SMART GREEN ROOFS: Cooling with Variable Insulation Pablo La Roche Cal Poly Pomona University & HMC Architects Eric Carbonnier HMC Architects Christina Halstead Cal Poly Pomona WREF/ASES 2012
Contents 1. Introduction 2. Objective 3. Climate 4. Experimental System 5. Results 6. Discussion 7. Conclusions
1. Introduction: Green Roofs A living or green roof is a roof that is substantially covered with vegetation. A green roof has positive effects on buildings by: Reducing the stress on the roof surface, Improving thermal comfort inside the building, Reducing noise transmission into the building, Reducing the urban heat island effect by reducing hot surfaces facing the sky, Reducing storm water runoff, Re-oxygenating the air and removing airborne toxins, Providing habitat for living organisms and wildlife habitat, Conservation of biodiversity. All of this while creating beautiful environments and attractive open spaces.
Green Roofs: Positive Thermal Effects The reduction of the external surface temperature by the vegetation. The reduction of the thermal transmittance of the assembly, mostly due to the effects of insulation, usually placed between the sustaining material and the roof slab. Vegetative matter cools by absorbing solar radiation through the biological processes of photosynthesis, respiration, transpiration, and evaporation.
Nocturnal Ventilative Cooling The building is ventilated at night, cooling its internal structural mass by convection. This mass acts as a heat sink during the daytime when the building is closed to the higher outdoor temperatures. Nocturnal ventilative cooling is a well known strategy that has been used for many years, mostly in warm and dry climates
+ Green Roofs + Night Ventilation Studies have demonstrated that a well planned and managed green roof (with insulation) reduces heat gains in the summer. Previous research (La Roche 2006, 2009)has demonstrated the advantage of using the mass of an uninsulated green roof as a heat sink in temperate or hot climates. By reducing the temperature of the outer surface of the roof and increasing thermal capacity in contact with the indoors, green roofs can contribute to the cooling of spaces.
Temperature (C) Performance of an Uninsulated Green Roof with Night Ventilation 40 outdoors control green roof insulated green roof no insulation 35 30 Average max outdoors Average max insulated roof Average max insulated green roof 25 Average max green roof no insulation 20 15 10 1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145 154 163 172 181 190 199 208 217 226 235 244 253 262 Time (Hours)
However, insulation is needed to keep heat out when it is too warm outdoors or to keep heat inside when it is too cold outside. This means that in more extreme conditions it would be helpful to have a well insulated, sealed building, including an insulated green roof. Is it possible to have a building that is not insulated and coupled to the thermal mass when night ventilation is needed and insulated from the exterior when conditions are more extreme?
2. Objective To determine if a green roof with a variable insulation system can cool a space using night ventilation while reducing heat losses in cold days.
3. Climate The tests were conducted at Cal Poly Pomona University, located in a hot and dry climate, about 45 miles inland in Southern California (Climate Zone 9).
4. Experimental System Several series in the fall 2011 and winter, spring 2012 are presented. These include changes in the operation of the ventilation system and the shading of the window. The planting material of the uninsulated green roof is thermally coupled with the interior, the insulated green roof and the white roof have 0.1m of matt insulation. The variable insulation roof also has insulation with an air space. Insulated Roof Insulated Green Roof Uninsulated Green Roof Variable Insulation Green Roof
Test Cells 1.2 x 1.2 x 1.2 meters. Stud wall construction (2 by 4 ), drywall on the inside, plywood on the outside and batt insulation in between. Exterior painted white. 0.61 m by 0.61 m (2ft by 2ft) windows facing south. (double glazed) Equipped with fans controlled by dimmers and timers to adjust the ventilation rate. 1½ (0.038 m) thick concrete pavers for the slab.
Construction of Experimental Cells The test cells were built by the author with Architecture and Master of Science in Regenerative Studies students in the Lyle Center for Regenerative Studies at Cal Poly Pomona, to study the performance of different types of passive cooling systems.
Controller System How is this variable insulation system? There is an insulated plenum under the green roof in which a fan is activated by temperature based rules. When the fan is on the plenum is ventilated and when it is turned off the ceiling acts as an insulator.
