Introduction Recently, the design of residential buildings has become an area of increased research due to the potential to reduce greenhouse gas emissions in this sector of building construction. According to the Energy Information Agency, in 2008 approximately 40% of the total U.S. energy consumption was due to buildings. Of that 40%, 22% was caused by residential consumption. * Through better design environmental factors such as a buildings use phase energy consumption can be decreased. There has been extensive amount of work done to determine what some of these design improvements may be. Smeds et al. notes that buildings with a comfortable indoor climate and a low energy use share the following characteristics applied in unison: an area to volume ratio resembling that of a globe, thermal insulation, an extremely air tight building envelope, use of passive solar gains through windows while avoiding transmission losses, and consideration of energy conservation measures in the first step in the design process.* On thermal insulation Smeds et al. continues to note that insulation thickness depends on building geometry and internal loads and that building a wellinsulated envelope is essential because the envelope is the most expensive building component to retrofit. * This project is primarily concerned with determining the energy consumption of a residential building of wood frame construction and of insulated concrete forms (ICF)an alternative construction. The energy use during building operation is responsible for far more energy consumption than either the pre-use or postuse phase*. To accurately assess the use phase of a building it is necessary to consider the following in an energy model: Heating, Ventilation, and Air Conditioning (HVAC) system Lighting Domestic Hot Water (DHW) use Appliance and plug loads However, operating phase energy use alone is not the sole determinant in the environmental impact of a building. To determine this impact, a life cycle assessment (LCA) will utilize the modeled building performance to asses each building’s: natural resource depletion, greenhouse gas emissions, and waste production. LCA comparison of wood frame to ICF multifamily buildings is one third of a research study being performed by the Massachusetts Institute of Technology (MIT) on behalf of Cemex a global supplier and manufacturer of concrete products. The concrete construction industry has reached a low-point in residential construction as the current housing market remains stagnant. Wood frame construction is the most common construction type for both single family and multifamily residential buildings* while ICF construction is a less widely used method* despite exhibiting considerable thermal mass and inherent insulation. To evaluate performance claims made by the industry to support ICF construction each building model will be simulated with climate data for Phoenix, AZ and Chicago, IL. Unfortunately, construction is not governed by an overarching building code and varies from state to state even to the county level. The U.S. Department of Energy divides the nation into eight climactic zones with further subdivision by county. Phoenix lies in climactic zone 2A while Chicago lies in 5B. The American Society of Heating, Refrigeration, Air conditioning Engineers (ASHRAE) has developed an extensive building code that accounts for the D.O.E. climactic zones. ICF construction is the same across zones while wood frame construction has a large margin of building practices. Each building is modeled to meet the requirements of the 2007 ASHRAE handbook for low rise buildings. Methods A single building type with a fixed geometry is studied. The building type is a low rise multifamily residential building. The building has a total floor area of 1076 m2 and is composed of four stories each 3.05 m high. There are a total of 24 apartment units of varying floor areas, six on each floor, and two main circulation cores. Glazing ratio is set to 18%. The occupancy, plug loads, activity schedule, DHW use schedule, HVAC use schedule, lighting schedule, and respective set points are modeled according to DOE Benchmark Multifamily building*. Construction of wood frame and mass buildings are made according to ASHRAE 90.1-2007*where applicable. Phoenix and Chicago are chosen as reference locations for each building. Site orientation is rotated 90° for each construction type for each city. Weather data is provided by D.O.E. for the reference sites, which provides solar radiation and outdoor temperature. Energy simulations are done using Design Builder, a UK based modeling software that uses the DOE’s EnergyPlus engine to perform model calculations. The climate data from D.O.E. is used as an input file to Design Builder. Simulation results from Design Builder yield annual and monthly values for the chiller system energy, heat generation energy, total electricity, total gas, sensible cooling, total cooling, and zone heating. Hourly data is examined in Chicago during a typical summer and typical winter week. Wood Frame multifamily building The exterior walls are a wooden frame construction using platform framing. Studs are a standard 2”x6” and set at 24 in. on center*. Fiberglass board is the insulating material. Windows are double glazed, clear, with an air gap. The roof is flat with fiberglass insulation entirely above deck. The floor construction is an unheated slab on ground without a basement or any walls below grade. The U-values of the components are shown in Table 1. Insulated concrete forms multifamily building Exterior walls are made of insulated concrete forms. The modeled forms used 2.5” of Extruded Polystyrene on both the interior and exterior, steel reinforcement both horizontally and vertically and eight inches of cast in place concrete*. Windows and frame are the same as for wood frame construction. Again, the roof is flat with insulation