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THERMAL PERFORMANCE COMPARISON OF ALTERNATIVE BUILDING ENVELOPE SYSTEMS:
An Analysis of Five Residences in the Community of CIVANO.
Nader V. Chalfoun, Ph.D. and Richard J. Michal P.E.
University of Arizona College of Architecture, Planning, and Landscape Architecture [email protected] and [email protected]
Abstract This paper describes the impact of alternative building material envelope systems on the overall thermal performance of five selected residences in Civano; a residential development in Tucson, Arizona. These systems included adobe, Integra block, strawbale, metal stud framing with rigid insulation (Heydon system) and structurally insulated panels (SIPs). After thorough on-site investigation and data collection by the House Energy Doctor (HED) team, the information, along with regional weather data, were input into the CalPas3 energy simulation software for thermal performance evaluation. While all of the five homes studied outperformed the average for Tucson home heating and cooling energy use and were within compliance with the 1995 Model Energy Code (MEC), three of the homes were found to exceed the energy cost per square foot source consumption guidelines required under the 1997 sustainable energy standards (SES). It was also determined that at least in the cases studied, the performance of each of the alternative building material envelope systems exhibited common design deficiencies while attempting to address issues not necessarily focused on energy conservation and solar technology. These deficiencies, related to orientation, placement of windows, insulation and shading, are also discussed in this paper. Introduction Energy consumption in the United States costs $450 billion each year. The growing awareness of the importance of energy conservation has resulted in savings in the United States of $160 billion annually since 1973 [1]. One of the areas related to energy conservation and of particular interest within the field of architecture is the development of non-traditional or alternative building materials. The purpose of this paper is to demonstrate the thermal performance, as measured by the students and faculty of the “House Energy Doctor” Program (HED), of five single family residences each utilizing alternative building materials. The five residences chosen for analysis by the HED program each represents a unique alternative building envelope system. Each of these systems is intended to improve the overall thermal performance and therefore reduce energy consumption. The authors assume that this paper will serve as a basis to begin to compare the effectiveness of each of these unique systems and to discuss additional factors that may have affected their performance. The House Energy Doctor The “House Energy Doctor” is a program at the University of Arizona’s College of Architecture that provides education and research experiences for students as well as no cost energy consultation and design prescription services for home owners. The advanced methods taught to students provide them with the necessary skills to conduct residential energy analysis and performance predictions utilizing computer simulation. Among others, the CalPas3 computer energy simulation software was used in this
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analysis [2]. The House Energy Doctor residential analysis is performed through a ten step procedure starting from the site survey and ending with conclusions and recommendations for improving energy consumption of buildings [3].
Figure 1: House Energy Doctor Students conducting site surveys (Photos by Dr. Chalfoun)
Project Description and Background During the spring semester of 2002, students and faculty in the HED program selected CIVANO as the site for investigation of the thermal performance of alternative construction materials. CIVANO is a residential development located in Tucson, Arizona intended in part to showcase passive solar and energy conservation designs in
Figure 2: CIVANO Community, looking north (Photo by Dr. Chalfoun, 2001)
residential housing. While CIVANO was originally envisioned to be the “Solar Village,” the project was subsequently modified to incorporate “Neo-Traditional” or “New Urbanism” design characteristics such as increased landscaping areas, narrower curvilinear streets, wider pedestrian pathways and sidewalks with landscaping buffers and the use of front porches to connect with the streetscapes and pedestrian pathways. It is the authors’ view that while these added design criteria provided some benefits for the community as a whole, they also may have resulted in some compromises in the thermal performance of some of the homes within the community. The five residences selected for the study, each utilizes a unique alternative building material envelope system. These systems included adobe, Integra block, strawbale, metal stud framing with rigid insulation (Heydon system) and structurally insulated panels (SIPs).