Plants Different types of plants were used and for these series succulent plants on a layer of soil 7.5 cm thick above 2.5 cm of gravel with a plastic liner underneath and above a metal plate supported by wooden joists. Drainage tubes in the gravel capture excess water and drain it outside.
5. Series and Results
September 8, 2011 The cells are not shaded and the fan is off. All cell temperatures are higher than DBT because of the large window to floor ratio 1:4. Best performance is in the plenum of the smart green roof followed by the uninunsulated green roof and worst is in the insulated roof followed by the variable insulation (smart) green roof..
October 14, 2011 The cells are shaded and not ventilated. Best performance as demonstrated by the lowest maximum temperature is in the plenum of the smart green roof and worst performance are in the control cell and the insulated green roof..
Feb 24, 2012 The cells are not shaded and are ventilated using smart system and fan. The variable insulation roof has the fan operating according to the rules stated while the rest of the cells operate with a fan from 9pm to 6am..
March 26, 2012 The cells are not shaded and are ventilated from 8 PM to 3 AM. The plenum fan is OFF The plenum fan mixes the air when the fans are on (night), otherwise the plenum temperature is much cooler than the air temperature inside the space..
April 16, 2012 All cells are night ventilated from 8PM to 3 AM and shaded. The plenum fan is ON continously The plenum and space temperature in the smart green roof are all very close together and perform better than the insulated green roof and the insulated cell..
6. Discussion The percentage of hours overheated is used to evaluate the performance of the series. Insulated Roof Insulated Green Roof Uninsulated Green Roof Variable Insulation Green Roof
Using ASHRAE Comfort Standard 55-2005 for free running buildings. Tn = 17.8 + 0.31 Tavg Tn = 17.8 + 0.31 * 20.45 = Tn = 24.14 C Tn = neutrality temperature Tavg= outdoor average temperature
Time Test cells were compared in a 24 hour period to determine overheated hours using adaptive comfort zone. Series Outdoor DB Control Cell Uninsulated Smart Roof Smrt Rf -Plenum Insulated Outdoor DB Control Cell Uninsulated Smart Roof Smrt Rf -Plenum Insulated 13.88 16.63 17.48 18.16 19.16 18.23 13.86 16.45 17.36 18.06 18.95 18.06 13.79 16.35 17.2 17.88 18.81 17.97 13.63 16.21 17.05 17.71 18.65 17.78 13.6 15.98 16.84 17.55 18.53 17.48 13.37 15.93 16.7 17.41 18.37 17.31 13.37 15.81 16.61 17.27 18.2 17.22 13.3 15.74 16.49 17.15 18.04 17.17 13.13 15.61 16.38 16.98 17.92 16.94 13.3 15.56 16.3 16.94 17.8 16.87 13.37 15.54 16.19 16.77 17.64 16.82 13.09 15.31 16.05 16.66 17.52 16.61 13.11 15.26 15.98 16.49 17.43 16.49 13.09 15.11 15.91 16.45 17.31 16.54 12.92 15.06 15.84 16.35 17.15 16.4 12.99 14.94 15.69 16.21 17.05 16.23 12.9 14.84 15.61 16.16 16.91 16.23 13.02 14.79 15.54 16.09 16.84 16.07 13.09 14.69 15.46 16 16.73 16.02 12.56 14.56 15.24 15.81 16.59 15.76 12.56 14.49 15.21 15.71 16.47 15.81 12.51 14.39 15.06 15.66 16.4 15.61 12.41 14.21 14.96 15.51 16.28 15.46 12.78 14.17 14.94 15.49 16.21 