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...measurements that these buildings are subject to larger infiltration rates than commonly believed (Persily 1998; Proskiw and Phillips 2001). Infiltration in commercial buildings can have many negative consequences, including reduced thermal comfort, interference with the proper operation of mechanical ventilation systems, degraded indoor air quality, moisture damage of building envelope components, and increased energy consumption. For these reasons, attention has been given to methods of improving airtightness both in existing buildings and new construction (Persily 1993). Since 1997, the Building Environment and Thermal Envelope Council of the National Institute of Building Sciences has sponsored several symposia in the US on the topic of air barriers for buildings in North American climates. Canada Mortgage and Housing Corporation has sponsored similar conferences in Canada. Others have also published articles on the importance of air leakage in commercial buildings (Anis 2001; Ask 2003; Fennell and Haehnel 2005; Lstiburek 2005). However, the focus of these conferences and publications has largely been air barrier technology and the nonenergy impacts of air leakage in buildings. In order to evaluate the cost-effectiveness of such measures to tighten buildings, estimates of the impact of air leakage on energy use are needed.
An earlier study estimated the national impact of infiltration in US office buildings based on a simplified method for calculating both the infiltration rates and the associated building energy use (Emmerich et al. 1995). The loads were calculated for a set of 25 buildings, each representing a certain percentage of the total office building stock of the United States. Twenty of these buildings represent the existing office building stock as of 1979 (Briggs et al. 1992) and five represent construction between 1980 and 1995 (Crawley and Schliesing 1992). Further work improved on this initial method by using airflows from multizone airflow simulations (Emmerich and Persily 1998) combined with a simple load calculation. More recently, a more detailed method of analysis to determine the impact of infiltration and ventilation rates on building energy usage was developed (McDowell et al. 2003). This approach is based on the coupling of a detailed multizone airflow model based on the CONTAM simulation program (Dols and Walton 2002) and the detailed multizone building energy modeling program TRNSYS (Klein 2000). This project demonstrated the ability of the coupled programs to estimate the annual heating and cooling energy use in the US office building stock as a function of infiltration and ventilation rates.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE 2004) Standard 90.1 Envelope Subcommittee has formed a task group to consider updating the building air leakage requirements in the standard to require a continuous air barrier system. An air barrier system is the combination of interconnected materials, flexible joint systems, and components of the building envelope that provide the airtightness of the building. Included in the current standard are detailed quantitative limits for air leakage through fenestration and doors but only general qualitative guidance for the opaque portion of the building envelope (ASHRAE 2004). For example, the standard requires sealing, caulking, gasketing, or weather-stripping such locations as joints around fenestration and doors, junctions between floors, walls, and roofs, etc. However, there is no quantitative air leakage limit specified for either the wall and other envelope components or the building as a whole. This is analogous to requiring that care be taken when installing insulation without requiring any minimum R-value. The 90.1 Envelope Subcommittee has been considering a quantitative air barrier requirement but needed more technical information to support their discussions.
METHOD OF ANALYSIS
To provide input to the ASHRAE 90.1 Envelope Subcommittee in its consideration of the potential energy savings and cost-effectiveness of an effective air barrier requirement, annual energy simulations and example cost estimates were prepared for three common, modern nonresidential buildings--a two-story office building, a one-story retail building, and a four-story apartment building. The apartment building was included in the study because the scope of Standard 90.1 includes multi-family structures of more than three stories above grade. The apartment building results are not included in this paper due to space limitations but can be found in the complete report of this work (Emmerich et al. 2005). The combined airflow-building energy modeling tool (McDowell et al. 2003) was used to estimate the energy impact of envelope airtightness in multiple US climate types. HVAC systems representative of the types used in these buildings were included in the building models. Other building model parameters were chosen such that the buildings would be considered typical new construction and meet current ASHRAE Standard 90.1 requirements.