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Table 1: Selected Residences Residence Lot# Envelope System Mechanical System 1. Strawbale 4 2-string straw bale 14x18x40 SEER12, HSPF7 Elec. heat pump 2. Adobe 123 14” Adobe + exterior insulation SEER18 AC & radiant floor heat 3. SIPs 46 4” SIPs + 1” foam SEER12 AC & hydroponic heat 4. Heydon 6 Steel Frame w/ rigid insul. SEER12, HSPF 7 Elec.heat pump 5. Integra Blockr 96 8” integra block SEER12 AC & gas furnace Figure 3: Map of Neighborhood 1 of the Community of CIVANO
96
4
123
6
46
4
Strawbale Residence The strawbale residence is a 1386 square foot single story post and beam straw bale home. The home includes two bedrooms, two baths a kitchen and family/living/dining room with concrete floors throughout. The roof system is galvanized corrugated steel on roof trusses with 10” blown-in cellulose insulation. The home has a two car detached garage. The mechanical system is a SEER12 electric heat pump for heating and cooling. Figure 4: Strawbale house
The envelope system incorporates 14”x18”x40” tied bales of straw infill within a post and timber structural frame to form 18” thick walls with an additional 2” of cement plaster on each side. The effective overall R value of this envelope system is approximately R-16. Figure 5: Detail of Bales Heydon System Residence Figure 6: The Heydon Envelop System
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The Heydon System residence is a 1,842 square foot single family dwelling. The home consists of three bedrooms, two bathrooms, a kitchen, a living room and a dining room with 10 foot ceilings throughout. The wall construction is the Heydon System discussed below. The exterior of the home is made up of cement plaster walls and clay tile roofing. The mechanical system is an electric heat pump with SEER12, HSPF 7 efficiency. The Heydon System consists of steel stud framing with 4” rigid polystyrene foam insulation panels incorporated both between and on to the exterior face of the steel framing members. The overall insulating value of this wall system is R-25. Integra Block Residence
Figure 8: The Integra Block Residence The Integra Block house is a 1,680 square foot single family dwelling built with Integra Block. The home is currently detached but is intended to share common walls with future homes on both its east and west elevations. The home is two stories at the south end and slopes down to a single story at the north end forming a large vaulted ceiling sloping down to the north over the family room in the center. The home consists of two bedrooms, two and a half baths, a kitchen, large central family/living/dining room and a study. . The mechanical systems include an 80% efficient 3.5 ton gas furnace and a SEER 12 air conditioning unit. Integra block is a concrete masonry unit system designed to minimize thermal bridging by filling the masonry cells with urethane foam insulation and reducing the webbing between the two faces of the block. The stated R value of a properly installed 8” Integra block system is between R-16 and R-20. The advantage of the Integra block system is its duel role of providing both high R value and thermal mass within the envelope system.
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Adobe Residence
Figure 10: The Adobe Residence The adobe residence consists of two dwelling units, a primary residence as well as a guest house above the garage. Both units are oriented eight degrees east of south and share a radiant floor heating system. The primary residence (which is the focus of this paper) is 1,600 square feet, with 10’-0” ceilings, three bedrooms and two full baths. Floor construction is concrete slab on grade with radiant floor heating.. Exterior wall construction is 14” adobe with 1” paper based exterior plaster. Two natural gas water heaters and a solar pre-heater are used for domestic water heating as well as the radiant floor heating system. The roofing is 26 gauges galvanized corrugated steel on roof trusses with 10” blown-in cellulose insulation. The primary residence is cooled by a SEER18 air conditioning unit. The type of adobe blocks used in the Calhoun residence measure 10”x4”x14”, weigh approximately 40 pounds each and have an R-4.5 insulation value. The overall envelope system is approximately 15” thick with exposed adobe to the interior and a 1’ paper based exterior plaster applied to the exterior (R-3). Adobe represents a high thermal mass material but the effective R value of the envelope system including air films, was only approximated to be R-8. SIPs Panel Residence The SIPs Panel Residence is a 1,280 square foot single family residence consisting of two bedrooms, two bathrooms, and a family/kitchen/dining room. At the time of the analysis the home did not have a garage. The wall and roof construction are of SIPS or structurally insulated panel systems. The cooling system is a SEER12 air conditioning unit and the heating system is a hydronic system which relies on the homes gas fired domestic water heater as a hot water source used to heat the air delivered into the house through heat exchangers.
