POST OCCUPANCY DESIGN INERVENTION TO IMPROVE COMFORT AND ENERGY PERFORMANCE IN A DESERT HOUSE

POST OCCUPANCY DESIGN INERVENTION TO IMPROVE COMFORT AND ENERGY PERFORMANCE IN A DESERT HOUSE Vidar Lerum Arizona State University P O Box 871605, Tempe, AZ, 85287-1605, USA vidar.lerum@asu.edu Venkata Ramana Koti Arizona State University P O Box 871605, Tempe, AZ, 85287-1605, USA venkata.koti@asu.edu ABSTRACT This paper reports on the methods used to analyze the effect on thermal comfort and energy performance as a result of incremental changes applied to the building envelope. The object of investigation was the House of Earth and Light, a residence built in the hot and dry climate of the low Arizona desert. As new owners occupied the house in 2003, they experienced severe overheating in the living room. A simple single path method was developed to determine the effect on surface temperatures as a function of changes in floor, glass walls, and roof R-values as well as changes in the indoor air temperature. The mean radiant temperature (MRT) was then calculated for each incremental improvement while the indoor air temperature was changed in order to achieve a target operative temperature (OT) of 78 F. The effect of the incremental improvements on the energy performance of the building was analyzed by means of hour-by-hour calculations using the Energy-10 simulation software. 1. INTRODUCTION Thermal comfort is related to both the physiological contentment of the inhabitant with a space and the necessary energy used to condition the space. In 1999, when the Building Owners and Management Association (BOMA) in partnership with the Urban Land Institute (ULI) surveyed 1,829 office tenants in which they were asked to rate the importance of 53 building features and amenities, 99% of them felt thermal comfort was an important feature (1). Fig. 1: House of Earth and Light: night view of the fabric roof. Despite the best intentions of the designer, nature has its own ways of reminding us of the potential thermal shortcomings of an assembly. Sometimes, in an attempt to address a wider universal ideal, formal and spatial considerations take priority over environmental concerns. Though applying post occupancy corrective measures are not the best approach, it sometimes becomes imperative. The single story residential building under consideration here has been widely published and commended for its transparency to the sky during day and night (2). The analysis described in this paper was limited to the living and dining room, a 36 by 24 foot steel and glass bridge with a concrete floor spanning across a dry wash (arroyo). As built by the original owners, a translucent fabric roof covered the entire house, including the bridge. The

outer perforated fabric layer was stretched over threedimensional steel trusses extending six feet to form an overhang on the south side. Except for the north and south facing glass walls of the bridge, all exterior walls and most of the interior walls were made of 18-inch thick slabs of concrete poured in plywood forms. The building is situated in Phoenix, which has a hot and dry desert climate with the highest outdoor air temperature measured in 2004 at 112 F. When the original owner built the house, they made several deviations from the architect s specifications: The living room floor slab, extending across the dry wash, was not insulated. Single pane glass was used in the floor-toceiling north and south walls, except for a few smaller double pane glass panels. The third (inner) roof fabric layer was not installed. 2. METHODS 2.1. Data acquisition Surface and air temperature data were recorded in the living room (the bridge ) on May 27 th, 2003 from before sunrise until after sunset. Outdoor air temperatures were recorded near the entrance on the south side, in the shade. A complete data set is available upon request. 2.2. Approach This particular monitoring day turned out to be hot and sunny with the ambient temperature reaching 111 F at 2 PM. Since the summer design temperature for Phoenix is 107 F and the thirty-year average maximum dry bulb temperature is 115 F, the conditions at 2 PM on May 27 th of 2003 may be seen as a design hour used to size the cooling system and to assess summertime thermal comfort. The part of the analysis that pertains to thermal comfort was therefore focused on the 2 PM hour on May 27 th, while the energy performance simulations were carried out using an 8760-hour TMY2 weather file for Phoenix, Arizona. 