Cool Roof Calculator Input

Here's some help for entering values in the DOE Cool Roof Calculator, either the CoolCalcEnergy or the CoolCalcPeak versions

There are 243 different locations built into the pull-down lists in the calculator, 235 for the US, Pacific territories and Puerto Rico and 8 Canadian cities. If you do not find your exact location listed, use one with weather similar to yours. This location may not be in the same state as yours. The city selector is keyed off the state selector. If you try to select a new city in the same state after running the calculator, you may see a blank city list. Temporarily select a different state and view its cities. Then select the original state and the city you want.

After you select your location on the calculator, you need to tell it some things about your low-slope roof and the solar radiation control you are considering. Solar radiation control entails either coating your existing black roof or replacing the black membrane with a non-black one.

The first roof descriptor is the R-value of your roof in U.S. units. The R-value gives the total thermal resistance of the roof. Most of the R-value in a low-slope roof is due to conventional insulation. A metal deck and the waterproofing membrane, even if its outside surface does solar radiation control, do not add any significant amount of R-value. Underneath many low-slope roofs is a simple plenum space and a dropped ceiling. This plenum space and dropped ceiling add a small amount of resistance, so if you have such an arrangement, you can increase the roof R-value by about 3. Here are some examples to help you pick your R-value (remember to add 3 if you have a simple plenum space with a dropped ceiling). If you have a very simple roof made from 1.5 in. of wood fiberboard and no other insulation, you'd have an R-value of about 5. If, instead of 1.5 in. of wood fiberboard, you have 2 in. of foam insulation, you'd have an R-value of about 13. If you have 4 in. of foam insulation, the R-value should be about 25. And with 5 in. of foam, the R-value should be about 32.

Simple plenum spaces can include well-insulated supply and return air ducts for the building conditioning system. More complicated situations are not allowed because of interactions between the duct system and the plenum air. The basic assumption of the calculator is that the heat flow through the roof deck is the only thing that directly affects the roof's share of the load on the building conditioning system.

After you've told the calculator about your roof's R-value, you need to describe the proposed solar radiation control system. Table 1 provides seven surface descriptions and combinations of solar reflectance and infrared emittance that were obtained after two years of weathering in the research done to support this calculator. An average white coating and an average aluminum coating are added. These data should help you decide appropriate solar reflectances and infrared emittances for the type of solar radiation control you are investigating. Coatings on existing black roofs and replacement membranes for solar radiation control have the same range of solar reflectances and infrared emittances. New membranes tend not to weather as rapidly as coatings. You can easily run the calculator with different values of the solar reflectance to see how annual savings change due to weathering. Print out results of each run.

Table 1. Solar reflectances (SR) and infrared emittances (IE) for surfaces weathered two years.

Once you've chosen the location and described your roof, economic factors are needed. For energy costs and equipment efficiencies, you need to tell the calculator your average price of electricity for the cooling season (in U.S.$ per kilowatt-hour), average coefficient of performance of the air conditioning system over the cooling season, and whether you heat by electricity or by burning a fuel (choose one or the other). If you heat with electricity, then you should input the price of electricity for heating (in U.S.$ per kilowatt-hour). If you use a fuel, input the price of that fuel (in U.S.$ per Therm). A Therm is 100,000 Btu of heating value. Natural gas prices are typically given in $/Therm or in $/MCF. If your prices are shown in $/MCF, divide by 10 to get the $/Therm. For example, natural gas with a price of $6.50 per MCF costs $0.65 per Therm. Heating oil prices are usually given in $/gal. Multiply this number by 0.71 to get $/Therm. For example, heating oil at $1.20/gal would be $0.85/Therm. You also need to input the seasonal average efficiency of the heating equipment.

For detailed information on state-by-state energy costs, see the Energy Information Administration websites. Local energy prices may be significantly different from statewide averages so the best source of information is local rates. If necessary, please consult the EIA websites before using the calculator to make a decision about appropriate energy prices for your location. Go to www.eia.doe.gov and choose appropriate locations on the website. For example, a spreadsheet of electricity prices in 2003 by state for the commercial sector can be found here. For heating oil through the 2004/2005 heating season by region and state, PDF document is available here. For wholesale prices use this document for residential prices. For annual prices through 2004 for natural gas in various sectors (select the state of interest from the pull down menu), the locator is found here.

As described above, the monetary unit displayed in the calculator is U.S. $. If you consistently enter all costs in another monetary unit, the results from the calculator will be correct values in this unit. Simply interpret the $ sign in the output as your monetary unit. Internal conversions are made among energy units, however, so they must be what the input requires.