15.51 12.21 Cool 14.05 14.79 15.31 16.07 15.34 12.56 14 14.76 15.24 15.95 15.24 12.71 13.93 14.74 15.16 15.88 15.19 12.31 13.84 14.54 15.06 15.81 15.09 12.31 13.77 14.56 14.99 15.71 15.14 12.26 13.74 14.49 14.91 15.61 14.96 12.26 13.65 14.41 14.84 15.56 14.84 12.14 13.58 14.29 14.74 15.46 14.81 12.11 13.53 14.24 14.66 15.34 14.74 12.16 13.46 14.21 14.61 15.29 14.71 12.11 13.37 14.14 14.51 15.21 14.71 12.21 13.37 14.07 14.49 15.11 14.54 12.01 13.28 13.95 14.39 15.01 14.51 11.81 13.23 13.91 14.26 14.91 14.41 12.06 13.28 13.91 14.29 14.86 14.39 11.99 13.18 13.86 14.21 14.79 14.34 12.44 13.25 13.98 14.21 14.76 14.41 13.16 13.37 14.14 14.34 14.79 14.54 14.24 13.58 14.76 14.94 14.94 15.11 14.81 13.65 14.94 15.01 15.04 15.36 16.09 13.7 15.01 15.04 15.06 15.46 17.08 13.72 15.01 15.11 15.11 15.46 18.16 13.79 15.06 15.16 15.16 15.54 17.99 13.84 15.21 15.24 15.24 15.74 18.93 13.91 15.34 15.26 15.29 15.86 18.46 14.07 15.51 15.36 15.39 16.09 19.77 14.24 15.74 15.49 15.51 16.33 19.4 14.46 15.91 15.61 15.64 16.59 20.71 14.76 16.07 15.79 15.84 16.8 22.1 15.11 16.35 15.93 15.95 17.05 22.08 15.54 16.56 16.09 16.14 17.36 22.8 15.95 16.8 16.3 16.33 17.69 22.43 16.42 17.05 16.52 16.54 17.97 23.46 16.91 17.27 16.73 16.75 18.39 23.99 17.41 17.59 16.94 16.98 18.79 25.08 17.95 17.9 17.15 17.22 19.16 25.79 18.51 18.18 17.43 17.48 19.56 26.51 19.16 18.46 17.66 17.73 19.94 26.89 19.82 18.81 17.9 17.97 20.13 28.69 20.56 19.12 18.18 18.25 20.59 28.84 21.29 19.42 18.44 18.55 21.11 29.31 Cmf 22.03 19.7 18.74 18.84 21.36 30.86 22.71 20.24 19.02 19.19 21.98 33.18 23.51 20.59 19.4 19.49 22.45 30.66 24.04 21.11 19.73 19.87 22.83 29.76 24.56 21.84 20.05 20.15 23.08 30.71 25.36 22.01 20.39 20.56 23.64 31.19 26.02 22.76 20.79 20.96 24.26 31.39 26.44 22.94 21.19 21.34 24.56 31.56 26.99 23.15 21.56 21.71 25.04 32.59 27.61 23.37 21.94 22.05 25.48 32.49 28.09 23.69 22.33 22.4 25.67 32.26 28.54 24.06 22.66 22.76 25.93 32.66 29.01 24.39 22.99 23.06 26.28 34.77 29.64 24.86 23.37 23.44 26.96 36.57 30.24 25.06 23.76 23.84 27.36 32.86 30.54 25.5 24.09 24.16 27.44 35.2 Hot 31.06 25.83 24.49 24.54 28.14 34.85 31.39 25.9 24.79 24.86 28.24 37.19 31.91 26.23 25.13 25.27 28.81 35.57 32.29 26.54 25.48 25.6 28.96 37.44 32.76 26.94 25.81 25.95 29.59 36.46 33.21 27.39 26.16 26.25 29.71 36.74 33.63 27.71 26.49 26.61 30.16 36.34 33.9 28.14 26.76 26.94 30.49 35.89 34.14 28.04 27.11 27.26 30.69 36.93 34.46 28.49 27.41 27.64 30.99 35.97 34.72 28.86 27.69 27.91 31.21 33.71 34.72 29.31 27.91 28.16 31.44 32.51 34.72 29.44 28.06 28.36 31.26 31.46 34.54 29.34 28.19 28.51 31.19 31.14 34.3 29.34 28.29 28.66 31.14 31.06 34.14 29.29 28.36 28.76 31.19 30.51 34.08 29.31 28.46 28.89 31.16 30.41 33.84 29.29 28.56 28.96 31.16 30.46 33.76 29.24 28.61 29.01 31.14 29.74 33.58 29.16 28.64 29.11 31.04 29.19 33.47 29.09 28.69 29.14 31.01 29.04 33.26 29.06 28.71 29.24 31.04 28.91 33.07 28.94 28.71 29.21 30.91 28.54 32.74 28.86 28.71 29.24 30.86 27.99 32.49 28.59 28.64 29.24 30.66 26.99 32.09 28.29 28.59 29.19 30.41 26.64 31.59 28.09 28.49 29.14 30.16 26.33 31.19 28.04 28.41 29.01 29.99 25.65 30.84 27.66 28.29 28.94 29.79 25.36 30.56 27.54 28.26 28.84 29.69 24.64 