Effect of Solar Radiation Control on Energy Costs
How does solar radiation affect the temperature of a roof surface and heat flow through the roof?
This solar radiation control fact sheet focuses on low-slope roofs for commercial buildings. The membranes or weatherproofing materials used for these roofs are opaque to solar radiation. When opaque roof surfaces are exposed to solar radiation, no solar radiation is directly transmitted through the roof. The sketch above in the heading depicts an opaque low-slope roof interacting with solar radiation. It illustrates that some of the sunlight is reflected away by the surface and the rest is absorbed. The fraction reflected is given by the solar reflectance of the surface, a number between 0 and 1 that applies to the solar part of the electromagnetic spectrum. Solar radiation includes the wavelength range from near ultraviolet to near infrared, spanning what is visible to the human eye. The sketch assumes a solar reflectance of about 0.85.
The solar radiation that is absorbed heats the surface. The absorbed energy is no longer solar energy. It is characterized by the temperature of the surface material, which is much lower than the equivalent solar temperature. Consequently, the surface emits radiation in the far infrared part of the spectrum. This infrared radiation is not to be confused with the reflected solar radiation. The amount emitted is in direct proportion to the surface's infrared emittance, a number between 0 and 1 that is generally different from the solar reflectance. A roof surface also exchanges energy by convection with the air above the roof and by conduction with the layer of the roof directly below the surface. Moisture effects, mainly evaporation and condensation of liquid water on the surface, are occasionally important for low-slope roofs.
The temperature of the roof surface is determined by a balance among energy gains and losses, including energy stored in the roof. The peak surface temperature strongly depends on the peak solar radiation and the solar reflectance of the surface. On a sunny day in late June, all of July or early August in the northern hemisphere, a black roof surface (solar reflectance less than 0.1) may reach peak temperatures exceeding 170°F (77°C). At the same time, a highly reflective white surface (solar reflectance greater than 0.8) could be less than 110°F (43°C).
For steady conditions, heat flow per unit area through a roof is given by the quotient of the temperature difference across the roof and the total thermal resistance of the roof. On a cloud-free day in the middle of summer for typical inside surface temperatures of 80°F (27°C), solar radiation control causes a decrease in the temperature difference across the roof from about 90°F to 30°F (from about 50°C to 17°C). If the total thermal resistance (R-value) of the materials present in the roof is small, a significant decrease in steady heat flux through the roof will occur. If the roof has significant thermal capacitance (thermal mass), absorbed energy will be stored in the roof and heat fluxes through the roof will be delayed and diminished, lessening the effect of solar radiation control. Cloudiness and other non-steady conditions also decrease the heat flux through the roof relative to the maximum steady value.
What affects the values of the solar reflectance and the infrared emittance?
The solar reflectance and infrared emittance of a surface are both dependent upon the kind of material that forms the surface and the condition of the surface. The effect of weathering is significant for solar reflectance. A typical white coating with initial solar reflectance exceeding 0.8 will likely have solar reflectance below 0.55 after a few years of exposure, if the surface is unprotected from airborne dust and contaminants. A typical aluminum coating with initial solar reflectance of about 0.6 will likely have solar reflectance about 0.4 after a few years of exposure. Typical white coatings seem to retain infrared emittances greater than 0.8 despite changes in solar reflectance. Typical aluminum coatings have infrared emittances from 0.3 to 0.5 when new and the infrared emittances increase to values from 0.5 to 0.7 due to weathering. These values of solar reflectance and infrared emittance are averages over the variations in surface temperature that roof surfaces undergo due to daily and seasonal climate changes.
Surface contamination and alterations cause the changes in radiation properties. Together they comprise what is called weathering. Contamination occurs over time due to atmospheric pollution and biological growth. Alterations occur due to many factors including ultraviolet radiation, temperature cycling due to sunlight, sudden temperature swings due to rain, moisture penetration, condensation and evaporation of dew, wind, freezing and thawing and effects of sleet, snow and hail. Rain and deliberate washing may temporarily help restore high solar reflectance but not to initial levels.