Energy simulations were performed using TRNSYS (Klein 2000), a transient system simulation program with a modular structure that was designed to solve complex energy system problems by dividing the problem into a series of smaller components. Each of these components can then be solved independently and coupled with other components to simulate and solve the larger system problem. Components (or "Types" as they are called in TRNSYS) may be as simple as a pump or pipe or as complicated as a multizone building model. The entire program is then a collection of energy system component models grouped around a simulation engine (solver). The modular nature of the program makes it easier to add content to the program by introducing new component models to the standard package. The simulation engine provides the capability of interconnecting system components in any desired manner, solves the resulting systems of equations, and facilitates inputs and outputs. The TRNSYS multizone building model (called "Type 56") includes heat transfer by conduction, convection, and radiation, heat gains due to the presence of occupants and equipment, and the storage of heat in the room air and building mass.
The infiltration in the buildings was modeled using a TRNSYS type based on an updated version of the AIRNET model (Walton 1989), which is included in the multizone airflow and contaminant dispersal program CONTAM (Dols and Walton 2002). CONTAM combines the best available algorithms for modeling airflow and contaminant transport in multizone buildings with a graphic interface for data input and display of results. The multizone approach is implemented by constructing a network of elements describing the flow paths (HVAC ducts, doors, windows, cracks, etc.) between the zones of a building. The network nodes represent the zones, each of which is modeled at a uniform temperature and pollutant concentration. The pressures in the zones vary hydrostatically, so the zone pressure values are a function of the elevation within the zone. The model accounts for the wind-driven and temperature-driven pressures that cause infiltration to occur. See chapter 27 of the ASHRAE Handbook--Fundamentals (ASHRAE 2005) for more detail on wind-driven and temperature-driven infiltration. The network of equations is then solved at each time step of the simulation.
The CONTAM airflow model and Type 56 energy model were coupled via the TRNSYS simulation engine. At each time step, each model would successively iterate to a solution and then pass needed data (i.e., airflows or temperatures) to the other model. This would continue until both models reached a satisfactory solution. McDowell et al. (2003) described the coupling of the TRNSYS and CONTAM models in more detail.
Simulations of annual energy use were run using TMY2 files (Marion and Urban 1995) for five cities representing different climatic zones of the US (Miami, FL; Phoenix, AZ; St. Louis, MO; Bismarck, ND; and Minneapolis, MN) and at three levels of airtightness representing different construction practices. The levels of airtightness were selected to represent (1) no air barrier, (2) target air barrier, and (3) best achievable levels. The airtightness values were based on a review of measured commercial building airtightness data (Persily 1998), ASHRAE Handbook data (ASHRAE 2005), and other sources. Each building was modeled with frame construction and with masonry construction. Thus, the matrix of simulations reported in this paper is 2 building types x 2 envelope construction types x 3 airtightness levels x 5 climates for a total of 60 simulation cases.
Building Descriptions: Thermal Properties, Setpoints, and Schedules
The building models were developed so that the envelope constructions would satisfy the requirements of ASHRAE Standard 90.1 (ASHRAE 2004). Emmerich et al. (2005) list the detailed wall, roof, slab, and window thermal properties used in the models.
Office Building. The two-story office building modeled in this study has a total floor area of 2250 [m.sup.2] (24,200 [ft.sup.2]) and a floor plan as shown in Figure 1. The building has a window-to-wall ratio of 0.2 with a floor-to-floor height of 3.66 m (12 ft), broken up between a 2.74 m (9 ft) occupied floor and a 0.92 m (3 ft) plenum per floor. The building also includes a single elevator shaft.
[FIGURE 1 OMITTED]
The internal gains for the occupied spaces are divided into three parts: lighting, receptacle loads, and occupants. These gains are all applied using a peak value and fraction of peak schedule. The lighting peak is 10.8 W/[m.sup.2] (1.0 W/[ft.sup.2]), the peak receptacle load is 6.8 W/[m.sup.2] (0.63 W/[ft.sup.2]), and the peak occupancy density is 53 persons/1000 [m.sup.2] (5 persons/1000 [ft.sup.2]). The fraction of peak schedules are shown in Figures 2 to 4.