30.26 27.26 28.19 28.74 29.49 23.27 29.94 27.01 28.09 28.61 29.34 22.52 29.64 26.74 27.94 28.51 29.11 21.86 29.34 26.76 27.81 28.34 29.01 21.11 29.04 26.37 27.69 28.19 28.76 20.71 28.76 26.18 27.54 28.04 28.59 20.13 28.44 26.07 27.41 27.84 28.44 19.68 28.11 25.74 27.29 27.69 28.14 19.21 27.81 25.39 27.11 27.51 27.76 18.84 27.51 25.34 26.99 27.34 27.69 18.34 27.19 25.06 26.84 27.16 27.46 17.99 26.91 24.94 26.71 26.96 27.24 17.76 26.64 24.89 26.54 26.81 27.14 17.52 23.71 23.27 24.71 25.76 25.32 17.13 22.76 22.57 24.01 25.22 24.59 16.94 22.05 22.29 23.51 24.79 23.94 16.73 21.54 21.98 23.13 24.41 23.49 16.42 21.04 21.71 22.76 24.06 22.99 16.16 20.64 21.34 22.48 23.76 22.48 16.14 20.29 21.21 22.22 23.46 22.22 15.79 19.89 20.89 21.94 23.18 22.08 15.66 19.59 20.56 21.66 22.92 21.66 15.36 19.3 20.24 21.36 22.66 21.44 15.09 18.95 20.13 21.06 22.4 21.09 14.94 18.65 19.87 20.86 22.19 20.71 14.71 18.32 19.68 20.66 21.98 20.51 14.69 18.2 19.52 20.41 21.74 20.24 14.56 17.99 19.35 20.15 21.51 20.22 14.41 17.73 19.12 20.01 21.31 19.87 14.31 17.5 19.05 19.8 21.11 19.73 14.14 17.36 18.79 19.61 20.86 19.49 14.07 17.15 18.63 19.42 20.66 19.16 14 17.03 18.46 19.21 20.46 19.02 34% 36.8% 24.3% 27% 27% 33%
In previous series over several summers the test cell with the uninsulated green roof always performed better than the test cell with the insulated roof and the white insulated roof. When the plenum fan was off the smart green roof behaved like the insulated roof and was warmer. When the plenum fan was on the smart green roof behaved like the uninsulated green roof and was cooler.
7. Conclusions More tests and analysis have to be done but results are promising. The following rule can control the exchange of air with the outside. It seems that the best performance is achieved with the plenum fan working continuosly at least during the selected periods. If t o < t i and t i > Cf_low and t i <Cf_high then fan ON else fan OFF. Where: t o is the temperature outside; t i is the temperature inside ; C f_low, is comfort low at 18.33 C (65 F); C f_high is Comfort high at 25.55 C (78 F).
Plenum Fan ON Air Change Fan OFF Plenum Fan ON Air Change Fan ON Proposed Summer Strategies
Plenum Fan ON Air Change Fan OFF Plenum Fan OFF Air Change Fan OFF Proposed Winter Strategies
Applicability in different climates Since the green roof is used as thermal mass, the uninsulated and variable insulation night ventilated green roof should work well in the zones for high mass and high mass with night ventilation. The green roof with variable insulation should extend the applicability of the green roof beyond the extents of the uninsulated green roof. The variable insulation green roof would cool a space in two ways: The plants provide a natural smart shading system, reducing solar gains to the thermal mass. The earth in contact with the space is cooled at night with ventilation and absorbs heat during the daytime.
Uninsulated Green Roof
Variable Insulation Green Roof
Next Steps Continue series and data analysis comparing the maximum average temperatures and using the temperature difference ratio to develop predictive equations.