Our experience with the entire range of commercially available coating materials in a three-year outdoor test in East Tennessee indicates that thorough washing of fully-weathered white coatings with a solution of trisodium phosphate in water restores about 40% of the average 0.27 decrease in solar reflectance due to weathering. Thorough washing of fully-weathered aluminum coatings restores about 55% of the average 0.20 decrease in solar reflectance due to weathering. Washing did not affect infrared emittance of white coatings but appeared to restore the initial infrared emittance of aluminum coatings. After the surfaces were washed, they again resumed weathering. In the half year we were able to observe this continuation of weathering for all the surfaces, the solar reflectances of the white coatings decreased about 0.03. The solar reflectances of the aluminum coatings decreased about 0.02. For four white coatings and three aluminum coatings that we were able to continue to observe for three years after cleaning, the solar reflectances decreased a total of 0.11 from cleaned values. These decreases are about what was restored by cleaning after the original three years of weathering.
Because of the dependence on surface condition, solar reflectance and infrared emittance are not properties of the surface material alone. Values for solar reflectance and infrared emittance need to be qualified by descriptions of the material and its condition. For example, a clean metal surface has a high solar reflectance and a low infrared emittance. An oxidized or rusty metal surface would likely have a lower solar reflectance and a higher infrared emittance. Various combinations of high to low solar reflectances and high to low infrared emittances are possible with different surface materials and conditions. The values are not related.
What is solar radiation control?
Solar radiation control for low-slope roofs follows from use of surface materials that have high reflectance in the solar part of the electromagnetic spectrum and high emittance in the infrared part of the spectrum. High means 0.75 or more on a scale from 0 to 1. Such materials are known as 'cool materials.' See http://EETD.LBL.gov/HeatIsland/CoolRoofs/ for a comprehensive discussion, by researchers in the Heat Island Group at Lawrence Berkeley National Laboratory, of the importance of both solar reflectance and infrared emittance and techniques to measure them. Since the amounts of solar energy absorbed and infrared energy reemitted by low-slope roofs are linearly proportional to solar reflectance and infrared emittance, respectively, materials with solar reflectances and infrared emittances less than 0.75 are able to do some solar radiation control.
The objective of solar radiation control is to decrease the cooling load on a building. For commercial buildings, the high intensity of summertime direct solar radiation on horizontal surfaces and the large area of low-slope roofs makes these roofs the primary target for solar radiation control. High solar reflectance for the roof surface causes much of the solar radiation to be reflected away before it can affect the energy balance for the roof. High infrared emittance enhances the ability of the roof to radiate some of the absorbed solar energy and energy from inside the building to the sky, which is helpful during the cooling season. Especially on clear nights, the equivalent sky temperature is much lower than the roof temperature. It is common for surfaces with high infrared emittances to be 5°F to 10°F (3°C to 6°C) cooler than the outside air temperature on clear nights. Surfaces with low infrared emittances can be that much warmer than the outside air temperature, which can help to decrease the heating load on a building during the heating season.
Is solar radiation control recommended for all low-slope roofs?
With current methods to achieve solar radiation control on low-slope roofs, solar radiation control is a passive technology. It works night and day, all year round, except during rainy periods or when the roof is covered by dew. A layer of water has high infrared emittance. This dominates the nighttime behavior of a dew-covered roof. Even if air temperatures are several degrees above freezing, thin layers of water exposed to the night sky will freeze on clear nights. Until solar energy evaporates ponds or dew from a roof, the roof temperature remains near the ambient air temperature. In effect, therefore, even though it complicates the energy effects, water on a roof enhances solar radiation control.
Since solar radiation control cuts down on the amount of solar radiation absorbed by a roof, there is less heat gain during sunny periods through a roof with solar radiation control than without it. This heat gain may be desirable during the heating season. The diminution of heat gain during the heating season by solar radiation control is commonly referred to as a heating penalty. Many commercial buildings are dominated by the internal loads due to equipment and people. Hence, solar radiation control is not necessarily undesirable even in climates with a large number of heating degree-days. Heating degree-days are a common measure of the potential for conduction heat losses through the building envelope. They do not indicate how large the heat gains from internal sources are relative to the heat losses through the building envelope. That needs to be determined on a case-by-case basis. Only heat losses through the building envelope are proportional to the heating degree-days.