The thermostats operate on a setpoint with setback/setup basis. The heating setpoint is 21.1[degrees]C (70[degrees]F) with a setback temperature of 12.8[degrees]C (55[degrees]F), and the cooling setpoint is 23.9[degrees]C (75[degrees]F) with a setup temperature of 32.2[degrees]C (90[degrees]F). The schedule for the setback/setup differs between weekdays (hours from 0600 to 2000 at setpoint), Saturdays (hours from 0700 to 1400 at setpoint), and Sundays (always at setup/ setback). However, for the first hour of operation at setpoint, the system does not bring any outdoor air into the zone. This hour is pre-occupancy of the building and is used to bring the zone back to setpoint from the setup/setback temperature.
Retail Building. The retail building modeled in this study is a one-story building with a total floor area of 1125 [m.sup.2] (12,100 [ft.sup.2]) and a floor plan as shown in Figure 5. The building has a window-to-wall ratio of 0.1 with a floor-to-floor height of 3.9 m (13 ft), broken up between a 3.0 m (10 ft) occupied floor and a 0.9 m (3 ft) plenum per floor.
[FIGURE 5 OMITTED]
The internal gains for the occupied spaces are divided into three parts: lighting, receptacle loads, and occupants. These gains are all applied using a peak value and fraction of peak schedule. The lighting peak is 16.2 W/[m.sup.2] (1.5 W/[ft.sup.2]), the peak receptacle load is 2.6 W/[m.sup.2] (0.24 W/[ft.sup.2]), and the peak occupancy density is 162 persons/1000 [m.sup.2] (15 persons/1000 [ft.sup.2]). The fraction of peak schedules are shown in Figures 6 to 8.
The thermostats operate on a setpoint with setback/setup basis. The heating setpoint is 21.1[degrees]C (70[degrees]F) with a setback temperature of 12.8[degrees]C (55[degrees]F), and the cooling setpoint is 23.9[degrees]C (75[degrees]F) with a setup temperature of 37.2[degrees]C (99[degrees]F). The schedule for the setback/setup differs between weekdays (hours from 0700 to 2100 at setpoint), Saturdays (hours from 0700 to 2100 at setpoint), and Sundays (hours from 0900 to 1900 at setpoint). However, for the first hour of operation at setpoint, the system does not bring any outside air into the zone. This hour is prior to building occupancy and is used to bring the zone back to setpoint from the setup/setback temperature.
Building Airflow Models
This section provides details of the airtightness levels used in the study. Three different airtightness levels ("no air barrier," "target," and "best achievable") were modeled in each building by changing the leakage characteristics in the CONTAM multizone airflow models for each building. The values for the "no air barrier" level varied for each location, while the "target" and "best achievable" construction cases were the same for all locations. The values for the "no air barrier" (i.e., baseline) case were established through an analysis of the available published airtightness data for buildings other than low-rise residential buildings.
The majority of the data were compiled in a 1998 summary (Persily 1998), supplemented by additional data from Florida commercial buildings, UK office buildings, and Canadian apartment buildings The entire dataset of 166 buildings (144 in North America and 22 in UK) and the references are presented in Appendix A of Emmerich et al. (2005). This dataset includes all data from the ASHRAE Handbook--Fundamentals (ASHRAE 2005) but adds data from over 150 additional buildings reported in 13 different studies. Since these data are intended to represent only the baseline case, no buildings known to have been constructed to a specific airtightness level are included. Most of the air leakage rates in the dataset were determined using ASTM E779 fan pressurization tests (ASTM 1999). Others were tested by very similar methods such as Canadian (CGSB 1986, 1999), international (ISO 1996), or British (CIBSE 2000) standards. Proskiw and Philips (2001) summarize and compare these and other current or proposed building airtightness testing methods.
[FIGURE 2 OMITTED]
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[FIGURE 8 OMITTED]
Based on the available information, the dataset was adjusted by excluding buildings older than 1960 (even though examination of...
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