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Figure 11: The SIPs Residence
Structurally Insulated Panel System (SIPs) The structurally insulated panel system (SIPs) is made up of sheets of approximately 4” to 6” urethane insulation sandwiched between two sheets of ½” oriented strand board. In addition to the structurally insulated panels, 1” rigid insulation was applied to the exterior with lathe and a cement plaster or Dryvit system and ½” drywall was applied to the interior. The overall effective R value of the envelope system is R-24 for the 4” walls and R-32 for the 6” roof panels.. Figure 12: Detail of SIPs Panels Data Collection, Simulation and Correlation During the Spring of 2002, site surveys were conducted in order to measure and document the physical characteristics and energy usage of each of the homes. Information collected during these site surveys included building orientation, building materials, building areas and volumes, shading, ground reflectance, mechanical systems, infiltration, occupancy patterns, etc. A summary of some of the information collected during site surveys is provided in Table 2 in Appendix 1. This information was then used to create a computer input model or “Basecase” of each home. CalPas3 energy simulation software along with Tucson regional weather data were used to generate thermal performance profile for each “Basecase.” The results were then correlated to each of the homes’ actual energy usage for the most recent twelve months. The following table shows the output results of the “Basecase” models for each of the homes.
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Table 3A: Basecase and Code Compliance Envelope System Area Source Consumption* Cost MEC SES Ft2 Heating
KBTU/ft2.yr Cooling KBTU/ft2.yr
$/ft2 pass pass
Straw Bale 1386 9.0 22.7 0.25 Yes Yes Heydon (steel & foam) 1834 9.0 16.3 0.23 Yes Yes Integra 1685 15.8 25.6 0.41 Yes No Adobe (Insulated) 1600 15.1 36.2 0.47 Yes No SIPs 1290 7.5 43.5 0.51 No No * Source consumption accounts for energy loss due to generation and transmission. All five of the residences outperformed the average annual energy cost of $0.75 to $2.00 per square foot for homes built in Tucson prior to the 1995 [4]. One of the homes failed to comply with the Metropolitan Energy Code (MEC) standards of $0.50 to $0.60 per square foot [5]. In addition, three of the homes did not comply with the Sustainable Energy Standard (SES) value of $0.23 to $0.36 per square foot. The SES requires 50% more energy savings than the MEC. All homes within CIVANO are required to show that they will be able to exceed the SES prior to their construction. It was determined that individual deficiencies existed in three of the homes, the Integra, adobe and SIPs. It was necessary to identify and account for these deficiencies if possible prior to attempting to analyze the performance of each of the alternative building envelope systems. Findings of Individual Deficiencies With regard to the first two of these residences, the adobe residence and the Integra block residence, individual deficiencies were identified and accounted for within the computer energy models to allow the simulation output results to be updated. Two deficiencies were identified within the adobe residence. The first was that the 24” perimeter slab rigid insulation had not been installed along the perimeter of the residence. The adverse heat loss/gain via conduction through the slab edge was further compounded by the use of the radiant floor heating system in the slab. The second deficiency was discovered during the on-site audit. During the blower door test infiltration/leakage levels twice the acceptable levels of 0.5 air changes per hour were recorded. It was determined that this infiltration was the result of warping of the vinyl window track moldings in the home’s south facing windows due to exposure to the sun. These two deficiencies combined were determined to result in increased heating and cooling source consumption levels of 42% and 27% respectively. The primary deficiency in the Integra block residence related to the performance of the envelope system itself. The Weber residence relies on the Integra block system for insulation and prevention of thermal bridging therefore no exterior insulation was added. Unfortunately, due to the use of improper materials and poor installation, it was found that the system was not achieving its specified insulating value. This combined with the lack of exterior insulation has led to significant thermal bridging and heat/loss gain through the exterior walls. This home was designed to share common walls with future homes along its east and west (long) facades which would reduce convective heat/loss gain to the outside. It was determined that the addition of the future homes sharing
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common walls with the Weber residence on the east and west facades would reduce the combined annual heating and cooling source consumption by 27.6% and 10.9% respectively. With regard to the SIPs residence, the cause of the deficiency was recently discovered (April 2003) to be a failure in the mechanical heating and cooling system. There was not attempt to revise the results while the problem persisted. The following table shows the updated output results for all of the homes with the exception of the SIPs residence: Table 3B: Adjusted Base Case and Code Compliance Envelope System Area Source Consumption Cost MEC SES Ft2 Heating
