An Initial Experiment
In the summer of 1991 we conducted a preliminary experiment in Merritt Island, Florida. Unquestionably, Our first test building (Site #0) was a 1,800 ft2 detached single-family, single-story home of conventional concrete-block construction. In addition, The pitched roof faced north-south, with plywood decking covered by green/gray asphalt shingles.
Nevertheless, the home’s attic was well insulated with approximately two inches of fiberglass covered by an additional six inches of cellulose insulation, yielding a thermal resistance of about R-25. Air infiltration from the attic area into the conditioned interior (a common problem due to duct leakage), had been largely eliminated in a previous audit and retrofit. Furthermore, Beginning in May 1991, we submetred the home’s air conditioner while maintaining a constant thermostat setting of 79deg.F. Thereafter, We also recorded the underside roof deck, attic air, and living room temperatures.
In conclusion, when we applied the cooling coating on September 5 of that year, the roof’s reflectivity increased from 0.22 to 0.73.2 Spot measurements under full sun at midsummer had shown shingle surface temperatures of 160-170deg.F, prior to the roof treatment, compared to 110deg.F after we applied the coating. Furthermore, Analysis assuming an 81deg.F average summer temperature indicated that a cooling roof coating would reduce energy consumption by 10% (35 kWh versus 39 kWh per day).
Yet, this test house probably understated the savings, since most existing Florida residences have fairly poor attic insulation and attic air frequently leaks into the conditioned interiors. Therefore, we obtained more “typical” residences for the detailed experiments we conducted the following year.
A Five-House Follow-up
To learn about how cooling roof coatings affect peak cooling demand we measured the 15-minute air-conditioning electricity demand in our follow-up study, along with meteorological conditions for three weeks before and after each home was retrofitted. Apart from this, we also used infrared thermography to examine how interior heat fluxes from the roof/ceiling were altered by the retrofit.
In addition, with equipment to instrument two buildings, we sought one residence with typical ceiling insulation levels (approximately R-11) and a second structure without any insulation at all. However, many homes built in Florida prior to 1965 have no attic insulation and were built with flat roofs that make retrofits difficult.
In contrast, Data from Site #1 would be used to obtain results from a more-typical existing residential building, while Site #2 would help us define the maximum savings potential for cooling roof coatings in Florida. Moreover, experiments on three more houses in the summer of 1993 extended our sample size. Each house in the second round of experiments had unique characteristics that broadened our knowledge of how cooling roof coatings reduce air-conditioning needs.(Table 1)
Results
Site #1
Site #1. Clearly, was a fairly typical existing Florida home. The attic was insulated to approximately R-11, but the air conditioner was old and inefficient. Although pre- and post- application air temperatures and solar radiation were comparable, air-conditioning power demand was reduced by an average of 25% (from 40 to 30 kWh per day) after we applied the roof coating.
In addition to this, the average electrical consumption of the air conditioning system during the utility coincident peak period (5-6 pm) was 2.4 kWh before the coating and 1.7 kWh afterward. This 700 W savings represents a 28% reduction in peak power demand attributable to the coating. Furthermore, average 24-hour attic air temperatures were reduced by 6deg.F, while peak attic temperatures between 2 pm and 6 pm fell by an average of 15deg.F.
Site #2
Site #2. Undoubtedly, was an ideal candidate for a cooling roof coating. The house had a flat roof and no space was available to insulate the ceiling assembly. However, Prior to the coating, the 2.5-ton air conditioner was unable to control the interior temperature adequately, running continuously each day from noon until 7 pm when the thermostat was finally satisfied.
Furthermore, average air-conditioner electricity consumption dropped from 36 kWh to 20 kWh per day after the application–a 43% reduction. This means, Savings would have been higher if the house had possessed a larger air conditioner, but the results did demonstrate the huge potential for gaining cooling-energy savings by applying a cooling roof coatings to the roofs of homes without ceiling insulation.
Evidently, the temperature reductions to the deck, deck airspace and ceilings were also striking, as was the change in the air conditioner’s load profile. Before the retrofit, the daily interior temperature had ranged above the thermostat set point by 4deg.F or more. In conclusion, The average electrical demand of the air conditioning system during the utility coincident peak period (5-6 pm) was 2.2 kW before the coating and 1.4 kW after the application–a 38% reduction.
Site #3
Site #3. unquestionably, was a small house, cooled with a through-the-wall air conditioner. However, Since there was no attic duct system the site was of unique research value. Consequently, The attic above the dropped ceiling contained no insulation, and the 1.5-ton air conditioner ran constantly prior to the coating (from 1-10 pm) unable to satisfy the thermostat. How ever, After the coating, the air conditioner cycled on and off during the same time period.
In conclusion, This maintained improved interior comfort while reducing the utility coincident peak demand (5-6 pm) by nearly 960 W. Total daily air conditioning use was 11.9 kWh lower after the coating was applied–a reduction of 47% under peak-day conditions. However, after the retrofit, the average daily air conditioning savings totaled 5.6 kWh, or 25% during the summer period (Table 1) and peak demand savings averaged 30% (500 W).