Average seasonal efficiency of cooling equipment and heating equipment depends upon the kind of equipment and its condition. Electric air conditioning in the U.S. is often rated by a seasonal energy efficiency ratio (SEER). Typical values for old equipment are 7 to 10 Btu/Watt-hour. New, very efficient equipment may have SEER values as high as 16 Btu/Watt-hour. Coefficient of performance is used in the calculator and is a fraction without units formed by dividing SEER by 3.4. Thus, old equipment has COPs from 2 to 3 while new, very efficient equipment may have a COP as high as 4.7 on average over the cooling season.

Heating equipment seasonal efficiency is entered as a fraction in the calculator. For fuel-burning equipment the fraction is always less than 1.0, ranging from 0.5 to 0.6 for old, inefficient equipment to 0.8 to 0.9 for typical new equipment. Condensing, natural gas furnaces may have seasonal heating efficiencies as high as 0.95. Electric resistance heating converts electricity directly into heat with an efficiency of 1.0. A better use of electricity for heating is to run an electric heat pump. Electric heat pumps in the U.S. are often rated by a seasonal heating performance factor (SHPF). It has the same units as SEER. Coefficient of performance is used in the calculator and is a fraction without units. For air-to-air heat pumps, a typical value is 1.5 with a value as high as 2.0 in mild climates.

If you have a large facility that pays a demand charge for electricity during the cooling season, a roof with solar radiation control can also cut down on the peak heat load relative to a black roof. The electricity supplier typically charges for your actual peak electricity demand during each month of the cooling season. The facility electricity meter measures the peak demand (the highest 15 minute average amount of kilowatts demanded during the month) and the supplier multiplies by a demand charge rate in $/kW. The resulting demand charge appears on the utility bill each month of the cooling season. The calculator estimates the reduction in peak electricity demand due to the proposed roof by dividing the reduction in heat flow during each month of the cooling season by air conditioner efficiency. The average efficiency you entered for energy savings is used. Because peak conditions are more severe than average conditions, air conditioner efficiency at peak cooling is actually slightly less than at average conditions. A conservative estimate of demand savings is obtained by using the average efficiency.

You don't need to tell the calculator about the building's indoor temperature. It works OK for buildings between 67 and 78°F (19 and 26°C). It's even OK if you keep the thermostat at different set points in the summer and winter as long as the thermostat settings are the same with and without solar radiation control.

The net savings is given in $ per square foot of roof area per year. If you want $ per square yard, multiply the $/ft² answer by 9. If you want $ per square meter, multiply it by 10.76.

This calculator directly shows you two ways to improve your roof to save the same amount of costs for energy. The first way is to install solar radiation control and that answer is shown in red on the calculator. The components of the total savings are listed and add up to the total and include demand savings, if applicable. The second way is to add more insulation to your roof. The calculator shows how much insulation you'd have to add to save the same amount of money for energy as you'd save if you installed the solar radiation control. See the blue line on the calculator for this information. Peak demand savings are not included in this line because more insulation under a black roof does not yield the same savings in demand as solar radiation control.

A third comparison can be obtained on the basis of energy savings only, not money savings. Make an extra run of the calculator in which you set the COP for cooling equal to 1.0. Choose electricity for heating and set its cost equal to that for summertime electricity. Set the heating efficiency equal to 1.0. Then calculate annual savings again. The new R-value for the black roof will be that required for energy use equal to that of the proposed roof with solar radiation control. Of course, with this comparison the money savings will not be realistic because they do not reflect your equipment efficiences and energy prices.

The rest of the output from the calculator is listed under Details. The heating and cooling degree-days and average solar irradiation are listed for the location that you chose. The calculator was developed from typical hour-by-hour annual weather data for each location. The heating and cooling loads calculated from these data are then listed for a black roof and your proposed roof with the same R-value. These loads are annual sums of the heat flow per square foot through the deck of the roofs under simple heating and cooling conditions. They are only for the roofs. Depending upon how you run your building, how its walls and windows are configured and how much internal load it has due to equipment and occupants, the actual heating and cooling loads will be very different from the roof loads. If you are using version CoolCalcPeak because you have savings in demand charges, the average heat load reduction is listed. It is the difference between a black roof and the proposed roof during the cooling season. This is a single number that came from the sum of the maximum heat load reductions each week during the cooling season divided by the number of weeks. This average heat load reduction is assumed to apply for as many months as the cooling season lasts. The maximum heat load reduction during the entire cooling season is also listed for your information to compare to the average heat load reduction.

Details on the development and validation of the scheme for estimated savings in energy charges due to solar radiation control on low-slope roofs are given in CoolCalcEnergyBackground. Similar details for the estimated savings in demand charges are given in CoolCalcPeakBackground.