How is solar radiation control achieved if it is desired?
Solar radiation control can be implemented during construction of a roof by selection of a membrane material with the desired characteristics of high solar reflectance and high infrared emittance. Since traditional built-up roofs are constructed by mopping down layers of roofing felts with asphaltic materials, they have radiation properties typical of asphaltic materials: very low solar reflectances (less than 0.1) and high infrared emittances (greater than 0.8). A layer of gravel on top of a built-up roof may have slightly higher solar reflectance and does add thermal mass, but it does not qualify the roof to be termed a roof with solar radiation control. Low-slope roof materials with composition like light-colored shingles for high-slope roofs (asphaltic materials with small light-colored granules embedded in the surface) have solar reflectance no higher than 0.25.
Single-ply roofing membranes are available with high solar reflectances and high infrared emittances. They can be installed with the same techniques as used for traditional black single-ply roofs made from ethylene propylene diene monomer (EPDM) or atactic polypropylene polymer (APP)-modified bitumen. Mechanical fasteners can be used to attach the roof insulation and the solar radiation control membrane to the roof deck in a sufficient number of spots to meet requirements for resistance to wind uplift and other structural requirements. Alternately, the insulation can be attached to the roof deck by fasteners or other means and the membrane can be adhered to the insulation with a suitable adhesive.
Applying a white acrylic coating with an airless sprayer.
After construction of a traditional black roof, solar radiation control can still be achieved. Liquid coatings can be sprayed, brushed or rolled on to the membrane. They dry to form a surface with radiation properties independent of those of the substrate. True solar radiation control coatings, with initial out-of-the-can solar reflectances greater than 0.75 and infrared emittances greater than 0.75, are generally white, water-based latex or acrylic products with titanium dioxide added to achieve high solar reflectance. The membrane needs to weather several weeks and/or special base coats must be applied to form a good bond between the coating and the roof.
Other coating materials and capsheets are available with initial solar reflectances generally less than 0.75 and infrared emittances anywhere from low to high values depending upon the material. Many have aluminum or other metal particles added to enhance the solar reflectance. A good bond between the coating and the substrate is also very important for these coatings and some weathering of the substrate and/or special base coats are needed to achieve it. Capsheets are a layer of bare or coated metal which is factory-applied to a substrate of asphaltic single-ply membrane material. The substrate with capsheet attached can be torch-adhered or adhesive-adhered to a fresh or weathered asphaltic roof membrane.
Effect of Solar Radiation Control on Energy Costs
This section introduces an interactive tool to assist commercial building owners and/or operators in estimating the maximum effect of solar radiation control on energy needs for a building under a low-slope roof. To generate data for the equations used in the estimating tool, a computer model was run over a wide range of climates for various roofs. No other part of the building envelope was modeled. The climates were represented by typical meteorological year data from the Renewable Resource Data Center, with web site http://rredc.nrel.gov/solar/old_data/nsrdb/tmy2/. The following locations were used to estimate energy savings (CoolCalcEnergy version). Their climates go from cooling-dominated to heating-dominated: Phoenix, Arizona; Miami, Florida; Tampa, Florida; Dallas/Fort Worth, Texas; Knoxville, Tennessee; Boulder, Colorado; Minneapolis, Minnesota and Anchorage, Alaska. To estimate peak demand savings (CoolCalcPeak version), instead of Anchorage, Alaska, Seattle, Washington, and Quillayute, Washington, were used along with the other climates. The low-slope roofs had light weight decks and insulation levels of R-4.8, R-12.6, R-25.2 and R-31.5 h·ft²·°F/Btu (R-0.9, R-2.2, R-4.4 and R-5.5 m²·K/W). Seven combinations of solar spectrum reflectance and infrared emittance were modeled. They corresponded to the range for different surfaces of measured values after two years in a comprehensive study of the thermal performance of twenty-four different coatings and four uncoated specimens.