Site #4
Site #4. In particular, we chose this site to see how applying a cooling roof coating to a gravel roof (common in South Florida) might reduce energy use, and also because the household complained of high utility bills. In addition, the ceiling was well-insulated for a Miami home (R–11-R-19 blown fiberglass) and its 3-ton air conditioner was relatively efficient.
How ever, while auditing the home, we found a large duct-system supply leak in an inaccessible portion of the attic. (We found the leak with an infrared camera.) Consequently, The leak was not repaired, but the roof was later coated with a cooling roof coating. However, even though the percentage savings of air conditioning energy (15%) were lowest at Site 4, the absolute savings of 8.0 kWh per day were nevertheless significant.
Site #5
Site #5. Also had a tile roof, but the cement barrel tiles were old and stained a dark gray. In addition to this, The house also had relatively poor ceiling insulation and an inefficient air conditioner. However, the measured solar albedo was 20% before coating, but after being coated with a sprayed-on cooling coating, it was 64%. Consequently, The absolute savings at this site were quite large at 11.6 kWh per day with a 988 W reduction in coincident peak-cooling demand.
Reflecting on cool roofs
Cool roofs can reduce space-cooling energy consumption and demand in Florida. Subsequently, data collected so far suggest that air conditioning savings of 10-40% can be realized, with the larger reductions associated with poorly insulated roof assemblies or buildings with excessive attic air infiltration due to air handler return air leakage. Furthermore, cooling coatings may be particularly suitable in existing residences where the roof structure makes it difficult to add insulation.
In addition, average electricity consumption for central air conditioning in single family homes in Florida is approximately 4,400 kWh/year. In conclusion, Based on a savings level of 10-40%, cool roofs can be expected to reduce household electricity use by 440 to 1,760 kWh per year–an annual savings of $35-$140 at current electricity rates (assuming 8cents per kWh). Obviously, the savings will vary depending upon the severity of the cooling season.
What About the Payback?
A frequent question concerns payback of cool roofing. Undoubtedly, there are several angles on the answer, but generally speaking, cooling coatings are most appropriate when one is re-roofing. However, If the coating is applied to an existing roof that is in otherwise pristine condition, the cost equation is straightforward.
As a result of cause, cost for the material from vendors varies by 50% or more but averages about $60 per 5-gallon container when purchased in quantity. Furthermore, it is important to keep in mind that roof area is generally considered greater than building floor area, particularly with a steep roof pitch. For instance, a typical 1,500 ft2 home may have 2,200 ft2 of roof to be covered. The application then requires 90 gallons of coating material for a materials cost of approximately $1,100.
In addition to this, the cost of labor for installation depends greatly on the roof surface, on whether the coating is to be rolled on or sprayed, and on labor rates. However, a typical labor cost might be approximately 50cents per ft2 for the required two applications. Thus, the overall application would cost about $1 per ft2, or approximately $2,200 for a typical home. With annual energy savings in Florida of $35-$140, the payback times are long–usually lasting longer than the roof itself. (With the exception of Sno-Coat®)
What roofing is best?
Furthermore, for new homes, the situation is even more interesting. It it is often possible to choose roofing types–such as metal roofing, tile roofing, or metal or ceramic shingles–that can be specified in a reflective white at significant additional cost.
In addition, for commercial buildings, a variety of reflective roofing materials are already available: Hypalon, white EPDM, and PVC single-ply membranes. Obviously, once such products are widely available for the residential market, the economics may be significantly altered as the cost of reflective roofing becomes inconsequential.
Notes
Firstly, reflectivity or albedo is the hemispherical reflectivity integrated over a particular wavelength band of the electromagnetic spectrum. However, For the purposes of this article, the terms reflectivity and albedo are used interchangeably and refer to the wavelengths encompassing the range of solar irradiance from 0.28 to 2.8 microns.
Secondly, surface solar reflectivity is measured using a precision spectral pyranometer with the device alternately faced upward towards the sun and downward towards the roof to determine the ratio of incident to reflected solar radiation.
Eskom and electricity
“The Eskom crisis has led to some increased awareness of so-called ‘green’ alternatives for power, but as yet there are very few initiatives to develop more holistic environmentally-friendly housing,” says Bester. However, “The country is so far behind when it comes to providing housing for the masses that green issues are at the bottom of the list. Also, This means that only homeowners building privately can actually give proper attention to the issues.”
To continue, In May 2005, a report by the South African National Biodiversity Institute (SANBI) stated that as a result of the ‘greenhouse’ effect, South Africa would experience a “steady warming of three degrees Celsius within the lifetime of the present generation of children”. Therefore, This means homes will need some form of insulation or will have to resort to energy-hungry air conditioning in an effort to keep the occupants cool.
“This has resulted in a wave of interest in environmentally-friendly building initiatives that are setting the stage for the homes of the future,” says Bester. “Furthermore, we are seeing more homes built that incorporate alternative sources of energy and climate proofing. subsequently, The homeowners will reap the benefits both through improved lifestyle and increased property values.”
In conclusion, Bester believes that the time will come when a property that does not comply with the energy efficient regulations of the future will be impossible to sell. “Although it may seem unnecessarily costly now to renovate for climate change,” he says, “ Suitable climate proofing will help reduce current energy costs as well as greatly improve the chances of selling the property in the future.”
