Online Tools to Predict Energy Savings
The Roof Savings Calculator is a simple and free online tool that allows users to calculate annual energy savings associated with choosing a cool roof instead of a dark roof. A portion of the interface is shown in Figure 7.
To use this tool, you will need to answer a few basic questions about your building and two proposed roofs. The results will show you how much energy savings you can expect to achieve by choosing a cool roof versus a dark- or less-cool roof, or by converting your existing roof to a cool roof. Several key factors affect the amount of energy you can save.
Climate has the biggest impact on energy savings. Cool roofs achieve the greatest cooling savings in hot climates (Climate Zones 1-3; see Figure 8) but can increase energy costs in colder climates due to reduced beneficial winter time heat gains. The DOE Cool Roof Calculator takes into account your local climate when you select the state and a city nearest your building.
Solar Reflectance and Thermal Emittance values of the roof surface help determine its temperature. Most roofs are not washed frequently, so we recommend using aged values to predict energy savings. If aged values for your roof are unknown, you can estimate the aged solar reflectance based on the initial solar reflectance by using this formula:
Aged Reflectance = 0.7 x (Initial Reflectance – 0.2) + 0.2
Some materials retain solar reflectance better than others, so use measured aged values whenever possible. Using initial values for thermal emittance is fine, since these values tend to change less over time.
Insulation reduces heat flow through the roof. A well-insulated roof will benefit less from a cool roof than a poorly insulated roof. Recent energy codes recommend R-20 insulation for nonresidential buildings in all U.S. climate zones, except for Climate Zone 1 (which is R-15). If you don’t know your roof’s insulation, you can use R-20 for a conservative savings estimate if your building was constructed in the past 15 years.
Utility Rates & Fuel Types vary by location and by customer. This tool allows you to specify what type of fuel you use for heating and cooling, as well as the price you pay. By clicking on the ? buttons, you can find recent average utility rates for your state.
Equipment Efficiency affects savings as well. Buildings with less efficient HVAC equipment will save more energy with cool roofs. If you plan to replace your inefficient HVAC equipment soon after replacing the roof, be sure to input the updated efficiency values.
Results are obtained after entering the above information and clicking the “Calculate” button. A calculation routine will predict your annual energy cost savings per square foot of roof area. Cooling Savings and Heating Savings (penalty) are presented, along with Net Energy Savings (penalty). Heating penalties are usually smaller than cooling savings, so most locations will yield a net energy cost savings for cool roofs.
Hypothetical Example 1: Roof Coating Comparison
Suppose your building has a 20,000 ft2 roof that is in reasonable condition, and you want to coat the roof to make it last five years longer. Your roofing contractor gives you two proposals: one for a dark coating and one for a cool coating. If the cool coating costs $0.10 more per square foot, is it worth it to spend the extra $2,000 ($0.10/ft2 x 20,000ft2) to have a cool roof?
- Begin by using the Roof Savings Calculator to compare the two roofs. Suppose you find that the cool roof lowers the cooling bill by $0.06/ft2 and increases the heating bill by $0.02/ft2. On average, you will save $0.04/ft2 (= $0.06/ft2 – $0.02/ft2) on energy.
- Next, calculate annual energy cost savings by multiplying the net savings by the roof area (20,000ft2). Each year you would save $800 on energy costs (= $1,200 cooling – $400 extra for heating). Since the coating lasts five years, you would expect to save $4,000 (= $800/yr x 5 yr) on energy over the life of the coating.
- Finally, to find the simple net savings, subtract the premium (if any) for the cool roof. In this case, the cool roof was $2,000 more, so the net savings would be $2,000 = (= $4,000 saved – $2,000 extra). In this case, the cool roof looks like the better investment.
Note: even though the cool roof saves money, the net savings would only cover a small fraction of the total coating installation.
Hypothetical Example 2: Roof Re-cover or Replacement
Suppose that the same roof has come to the end of its useful life and you must replace it. You are evaluating a dark and a white membrane, each with a 15-year warranty. As before, the cool option costs $0.10/ft2 more than the dark membrane, and the cooling savings and heating penalty are the same as Example 1. This time, the predicted simple net savings ($10,000) are much larger because the new roof lasts three times longer than the coating in Example 1.
