Roofing Types and Their Effects Roofing Types and Their Effects on the Thermal Properties of on the Thermal Properties of

RunoffRunoff

Robert W. Peters, Julie G. Price, Ronald D. Robert W. Peters, Julie G. Price, Ronald D. Sherrod, Stephen A. Watts, Julia M. Gohlke, Sherrod, Stephen A. Watts, Julia M. Gohlke,

Jason T. Kirby, and Matt WinslettJason T. Kirby, and Matt Winslett

University of Alabama at Birmingham University of Alabama at Birmingham (UAB)(UAB)

Birmingham, AlabamaBirmingham, Alabama

Paper presented at theAlabama Water Resources ConferenceOrange Beach, AlabamaSeptember 8–9, 2011

Goal of the ResearchGoal of the Research

• The overall goal of the presented The overall goal of the presented research involving mini-roofs on the research involving mini-roofs on the University of Alabama at Birmingham University of Alabama at Birmingham (UAB) campus is to identify a suitable (UAB) campus is to identify a suitable roofing material combination which roofing material combination which reduces energy cooling costs. Other reduces energy cooling costs. Other considerations involve considerations involve implementation costs, durability, and implementation costs, durability, and maintenance.maintenance.

Ongoing UAB Research of Roofing Ongoing UAB Research of Roofing MaterialsMaterials

• During the past three years, UAB has During the past three years, UAB has conducted research to determine an conducted research to determine an efficient roofing material combinationefficient roofing material combination that that decreases the building heat load coupled decreases the building heat load coupled with a minimal environmental impact.with a minimal environmental impact.

• UAB has been investigating external and UAB has been investigating external and internal temperatures to determine a way internal temperatures to determine a way to lower utility costs.to lower utility costs.

• UAB is also interested in the interaction of UAB is also interested in the interaction of roof temperature and rainfall.roof temperature and rainfall.

BackgroundBackground

• Heat that is being Heat that is being transferred into the transferred into the roofing structure is roofing structure is primarily transferred primarily transferred through conduction through conduction and convection, and convection, affecting building affecting building surface and internal surface and internal temperatures.temperatures.

Rooftop PerformanceRooftop Performance

• Due to the hot summers prevalent in the Due to the hot summers prevalent in the southeastern U.S. during which the southeastern U.S. during which the tempera-tures can approach 100tempera-tures can approach 100F, along F, along with conditions of high humidity, it is with conditions of high humidity, it is crucial to have a roof which minimizes crucial to have a roof which minimizes cooling costs.cooling costs.

• By increasing the albedo of a structure, By increasing the albedo of a structure, we increase how reflective the object is we increase how reflective the object is to the solar radiation, which will result in to the solar radiation, which will result in a lower surface temperature and less a lower surface temperature and less heat being transferred into the building.heat being transferred into the building.

• Vegetative roofs have up to 30% of total Vegetative roofs have up to 30% of total rooftop cooling being due to plant rooftop cooling being due to plant transpiration.transpiration.

Mini-RoofsMini-Roofs• During this study, 15 mini-roof During this study, 15 mini-roof

combinations were observed for trends in combinations were observed for trends in internal temperatures.internal temperatures.

• Each mini-roof is 8-ft long x 4-ft wide x 4-ft Each mini-roof is 8-ft long x 4-ft wide x 4-ft deep. All roofs are insulated with 2.0-in. of deep. All roofs are insulated with 2.0-in. of extruded polystyrene.extruded polystyrene.

Mini-Roofs (cont’d)Mini-Roofs (cont’d)

• Each roof contains an internal Each roof contains an internal temperature sensor to determine the temperature sensor to determine the temperature inside the building without temperature inside the building without air-conditioning; the temperature was air-conditioning; the temperature was recorded every 10-minutes during the recorded every 10-minutes during the past 3+ years.past 3+ years.

• This data was then automatically sent This data was then automatically sent to a data logger and placed into an to a data logger and placed into an Excel file for review later.Excel file for review later.

Photos of Mini-RoofsPhotos of Mini-Roofs

Roof 10: vegetated roof

Roof 8: concrete pavers with white epoxy coating

Roof 3: black membrane roof

Roof 6: conventional black granular roof

Table 1. Mini-Roof Table 1. Mini-Roof Descriptions.Descriptions.

Surface Temperature Surface Temperature MeasurementsMeasurements

• The surface temperatures of the The surface temperatures of the mini-roofs were measured using mini-roofs were measured using an infrared digital thermometer an infrared digital thermometer [InfraPro® 4 (P/N 35639-30) [InfraPro® 4 (P/N 35639-30) Oakton, China]. The temperature Oakton, China]. The temperature range is range is 25 to 140025 to 1400ooF (F (32 to 32 to 760760ooC), with an accuracy of C), with an accuracy of 1%.1%.