KBTU/ft2.yr Cooling KBTU/ft2.yr
$/ft2 pass pass
Straw Bale 1386 9.0 22.7 0.25 Yes Yes Heydon (steel & Foam) 1834 9.0 16.3 0.23 Yes Yes Integra 1685 7.3 31.5 0.34 Yes Yes Adobe (Insulated) 1600 7.3 31.5 0.35 Yes Yes SIPs 1290 7.5 43.5 0.51 No No Findings Based upon the data presented in table 4 (Appendix 2), two fundamental points are apparent. First the four homes under consideration can be divided into two groups, high thermal mass/low R-value and low thermal mass/high R value. The strawbale and the Heydon system residences have higher R values (R-16 and R-25 respectively) and exhibit similar thermal performance (cooling loads of 36.8 MMBtu and 34.9 MMBtu respectively). The adobe and Integra block residences both have low R values (although the Integra block is intended to have a higher R value) of R-9 and R-8 respectively and are closer in their thermal performance with adjusted cooling loads of 45.3 MMBtu and 40.4 MMBtu respectively. The second fundamental point is that all four residences suffer from adverse solar orientation. The strawbale, Integra block and adobe are all oriented with their long axis facing east and west. This results in increased exposure to extreme summer solar gain from the east and west and reduced beneficial winter solar gain from the south. While the Heydon system residence appears to be more appropriately oriented with its long axis closer to facing due south, unfortunately the lack of windows prevent it from benefiting from winter solar access. Most of the south facing window area in this residence is shaded. In fact of the total 104 square feet of south facing window area 77square feet or 74% is covered under front and rear porches so that only two windows or a total of 27 square feet of south facing window area is allowed access to beneficial winter sun. Conclusions While the original intent of the authors was to attempt to isolate and compare the impact of alternative envelope systems on the thermal performance of the five residences, it is apparent that additional study and modeling iterations will be required in order to sufficiently factor out other variables. It is also apparent that in the future it may be necessary to select other residences for study. Three of the homes studied, the SIPs, adobe and Integra Block residences, may
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not be ideally suited for the study of the performance of envelope systems due to the existence of individual design or construction deficiencies as previously discussed. As a result of this effort, however, the authors are interested in further examining not only the relationships between envelope systems and thermal performance but the relationship and the possible interdependency between envelope system, fenestration (window placement and size), and building orientation. It appears that orientation and fenestration may have a significant influence on the high thermal mass envelope systems. If this is the case then it may prove as a significant factor in identifying potential energy savings in the placement and specification of buildings in future developments. As discussed previously, the development of CIVANO was originally intended to be the “Solar Village” to demonstrate passive solar and solar technologies. This concept was subsequently modified to allow the incorporation of aspects of “New Urbanism” or “Neo traditional” neighborhood design. The resulting plotting adversely impacted the solar orientation of the strawbale residence (lot 4), the Integra block residence (lot 96), and the adobe residence (lot 123). Of the five residences studied, this adverse orientation appears to have had a more adverse impact on the residences utilizing thermal mass alternative building envelope systems than the low mass high insulation residences. In future studies the authors would like to: 1) Identify any foregone energy savings resulting from adverse solar orientation of the homes in the community of CIVANO, and 2) Quantify the impact of orientation and fenestration on the thermal performance of high thermal mass low R value envelope systems and compare this against the impact of orientation and fenestration on low mass high R value envelope systems. References 1. Harvey, H. and Keepin, B. (1991). Energy From Crisis to Solution. The Energy
Foundation, January, San Francisco, CA, U.S.A. 2. CalPas3 (1982). Program and Users Manual. Berkeley Solar Group. Berkeley,
CA. U.S.A. 3. Chalfoun, N.V. (1991). The “House Energy Doctor©”; An Educational, Research
and Community Service Program at the College of Architecture, The University of Arizona. 1991 Design for Desert Living Symposium, Jul. 21-26, Tucson, AZ, U.S.A.
4. Chalfoun, N.V., Yoklic, M.R., and Kent, K.J. (1991). Passive Solar and Energy Optimization for a residential home type in Tucson, Arizona. A Case Study for the Solar Village Project. International Solar Energy Society (ISES), Auguest 17-24, Denver, Colorado, U.S.A.
