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.