This information was produced by The Building Envelopes Program at Oak Ridge National Laboratory (ORNL) is a program within the Buildings Technology Center (BTC), the premier U.S. research facility devoted to developing technologies that improve the energy efficiency and environmental compatibility of residential and commercial buildings.
How does solar radiation affect the temperature of a roof surface and heat flow through the roof?
This 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 in the heading for this fact sheet 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 which 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), 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 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 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, the solar reflectance of the white coatings decreased about 0.03. The solar reflectance of the aluminum coatings decreased about 0.02.
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 reflectance 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 which 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 the comprehensive research conducted by the Heat Island Group at Lawrence Berkeley National Laboratory, on 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 reflectance and infrared emittances less than 0.75 are able to do some 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 radiation control recommended for all low-slope roofs?
With current methods to achieve radiation control on low-slope roofs, 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 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 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, 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 reflectance (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 reflectance 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 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.
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 radiation control coatings, with initial out-of-the-can solar reflectance 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 cap sheets are available with initial solar reflectance generally less than 0.75 and infrared emittances anywhere from low to high values depending upon the material. Many have aluminum 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. Cap sheets are a layer of bare or coated metal which is factory-applied to a substrate of asphaltic single-ply membrane material. The substrate with cap sheet attached can be torch-adhered or adhesive-adhered to a fresh or weathered asphaltic roof membrane.
