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Abstract
Cool roof membranes can be a critical component of a proactive roof maintenance program that results in lower lifetime membrane temperatures during sunny periods. The lower membrane temperatures, in turn, reduce the air conditioning loads of the building and potentially lengthen the service life of the roofing system. If the building is located where cooling loads predominate, peak load reductions and net annual energy savings are also realized. In mixed climates with both significant heating and cooling loads, the wintertime effect reduces the energy benefit because the desirable roof heat gain in winter is diminished somewhat by the higher solar reflectance of the membrane. Determining how exposure to the various climatic elements affects the reflectance and emittance of single-ply membranes is of paramount importance for promoting the energy efficiency and accelerating the market penetration of reflective roof products in commercial and industrial applications. Ultraviolet radiation, atmospheric pollution, microscopic growths, acid rain, temperature cycling caused by sunlight and sudden thunderstorms, moisture penetration, condensation, wind, hail, and freezing and thawing all contribute to the loss of reflectance of a roof’s exterior surface. However, data describing the impact of the weather are extremely sparse simply because of the time and patience required to collect and interpret the data. Temperature, heat flow, reflectance, and emittance field data have been electronically cataloged for a full 3 years for 18 different single-ply membrane roofs exposed to the climate of east Tennessee on an outdoor test facility, the Envelope Systems Research Apparatus (ESRA). Our results gleaned from the ESRA show that the surface of the white thermoplastic roof systems lost about 30 to 50% of their reflectance after 3 full years of field exposure. Our findings show that airborne particles are responsible for the loss in roof reflectance, and area also the vehicles for delivering microorganisms to the surface as these particles are deposited on the membrane. Microorganisms grow on the surface, sprouting thin root-like filaments called hypae that are covered with enzymes. The roof becomes wet during the evening hours because the surface temperature falls below the dew point temperature of the ambient air. The enzymes dilute in the condensate and dissolve edible food from the surface of the membrane. The hyphae grow in biological film-like mat that is hydrophilic and keeps the surface moist even when the air is dry. The hyphae also act as a net enhancing the continued deposition of dirt onto the surface, which in turn leads to larger losses in reflectance. Soiling may be exacerbated in certain thermoplastic membranes whose formulations contain edible foodstuffs for the biomass. The cost of additional polyisocyanurate insulation required by a smooth built-up roof (BUR), constrained to have the same annual operating cost of energy as a high-reflectance roof, was compared to the cost for white thermoplastic membranes to judge the affordability of cost premiums for the membranes. Analysis showed a synergistic cost benefit because of the combined effect of R-value and reflective roofing that peaks R-value increases from R-5 up t about R-20; however, continuing to increase R-value beyond R-20 causes the effect of insulation to mask the effect of a reflective roof. Results show that consumers in Phoenix, AZ, and Knoxville, TN, can easily afford cool membranes as compared to the additional cost of insulation for a smooth BUR. The annual roof energy for a soiled thermoplastic membrane exposed in a predominately cooling climate was observed to increase by 50% of the roof energy for the same thermoplastic membrane constrained to have no loss in reflectance. The increase in energy decreases through 3 years of exposure as the loss of reflectance levels off. In a more moderate climate, the cooling-energy savings are offset by the heating-energy penalty, and it appears that the ratio of cooling degree-days to heating degree-days exceeding 0.4 may roughly represent the boundary for periodically washing cool roof membranes. Washing a reflective membrane in moderate to cooling-predominant climates will save roof energy by offsetting the impact of soiling of the membrane.
Cool roof membranes can be a critical component of a proactive roof maintenance program that results in lower lifetime membrane temperatures during sunny periods. The lower membrane temperatures, in turn, reduce the air conditioning loads of the building and potentially lengthen the service life of the roofing system. If the building is located where cooling loads predominate, peak load reductions and net annual energy savings are also realized. In mixed climates with both significant heating and cooling loads, the wintertime effect reduces the energy benefit because the desirable roof heat gain in winter is diminished somewhat by the higher solar reflectance of the membrane. Determining how exposure to the various climatic elements affects the reflectance and emittance of single-ply membranes is of paramount importance for promoting the energy efficiency and accelerating the market penetration of reflective roof products in commercial and industrial applications. Ultraviolet radiation, atmospheric pollution, microscopic growths, acid rain, temperature cycling caused by sunlight and sudden thunderstorms, moisture penetration, condensation, wind, hail, and freezing and thawing all contribute to the loss of reflectance of a roof’s exterior surface. However, data describing the impact of the weather are extremely sparse simply because of the time and patience required to collect and interpret the data. Temperature, heat flow, reflectance, and emittance field data have been electronically cataloged for a full 3 years for 18 different single-ply membrane roofs exposed to the climate of east Tennessee on an outdoor test facility, the Envelope Systems Research Apparatus (ESRA). Our results gleaned from the ESRA show that the surface of the white thermoplastic roof systems lost about 30 to 50% of their reflectance after 3 full years of field exposure. Our findings show that airborne particles are responsible for the loss in roof reflectance, and area also the vehicles for delivering microorganisms to the surface as these particles are deposited on the membrane. Microorganisms grow on the surface, sprouting thin root-like filaments called hypae that are covered with enzymes. The roof becomes wet during the evening hours because the surface temperature falls below the dew point temperature of the ambient air. The enzymes dilute in the condensate and dissolve edible food from the surface of the membrane. The hyphae grow in biological film-like mat that is hydrophilic and keeps the surface moist even when the air is dry. The hyphae also act as a net enhancing the continued deposition of dirt onto the surface, which in turn leads to larger losses in reflectance. Soiling may be exacerbated in certain thermoplastic membranes whose formulations contain edible foodstuffs for the biomass. The cost of additional polyisocyanurate insulation required by a smooth built-up roof (BUR), constrained to have the same annual operating cost of energy as a high-reflectance roof, was compared to the cost for white thermoplastic membranes to judge the affordability of cost premiums for the membranes. Analysis showed a synergistic cost benefit because of the combined effect of R-value and reflective roofing that peaks R-value increases from R-5 up t about R-20; however, continuing to increase R-value beyond R-20 causes the effect of insulation to mask the effect of a reflective roof. Results show that consumers in Phoenix, AZ, and Knoxville, TN, can easily afford cool membranes as compared to the additional cost of insulation for a smooth BUR. The annual roof energy for a soiled thermoplastic membrane exposed in a predominately cooling climate was observed to increase by 50% of the roof energy for the same thermoplastic membrane constrained to have no loss in reflectance. The increase in energy decreases through 3 years of exposure as the loss of reflectance levels off. In a more moderate climate, the cooling-energy savings are offset by the heating-energy penalty, and it appears that the ratio of cooling degree-days to heating degree-days exceeding 0.4 may roughly represent the boundary for periodically washing cool roof membranes. Washing a reflective membrane in moderate to cooling-predominant climates will save roof energy by offsetting the impact of soiling of the membrane.
Date
8/2002
8/2002
Author(s)
W Miller; M Cheng; S Pfiffner; N Byars
W Miller; M Cheng; S Pfiffner; N Byars
Page(s)
75
75
Source
Oak Ridge National Lab
Oak Ridge National Lab
Keyword(s)
field performance; single ply; reflectivity; weathering; cool roofing; ESRA
field performance; single ply; reflectivity; weathering; cool roofing; ESRA