entirely above deck. There is no cavity beneath the unheated slab on ground floor construction. Component U-values are shown in Table 1. Heating, ventilation and air conditioning Mechanical and natural ventilation are modeled as off. A unitary fan coil system is used with electric cooling and gas heating. A four pipe fan coil is simulated and does not include an economizer. In lieu of mechanical ventilation, a total building air infiltration of 0.7 ach and 0.56 ach is used for wood frame and ICF construction respectively. Performance coefficients set to ASHRAE standard and electricity use, gas use, chiller energy and heat generation energy simulated by Design Builder are shown in Fig 3-4. Annually the ICF construction is seen to have an average savings of 214% in the energy necessary for heat generation as well as a 9.6% reduction in the total energy from gas expended. The ICF home however uses 4.77% more energy through the chiller, but still saves .2% of the total energy from electricity. Chicago Simulated space heating/cooling demand Simulated total energy demand, chiller energy & heat generation energy Results Phoenix Simulated space-heating/cooling demand A comparison of the wood frame construction to ICF construction space heating and cooling demand simulated by Design Builder is shown in Fig. 1-2. Annual comparison of the wood frame construction to the ICF construction shows savings as much as 254% in heating demand while increasing cooling demand by as much as 5.16% for the building oriented at 90°. On average the sensible cooling and total cooling load were increased by 4.74% in the ICF building while zone heating was decreased 214%. Simulated total energy demand, chiller energy & heat generation energy A comparison of the wood frame construction to ICF construction total Discussion The performance modeling of buildings is a necessary step in building design. Modeling allows engineers and architects the ability to decide which variables of design are influencing the use-phase energy consumption. As mentioned earlier, there are a variety of building factors that must be considered simultaneously to improve the energy efficiency of a building. In this project the main tested factors were building insulation and air infiltration. Thermal insulation of wood frame homes is a variable that changes with the building code followed. For the purposes of this project, ASHRAE 90.1-2007 was used and the prescribed minimum insulation values could be seen to closely mimic the insulation values for ICF construction. There is difficulty in transferring the code standard into an input file to be analyzed by the simulation software. A layer by layer replication of actual building practice is not possible in the Design Builder software because the framing and insulation in the cavity section of a wall or roof cannot be defined. To find the insulation of the cavity, two possible methods are lifted from ASHRAE 90.1-2007. The first method uses the rated Rvalue of the insulation described for a given climate zone to look up what the effective insulation in a cavity will be based on framing type. This method is simple to use as the effective cavity insulation values reside in A9.4C of the code. It is assumed that ASHRAE has calculated the 3dimensional heat transfer necessary to create such a table and therefore removes the need to do such calculations. In Design Builder the cavity is modeled as a single fiberglass layer with an R-value equivalent to Table A9.4C listing for the effective cavity Rvalue. Another method is described in ASHRAE and that is to calculate the cavity R-value through parallel addition of Rvalues in the wall or roof assembly. The percentages of cavity space that are 2”x6” wood stud and insulation are multiplied by their respective component R-values based on a specific framing factor. In Design Builder a new material class is created with the specified R-value being the value previously calculated. Layer by layer build up is done around the modeled cavity layer to produce the wall construction. The buildings in this project were modeled using the first method for determining the cavity insulation. Ventilation and infiltration is also a major determinant in how much energy the modeled HVAC system will use. Ventilation is scheduled air that is brought in to the environment through the building fans while infiltration is outside air that enters the building directly through the environment without pre-treating. Air infiltration is due to imbalances in pressure at various points in the building envelope and the three sources of pressure are: wind pressure, stack pressure and HVAC fan pressure*. These simulations the air infiltration was dominated by wind pressure due to each building being low rise and the absence of mechanical pressurization of the residential building. Several national laboratories and the National Institute of Standards and Technologies have performed investigations of building air tightness*. Design Builder allows modeling of air infiltration in terms of air changes per hour. Currently the author is working on calculating the air leakage of the chosen building geometry. The infiltration values used are based on an Oak Ridge National Laboratory study that assumed 0.7 AC/h for a wood frame single family home and compared this to a ICF single family home, which used a 20% reduction of uncontrolled air infiltration*. Air infiltration testing of single family ICF homes has been undertaken by the Building Technology lab to arrive at measured values of air infiltration. Acknowledgements I would like to thank Ms. Monica Orta and Dean Christopher Jones of the Office of the Dean of Graduate Education for hosting MSRP as well as the PA’s and staff. I also would like to thank Dr. Ochsendorf and Dr. Norford for mentoring me during the project and my supervisors Ms. Andrea Love, Ms. Marzena Fydrych, and Mr. Jason Tapia. References