The interactive estimating tool does not apply to high-slope roofs with a ventilated attic space between the roof deck and the attic floor. Solar insolation is slightly different than it is for a horizontal roof. More importantly, the ventilated attic space affects the conditions for heat transfer through the attic insulation. An estimating tool has been developed at the Oak Ridge National Laboratory for steep-slope roofs with duct-free attic spaces. It is on our web site as the DOE Steep-Slope Calculator. For radiant barriers in attics, see the discussion on our web site at web.ornl.gov/sci/roofs+walls/radiant/rb_01.html. For the results of recent measurements of whole house cooling energy use in Florida with various high-slope roofs on identical houses see an article on the Florida Solar Energy Center web site.
The interactive estimating tool for low-slope roofs allows for simple plenum spaces above drop ceilings. The small thermal resistance that they add must be figured in the thermal resistance of the roof assembly . Simple plenum spaces can include well-insulated supply and return air ducts for the building conditioning system. More complicated situations are not allowed because interactions between the duct system and the plenum air are not included. The basic assumption of the estimating tool is that the heat flow through the roof deck directly affects the load on the building conditioning system.
For the estimating tool, cooling load caused by a unit area of the roof on the building cooling equipment was defined as the annual sum of heat flow through the roof deck when outside air temperature exceeded 75°F (24°C). Differences between cooling load for an uncoated roof and a coated roof were searched for maxima each week of the cooling season to determine savings in peak demand due to solar radiation control. Heating load caused by a unit area of the roof on the building heating equipment was defined as the annual sum of heat flow through the roof deck when outside air temperature was below 60°F (16°C). Inside air temperature below the roofs was held at 72.5°F (22.5°C) and no thermostat setup or setback was modeled. Judging from results with the model of a warehouse in an annual energy use estimator for whole buildings, including severe thermostat setup and setback during unoccupied hours, the loads used for the estimating tool show the maximum amount of energy savings from solar radiation control on roofs of buildings with small internal loads.
Inside air temperatures ±5°F (±3°C) different from 72.5°F (22.5°C) are allowed because the estimating tool does calculations for a roof with solar radiation control relative to one without solar radiation control. As long as the inside air temperature is the same with and without solar radiation control, the estimates with the tool are valid. This includes effects of different thermostat set points during the heating and cooling seasons.
In the estimating tool, polynomials are used to fit the results from the roof-only model. This permits users of the estimating tool to input parameters of interest that are not exactly the values used in the model. The cooling loads are fit as a function of solar reflectance and infrared emittance of the roof surface, roof thermal resistance, and cooling degree days and average daily solar irradiation for the location. The heating loads are fit as a function of solar reflectance and infrared emittance of the roof surface, roof thermal resistance, and heating degree days for the location. Savings in peak demand are fit as a function of solar reflectance and infrared emittance of the roof surface, roof thermal resistance, and average daily solar irradiation for the location.
A link to an InputHelp file is given in both the CoolCalcEnergy and CoolCalcPeak versions of the estimator. It is to help you prepare the input for your situation, especially the appropriate fuel costs for your location. If you would like an estimate of the annual energy cost savings (plus demand cost savings, if applicable) with solar radiation control or the amount of conventional insulation in a roof without solar radiation control for the same annual energy cost savings, use the Cool Energy Estimator.
What besides energy savings justifies solar radiation control for low-slope roofs?
The savings in annual energy costs, demand costs (if applicable) and other cost savings need to be weighed against the local cost to purchase and install the coating or membrane system. The focus in this fact sheet is on protective coatings or membranes with reflective materials (such as aluminum or titanium dioxide) that are added to save on energy needed to cool buildings under the protected roofs. The other cost savings associated with the protective nature of the coating or membrane system may be more significant than the savings in annual energy costs, especially outside a cooling-dominated climate.
Protective coatings have a long history of use before air-conditioned buildings with cooling energy requirements became commonplace. See the Internet web site of the Roof Coatings Manufacturers Association http://www.roofcoatings.org/ for a brief history of protective coatings and their evolution to the variety of products available today. These products range from low viscosity, non-fibered coatings for penetrating primers and damp proofing coatings to high viscosity, heavy-bodied cements for adhering waterproofing membranes and components and for patching and repairing leaks. In the middle of this range are medium viscosity, fibered and non-fibered products for interply adhesives and top coatings.