• Surface temperature Surface temperature measurements were collected measurements were collected during June 2010 on the various during June 2010 on the various roofing materials.roofing materials.

Surface Temperature Surface Temperature Measurements (cont’d)Measurements (cont’d)

• During the course of the day, roofing surface During the course of the day, roofing surface temperatures ranged from 20.6temperatures ranged from 20.6ooC (69.4C (69.4ooF) to F) to 82.282.2ooC (180.0C (180.0ooF). [Internal temperatures from F). [Internal temperatures from the mini-roofs, collected in ten minute the mini-roofs, collected in ten minute intervals, had temperatures ranging from 74⁰F intervals, had temperatures ranging from 74⁰F to 95⁰F].to 95⁰F].

• The vegetative mini-roofs exhibit surface The vegetative mini-roofs exhibit surface temperatures ranging between 21.7temperatures ranging between 21.7ooC to C to 52.252.2ooC, while their large counterpart, the pilot C, while their large counterpart, the pilot roof on top of Hulsey Center, exhibits higher roof on top of Hulsey Center, exhibits higher tempera-tures ranging from 31.7tempera-tures ranging from 31.7ooC to 61.1C to 61.1ooC.C.

Mini-Roof Surface Temperature Mini-Roof Surface Temperature MeasurementsMeasurements

Typical Temperature Typical Temperature Fluctuations during the Study Fluctuations during the Study

PeriodPeriod

Surface Temperature Surface Temperature MeasurementsMeasurements

• Because the temperature seemed to vary Because the temperature seemed to vary cyclically during the course of a year, the data cyclically during the course of a year, the data was fitted to a sine wave function.was fitted to a sine wave function.

• After a Fourier transform of the data and analysis After a Fourier transform of the data and analysis through various software programs, the sine through various software programs, the sine functions for each mini-roof were determined to functions for each mini-roof were determined to describe the thermal trends. This function is describe the thermal trends. This function is applied to the exterior surface temperatures for applied to the exterior surface temperatures for estimation since the interior and exterior surfaces estimation since the interior and exterior surfaces experience the same source of thermal loading.experience the same source of thermal loading.

• The roofs had amplitudes ranging from 19.5 to The roofs had amplitudes ranging from 19.5 to 20.720.7oo and phases angles roughly 4.2 to 4.3 and phases angles roughly 4.2 to 4.3 radians.radians.

Fitted Sine Wave Regression Fitted Sine Wave Regression for a Mini-Roof Systemfor a Mini-Roof System

General ObservationsGeneral Observations

• Generally, the darker the roof, the hotter it Generally, the darker the roof, the hotter it is at a given time, no lag was observed is at a given time, no lag was observed when compared to a lighter roof of similar when compared to a lighter roof of similar composition.composition.

• Roofs of the same type perform roughly Roofs of the same type perform roughly the same internally and externally, but the same internally and externally, but roofs of different materials (e.g., green vs. roofs of different materials (e.g., green vs. conventional) were statistically different.conventional) were statistically different.

• The differences in roof temperatures are The differences in roof temperatures are greater in the summer months than any greater in the summer months than any other time of year. other time of year.

General Observations (cont’d)General Observations (cont’d)

• During the winter, autumn, and spring During the winter, autumn, and spring months, more roofs begin to exhibit months, more roofs begin to exhibit similar internal thermal properties due similar internal thermal properties due to diurnal temperature and lower to diurnal temperature and lower radiation thermal input.radiation thermal input.

• The rock roofs maintain lowest surface The rock roofs maintain lowest surface temperatures, and membranous roofs temperatures, and membranous roofs were the hottest with vegetative roofs were the hottest with vegetative roofs behaving similar to rock roofs.behaving similar to rock roofs.

Modulation of Mini-Roof Modulation of Mini-Roof Temperature Measurements Temperature Measurements

Associated with RainfallAssociated with Rainfall

• From the surface temperature From the surface temperature measurements, rapid cooling due to rain measurements, rapid cooling due to rain was a very effective temperature was a very effective temperature regulator.regulator.

• Rainwater has a very high specific heat Rainwater has a very high specific heat capacity, which makes water a very capacity, which makes water a very effective cooling agent.effective cooling agent.

Modulation of Mini-Roof Modulation of Mini-Roof Temperature Measurements Temperature Measurements

Associated with Rainfall Associated with Rainfall (cont’d)(cont’d)

• The precipitation events cause a The precipitation events cause a sudden drop in temperature at random sudden drop in temperature at random intervals during the year. Such events intervals during the year. Such events add to the randomness and noise add to the randomness and noise observed in the graphs of the raw data.observed in the graphs of the raw data.