2.3. Surface temperatures Fig. 2: Typical cross-section showing the three-layered fabric roof assembly, as designed. Note; this section is not cut at the living room (bridge). Since the house had not been built entirely according to specifications, the goal of this research was to investigate potential incremental improvements in thermal comfort and energy performance as a function of bringing the floor, glass walls, and roof up the standard that the architect had specified. In accordance with claims presented by the architect, a hypothesis was formulated as follows: Indoor thermal conditions in the living room could be brought into the comfort zone on a summer design day (or design hour) if the specified incremental improvements were put in place. A single path steady state heat flow method was developed to simulate changes in surface temperatures as a function of lowering the indoor air temperature and increasing the R-values of the floor, glass walls, and roof assemblies. Since the R-values of the individual components were known and each layer of the assembly would have a temperature difference based on the gradient of R-values, the R-values were tweaked until the calculated surface temperature matched the measured. This straightforward approach proved valid for the floor assembly, as seen in Fig. 3. Since the single path heat flow method did not account for thermal bridging represented by the steel framing of the glass walls, and similarly did not account for heat gain from radiation represented by ground reflection, the outdoor air temperature was elevated 10 F as a means of calibrating the glass wall module of the model. Similarly, the outdoor air temperature was elevated 29 F in order to calibrate the roof module of the model. This elevated outdoor air temperature for the roof calculations may be seen as an attic temperature of the air mass trapped between the outer perforated fabric (shade cloth) and the inner weatherproofing fabric.

using an MRT calculator that allows for simulating step changes in the indoor air temperature, see Fig. 6. Fig. 3: Calibration of R-value of the existing floor assembly using measured surface and air temperatures. All temperature values are in F. With the single path method calibrated, we could now simulate changes in the surface temperatures as a function of lowering the indoor air temperature. Fig. 4 shows how the surface temperature of the floor would be lowered to 87 F if the indoor air temperature was lowered to 71 F. The temperature difference between the indoor air and the floor surface would increase from 9 F at 88 F indoor air temperature to 16 F at 71 F indoor air temperature. Fig. 4: Calculated effect on surface temperature as a function of reduced indoor air temperature. All temperature values are in F. The next step was to introduce an improvement in the R- value of the assembly. Fig. 5 shows how the temperature difference between the indoor air and the floor surface could be reduced to 1 F by adding 13 inches of fiberglass insulation to the underside of the floor slab. Fig. 5: Calculated effect on surface temperature of reduced indoor air temperature and 13 inches of fiberglass insulation added to the underside of the concrete slab. All temperature values are in F. 2.4. Mean radiant and operative temperatures Based on the measured and calculated surface temperature data, mean radiant temperature (MRT) and the resultant operative temperature (OT) were calculated Fig. 6: The MRT calculator was designed to allow for simulating the effect of variable indoor air temperature. Mean radiant temperature as experienced by a seated user, see ASHRAE Handbook of Fundamentals, equation 53. The value cells for floor, glass walls, and roof surfaces temperatures were linked to the single path calculations in order to take advantage of the dynamic relationship between changing air temperatures and the temperatures of all interior surfaces. The surface temperatures of the interior (thermally neutral) east and west mass walls were assumed to be 1 F above the indoor air temperature, as observed on the day the space was monitored. As can be seen in Fig. 6, the data at 2 pm shows a temperature difference of 9.7 o F between the calculated MRT and the maintained indoor air temperature, and a resultant operative temperature of 92.8 o F. Note that the interior surface temperature of the fabric roof was measured as 119 F while the outdoor air temperature near the ground was 111 F. The asymmetry in the space was 30 F between the warmest surface (ceiling) and the relatively cooler interior mass walls. Studies have shown that a lower percentage of people express dissatisfaction with a cool ceiling and a warm wall, and a higher percentage of people express dissatisfaction with a warm ceiling and cool walls (3), see Fig. 9. Operative temperature (OT), which is one of the factors primarily responsible for thermal comfort, is an average of the MRT and the indoor air temperature (IAT). Hence thermal comfort requires that OT falls within the comfort zone on the psychrometric chart and also that the MRT and the IAT are fairly close to each other. A high MRT will require lower IAT in order to bring the operative temperature within the comfort zone. This in turn requires lower supply air temperature (from the AC unit) and higher air velocity (since more heat needs to be removed per unit of time).