Solar radiation control coatings fall in this medium viscosity category. Protective coatings, in general, use resins ranging from bituminous (such as asphalt) to polymeric (such as acrylic) with reinforcing fillers and a carrier solvent or water emulsification. Reflective pigments are added for solar radiation control. The coating is left behind as a cured, water resistant film when the carrier solvent or water evaporates.
An important purpose of the coating is to protect the membrane it covers from effects of the ultraviolet (UV) radiation in sunlight. UV degrades asphaltic and some rubber materials, causing them to become brittle and unable to flex during thermal stressing. Ultimately this leads to cracks in the membrane itself or its failure to adhere to flashings. Coatings with optional reflective pigments also protect the membrane from high peak temperatures, lessening the thermal stresses.
Solar radiation control offers the most savings when little or no thermal insulation is present in the roof. Even with solar radiation control, thermal insulation is needed to decrease heating loads and help to decrease cooling loads. Solar radiation control does not help to decrease heating loads. Often there is a heating penalty associated with its use. The cost of a roof, especially with moderate to high levels of thermal insulation, is a significant investment. Any projected net savings from solar radiation control helps to justify its installation in addition to insulation.
Often the only cure for water leaks into the building interior is extensive repair or replacement of the membrane. Water held in many types of roof insulation makes them ineffective as thermal insulation. Complete replacement of a damaged roof and disposal of wet insulation is more expensive than construction of a new roof with the same specifications. Even if energy savings due to decreased cooling loads are minimal, protective coatings have the potential to prolong the life of the membrane and add barriers or repair existing barriers against water leakage into the roof system. In this context, energy cost savings can be viewed as a bonus in addition to the other benefits of solar radiation control.
How is solar radiation control installed?
If solar radiation control is achieved by replacing the existing roof membrane by a new membrane with solar radiation control characteristics, roofing industry-accepted low-slope roof installation procedures are followed. An excellent guide to specifications and construction details for low-slope roof installation is the NRCA Roofing and Waterproofing Manual, Fifth Edition, Volumes 1 and 2, published in 2001 by the National Roofing Contractors Association, Rosemont, Illinois. See also the NRCA website for low-slope roofing on the Internet: http://nrca.net/technical/lowslope/.
Cleanliness is a special concern during installation of new membranes with solar radiation control characteristics. These membranes are light-colored and they are easily and permanently soiled by asphaltic materials. Foot traffic that tracks such materials from other areas on the roof must be avoided. Materials used to adhere insulation before the new membrane is installed may also have the potential to stain the top of the new membrane. If so, they must dry thoroughly before the new membrane is laid out on the roof. Besides being unsightly on a light-colored roof, every spot of asphaltic material lowers the solar reflectance of the affected area to less than 0.1.
Applying a white latex elastomeric coating with rollers.
Coating an existing roof to achieve solar radiation control is a less complicated procedure. For any coating process, the key to success is thorough cleaning and drying of the substrate. Roof coatings are especially formulated to adhere to a variety of roof membranes. They are not the same as paint. The final coating thickness for white latex or acrylic coatings is usually about 0.015 in. (0.38 mm) and is less for many aluminum coatings. Only minor surface defects can be bridged without compromising the integrity of the coating. Defects in the coating are potential sites for cracking and peeling. Many coating products are intended to seal the area they cover against leaks in the original membrane. Coating defects compromise this function before cracking or peeling becomes apparent.
The large area of most low-slope roofs rules out brush-coating as a means to apply the coating. Even so, brushing is often used near flashings and edges to precoat for additional sealing. On large areas of a smooth-surfaced roof, roller application of coatings is possible. A typical roller application is shown in Figure 1. On a rough-surfaced roof or for the most uniform application on any substrate, spray application of coatings is done. See Figure 2 where an airless sprayer is being used to apply a white coating to a gravel-topped built-up roof.