5. CABO, Pima County Model Energy Code (1995). 6. CIVANO, Sustainable Energy Code (1998).
TABLE 2 SITE SURVEY DATA
House Energy Doctor
Strawbale Heydon syst Integra block Adobe block SIPs system
CATEGORIES12° E of South 16° E of South 24° E of South 8° E of South 20° E of South
DESCRIPTION BASE BASE BASE BASE BASESIZE Floor Area 1,386 1834 1685 1600 1290
Volume 13,860 18,340 21165 16000 15480
Surface Area (walls) 1,886 2040 3360 1672 2009
Surface Area (Roof) 3,449 3367 1103 1814 1290
Total Surface Area (W+R) 5,335 5407 4463 3486 3299
Surface to volume ratio 38.5% 29.5% 21.1% 21.8% 21.3%INSULATION Roof R Value R-44 R-39 R-42 R-25 R-30
Wall R Value R-16 R-25 R-9 R-8 R-24Perimeter Slab Insulation No No Yes No NA
FENESTRATION South Window Area 65 104 125 36 184North Window Area 53 87 45 39 10East Window Area 34 91 0 37 40West Window Area 69 53 178 15 27Total Window Area 221 335 348 127 261south window/flr area ration 4.7% 5.7% 7.4% 2.3% 14.3%Total window/floor area ratio 15.9% 18.3% 20.7% 7.9% 20.2%Double glazed low-e Yes Yes Yes Yes Yeswindow material vinyl vinyl vinyl wd/vinyl vinyl
REFLECTIVITY Roof Reflectivity 36% 36% 50% 36% 60%Wall Reflectivity 37% 50% 32% 40% 35%Ground Reflectivity 33% 33% 33% 33% 45%
BLOWER DOOR Infiltration (air changes/hr) 0.356 0.4 0.5 1 0.6MECHANICAL Cooling system ht pmp ht pmp AC AC ACSYSTEM Heating system ht pmp ht pmp furnace radiant hydro
TABLE 4 THERMAL PERFORMANCE FINDINGS
House Energy Doctor
Strawbale Heydon syst Integra block Adobe block SIPs system
CATEGORIES12° E of South 16° E of South 24° E of South 8° E of South 20° E of South
DESCRIPTION BASE BASE BASE Adjust* BASE Adjust* BASESIZE Floor Area 1,386 1834 1685 1600 1290
Volume 13,860 18,340 21165 16000 15480
Surface Area (walls) 1,886 2040 3360 2175 1672 2009
Surface Area (Roof) 3,449 3367 1103 1103 1814 1290
Total Surface Area (W+R) 5,335 5407 4463 3278 3486 3299
Surface to volume ratio 38.5% 29.5% 21.1% 15.5% 21.8% 21.3%INSULATION Roof R Value R-44 R-39 R-42 R-25 R-30
Wall R Value R-16 R-25 R-9 R-8 R-24Perimeter Slab Insulation No No Yes No Yes NA
FENESTRATION South Window Area 65 104 125 36 184North Window Area 53 87 45 39 10East Window Area 34 91 0 37 40West Window Area 69 53 178 15 27Total Window Area 221 335 348 127 261south window/flr area ration 4.7% 5.7% 7.4% 2.3% 14.3%Total window/floor area ratio 15.9% 18.3% 20.7% 7.9% 20.2%Double glazed low-e Yes Yes Yes Yes Yeswindow material vinyl vinyl vinyl wd/vinyl vinyl
REFLECTIVITY Roof Reflectivity 36% 36% 50% 36% 60%Wall Reflectivity 37% 50% 32% 40% 35%Ground Reflectivity 33% 33% 33% 33% 45%
BLOWER DOOR Infiltration (air changes/hr) 0.356 0.4 0.5 1 0.5 0.6MECHANICAL Cooling system ht pmp ht pmp AC AC 0.6 ACSYSTEM Heating system ht pmp ht pmp furnace radiant 0.6 hydro
THERMAL Cooling load (kBtu) 36815 34910 50,906 45,374 46,314 40,364 NAPERFORMANCE Heating load (kBtu) 12601 16513 19,274 13,949 16,142 8,912 NASOURCE Cooling (kBtu/s.f.) 22.8 16.3 25.5 22.8 36.2 31.5 NACONSUMPTION Heating (kBtu/s.f.) 9.1 9 15.9 11.5 13.3 7.3 NA
Total Cost (heating+cooling) $362 $431 $694 $569 $705 $561 NATotal Cost / s.f. $0.26 $0.23 $0.41 $0.34 $0.44 $0.35 NA
* Values adjusted to factor out individual design and/or installation deficiencies within the Integra block and adobe residences.