Six variants (conditions) were defined relative to the existing condition as monitored (variant 1) and incremental improvements represented by lower IAT and higher R-values (variants 2-6). Surface temperatures, MRT, and OT were calculated for variants 2 through 6. The target OT was set at 78 F for these variants. 2.5. Simulating energy performance The annual energy performance of the 36 by 24 foot space with thermally neutral end walls was simulated in Energy- 10 for all variants 1 through 6. Though initially equest was considered for simulation, it was realized that modified surface temperatures could be determined using the single path method explained earlier. Since only a single zone was considered for this analysis, Energy-10 could be used to perform hour-byhour load calculations and simulations of the annual energy use. Fig. 7: Solid roof with skylights, as built 2005. The results from the Energy-10 simulations were kept as a series of bar charts as exemplified in Fig. 8. A 36 feet long and 24 feet wide space with a height of 10 feet was defined in Energy-10. The 10 feet height accounted for the 9-foot high glazing on the north and south with a 1-foot high bond beam above. The concrete masonry walls on the east and west were considered thermally neutral with an R-value of 1000. The auto-build function in Energy-10 sets up a reference case roughly representing standard practices and a low-energy case roughly representing best practice energy efficient design. While simulating the six variants of the incremental change method, the lowenergy case can be seen as a benchmark. As compared to the low-energy case (benchmark) and the space as monitored (v-1), incremental improvements were defined as follows: Lowering IAT to 71 F in order to obtain an OT of 78 F (v-2), adding insulation to the floor (v-3), modifying the existing single glazing to a double low-e glazing (v-4), and adding a second fabric layer with an intermediate air space to the roof (v-5). These variants are all defined within the basic premise of maintaining the visual appearance and architectural quality of the building. Variant 6 represents the actual situation today (2005) after the new owners decided to replace the fabric roof with sections of solid standing seem covered opaque roof panels intersected by stripes of low-e glazed skylights at the steel trusses, see Fig. 7. Fig. 8: Annual cooling energy use in MMBtu, as simulated for the auto-build reference case, the lowenergy (benchmark) case, and the six variants of the incremental change analysis. 3. RESULTS A summary of the results from the single path method, the MRT-calculations, and the Energy-10 simulations is included in Table 1at the end of this paper. 3.1. Conditions in the space as monitored The indoor air temperature measured at 2 pm was 88 o F. This condition reflects the fact that the AC unit was unable to maintain a lower IAT due to the excessive heat gain. The OT was estimated at almost 93 F and the asymmetry was 30 F between the interior mass wall surfaces and the warm ceiling surface. Obviously, this was not a condition of thermal comfort.