Effect of Rainfall on Surface Effect of Rainfall on Surface Temperature of Mini-Roof No.3Temperature of Mini-Roof No.3

Rainfall events occurred on days: June 1, 2, 4, 5 and July 6, 13, and 16.

Surface Temperatures on Mini-Roof Surface Temperatures on Mini-Roof No.3 No.3 (black membrane roof)(black membrane roof)

Effect of Rainfall on Surface Effect of Rainfall on Surface Temperature of Mini-Roof No.6 Temperature of Mini-Roof No.6

(conventional black granular roof)(conventional black granular roof)

Surface Temperatures on Mini-Roof Surface Temperatures on Mini-Roof No.6No.6

Effect of Rainfall on Surface Effect of Rainfall on Surface Temperature of Mini-Roof No.8Temperature of Mini-Roof No.8 (concrete (concrete

pavers with white epoxy coating)pavers with white epoxy coating)

Surface Temperatures on Mini-Roof Surface Temperatures on Mini-Roof No.8No.8

Effect of Rainfall on Surface Effect of Rainfall on Surface TemperatureTemperature

of Mini-Roof No.10 of Mini-Roof No.10 (vegetated roof)(vegetated roof)

Surface Temperatures on Mini-Roof Surface Temperatures on Mini-Roof No.10No.10

Summary and ConclusionsSummary and Conclusions

• The roofs are statistically different energy The roofs are statistically different energy performance-wise in terms of the performance-wise in terms of the temperature amplitude, while the phase temperature amplitude, while the phase angles are statistically the same.angles are statistically the same.

• Most roofs are significantly different in Most roofs are significantly different in terms of the average mean roof terms of the average mean roof temperature, but the difference is most temperature, but the difference is most pronounced in the summer months due to pronounced in the summer months due to the higher solar intensity; during the other the higher solar intensity; during the other times of the year, the roofs behave more times of the year, the roofs behave more similarly to one another.similarly to one another.

Summary and Conclusions Summary and Conclusions (cont’d)(cont’d)

• The color black absorbs all frequencies of visible light The color black absorbs all frequencies of visible light which causes the roof to become hotter than the which causes the roof to become hotter than the lighter colored roofing combinations. This property is lighter colored roofing combinations. This property is most well observed in Roof No. 6 which is the hottest most well observed in Roof No. 6 which is the hottest roof primarily due to its color and texture.roof primarily due to its color and texture.

• The coolest roofs were the roofs composed primarily The coolest roofs were the roofs composed primarily of a rock or stone material and were of light color.of a rock or stone material and were of light color.

• Vegetative roofs resulted in mini-roof surface and Vegetative roofs resulted in mini-roof surface and internal temperatures comparable to reflective roofs; internal temperatures comparable to reflective roofs; their performance is enhanced by evapo-transpiration.their performance is enhanced by evapo-transpiration.

Summary and Conclusions Summary and Conclusions (cont’d)(cont’d)

• Differences in the thermal properties is also Differences in the thermal properties is also a product of the surface area/volume ratio a product of the surface area/volume ratio of the roof and not just the material of the roof and not just the material composition itself.composition itself.

• Rainfall had a significant effect on the Rainfall had a significant effect on the cooling of the mini-roofs:cooling of the mini-roofs:

For black roofs, For black roofs, T > 60T > 60ooFF

For concrete pavers with a white epoxy For concrete pavers with a white epoxy coating, coating, T ~ 33T ~ 33ooFF

For vegetative roofs, For vegetative roofs, T ~ 33T ~ 334040ooFF

Future Research DirectionsFuture Research Directions

• Expand the research to develop an Expand the research to develop an overall sustainability assessment that will overall sustainability assessment that will compare different roof types in terms of compare different roof types in terms of economic, social and environmental economic, social and environmental impacts.impacts.

• Investigate quantifiable impacts on the Investigate quantifiable impacts on the Birmingham urban heat island and air Birmingham urban heat island and air pollution indices and associated pollution indices and associated quantifiable health outcomes.quantifiable health outcomes.

Future Research Directions Future Research Directions (cont’d)(cont’d)

• Monitor temperature of roof runoff in Monitor temperature of roof runoff in real-time to identify whether real-time to identify whether temperature shock loads emanate from temperature shock loads emanate from discharges into the natural environment discharges into the natural environment (e.g., discharge into ecosystems, etc.).(e.g., discharge into ecosystems, etc.).

Acknowledgments

• The authors acknowledge the The authors acknowledge the technical assistance provided by UAB’s technical assistance provided by UAB’s Facilities Management Department in Facilities Management Department in the conduct of this research.the conduct of this research.

• The authors also acknowledge the The authors also acknowledge the technical assistance of Dr. Ian Hosch technical assistance of Dr. Ian Hosch for his input on the sine wave analysis for his input on the sine wave analysis of the temperature variations.of the temperature variations.

•Questions???Questions???