost calculator. It is intended to compare an existing roof assembly with a proposed new roof assembly. Also keep in mind the results are only as good as the information you input. Remember to include items such as roof openings, ceiling plenums, appropriate fuel costs, etc.

Q. I am designing an office building in Dayton, Ohio, and the city's adopted energy code references ASHRAE 90.1-2004. I want to save my client some money by using less roof insulation. I am specifying a highly reflective roof membrane to use the exception to the minimum thermal insulation requirements. I created a roof assembly with a TPO membrane that has a total solar reflectance of 0.70 and minimum total thermal emmitance of 0.75. However, the required R-value did not get reduced.

A. In order to use the exception, a building has to be located in climate zones 1, 2 or 3 (these zones represent very warm climates). The city of Dayton is in Montgomery County which is in climate zone 5. Also keep in mind you cannot use the exception with roof assemblies over ventilated attics or semiheated spaces.

Q. I would like to use the rate my customer pays for natural gas. I have a copy of the bill, but the units are in CCF. EnergyWise Roof Calculator Online uses a heating energy cost of $/therm. How do I determine the right value?

A. A therm is a unit of heat energy and CCF represents 100 cubic feet. One therm equals 100 cubic feet. So you would input the rate as shown on the bill. Some gas companies use MCF which equals 1000 cubic feet. In this case, you would divide the rate by 10.