3.2. Lowering the IAT A target value for the OT of 78 F could theoretically be achieved by lowering the IAT to 71 F with no improvements to the building envelope. This would lower the MRT to approximately 85 F, but the asymmetry would increase to 40 F since the ceiling fabric temperature would still be around 112 F. The simulated annual energy requirement for heating, cooling, and fans would increase to more than the double, from 58 MMBtu to 117 MMBtu. For comparison, the similar value for the benchmark low-energy case was only 13 MMBtu. 3.3. Insulating the floor A layer of 13 fiberglass insulation was added to the underside of the floor slab in order to reduce the surface temperature and decrease the heat loss. This would allow us to maintain the OT at 78 F with a slightly increased IAT at 72 F, while lowering the MRT only about 1 F. The simulated annual energy requirement for heating, cooling, and fans was reduced to about 93 MMBtu. 3.4. Replacing single pane glass with double pane low-e glazing The R-values of the north and south glass walls were improved by replacing all single pane glazing with double pane low-e glass. This would allow for an increase in the IAT to 74 F while maintaining an OT of 78 F. Again, the MRT would decrease only 1 F, but the simulated annual energy demand for cooling, heating, and fans would decrease to 80 MMBtu. 3.5. Adding a second layer of fabric with an intermediate air space to the roof An improvement of the fabric roof, as originally specified by the architect, was now added to the already improved floor and glass wall assemblies. The calculated values show that this improvement would reduce the ceiling fabric surface temperature to around 96 F. While keeping the OT at 78 F, the MRT would decrease to 81 F and the IAT could be increased to 75 F. The calculated asymmetry between the warm ceiling surface and the cooler interior mass walls would go down to 20 F and the simulated annual energy requirement for cooling, heating, and fans would decrease to about 61 MMBtu. This condition represents an estimate of the thermal comfort at the summer design hour of the bridge space as specified by the architect, with a corresponding annual energy requirement to condition the space accordingly. With a simulated annual cooling energy demand at about four times the benchmark low-energy case in Energy-10, it is safe to assume that the airflow rate and the forced air velocity would be quite high. One could still argue that a reasonable level of thermal comfort could be achieved in a space as specified, but this marginal level of comfort would come at a cost of annual energy for cooling, heating, and fans at about four times the benchmark low-energy case. 3.6. Replacing the fabric roof with solid roof panels intersected by stripes of skylights The new owners decided to replace the fabric roof with solid (opaque) standing seem covered roof panels intersected by stripes of skylights at the steel trusses. The exact specifications of this new roof assembly were not known at the time of writing this report, but it is estimated that the new roof assembly has an R-value of 24. Keeping the OT at 78 F, the ceiling surface temperature (opaque panels) was now estimated at 78 F. This is a radical improvement from all the previous variants 1 through 5. The IAT could now be increased to 76 F. The north and south glass wall surfaces would still be quite warm, but the asymmetry would decrease to about 9 F between the floor surface and the interior surface of the south facing glass wall. The simulated annual energy requirement for cooling, heating, and fans would roughly be cut in half to 28 MMBtu. 4. DISCUSSION In order to verify the hypothesis that thermal comfort could be achieved by implementing the improvements specified by the architect, one needs to assess the estimated percentage of dissatisfied users. This approach is widely accepted in the field (3). Fig. 9 shows that the calculated surface temperatures of variant 5, representing a radiant asymmetry of about 20 F, would produce approximately 25% dissatisfied users. This is a significant improvement over variant 1 and 2, which could produce up to 80% dissatisfied users. One could argue that the 2 PM summer design hour as observed on May 27th, 2003 represents an extreme condition and that a reasonable degree of comfort is achieved for most times during a typical year. Until other additional improvements have been explored, however, we conclude that the hypothesis has not been verified.

The failure to verify the hypothesis with certainty, along with an estimated high cost of installing a third layer of fabric, lead the new users to make the decision to replace the fabric roof. The methods used in this analysis proved to be a set of adequate, yet relatively simple and straightforward tools for this type of task where both thermal comfort and annual energy performance were investigated. Upon the approval of the new owners, data acquisition will be set up again for one day during early summer 2005. This new data set, along with accurate specifications of the actual improvements as built 2005, will be used to assess the energy performance and thermal comfort of the space with a higher degree of accuracy. Fig. 9: Radiant temperature asymmetry. Reproduced from Stein & Reynolds page 52 (3). Table 1: Summary of surface temperatures (measured and calculated) and energy performance (simulated using Energy-10). REFERENCES (1) The Regents of University of California. Thermal Comfort of UFAD Systems. Sourced on 23 rd November 2004 from http://www.cbe.berkeley.edu/underfloorair/thermalcomfor t.htm (2) Several magazine articles: Dimensions, Vol.12; Dwell, October 2000; Dwell, 2001; Architecture, May 2002; and others. (3) Stein B, John S. Reynolds, Mechanical and Electrical Equipment for Buildings, 9th Edition.