Residential Retrofits Achieve Net-Zero Energy

An approach to lowering carbon footprints in older homes

As climate concerns continue to dominate the global agenda, net-zero energy has become a highly sought after goal in today's buildings. Net-zero energy buildings, which produce as much energy as they use over the course of a year, stand to transform how energy is generated and used in the built environment. Sophisticated construction technologies, renewable energy systems, and rigorous research are increasingly making net-zero energy buildings feasible.

In the residential sector, the move to net-zero energy homes is gaining steam, with states taking regulatory measures to encourage their construction, and national home builders adding net-zero homes to their portfolios. While many homes built today do offer impressive energy savings over years past, net zero is not the exclusive purview of new residential construction. The massive stock of existing buildings has received little in the way of energy retrofits, and represents tremendous potential for significant energy savings and even net-zero targets—important goals both in terms of a sound monetary investment for the owner and a reduction in the structure's carbon footprint and contribution to greenhouse gases.

What better way to examine the potential of residential retrofits than through a real-world example. Researchers and building industry partners involved in a deep energy retrofit of an older Midwest dwelling have shown that the right retrofit actions can reduce energy use by at least 50 percent, and are demonstrating a path to net-zero energy production. This article will focus on that experience, detailing the baseline conditions as well as the model that was developed and compared to collected data at the project site. Finally, the article will show how the model was modified and used to predict the effectiveness of various retrofit practices through minimizing the annualized energy-related cost in order to select the optimum retrofit package.

Existing Housing Stock—A Massive but Challenging Opportunity for Energy Savings

According to the U.S. Department of Energy (DOE), residential housing units account for 22 percent of the total primary energy usage in the U.S.¹ The average age of a single-family home in the U.S. is 34 years old, meaning that much of the existing housing stock was constructed in an era that offered both relatively inexpensive energy and did not consider carbon dioxide as a form of pollution that contributes to global warming. Consequently, simple energy-efficiency measures are sorely lacking in these homes— a problem with far-reaching consequences, but also an opportunity for substantial energy savings.

Buildings, in general, represent a substantial amount of energy requirements in the economy, making them of critical importance in reducing greenhouse gas emissions. Many experts and studies have weighed in on that point. “Improving the efficiency of buildings, which account for 40 percent of U.S. energy use, is truly low-hanging fruit,” maintains former U.S. Secretary of Energy Dr. Steven Chu. Global management consulting firm McKinsey and Company says, “Energy efficiency offers a vast, low-cost energy resource for the U.S. economy, but only if the nation can craft a comprehensive and innovative approach to unlock it.”

According to a study by the Regulatory Assistance Project (RAP),² a global, non-profit team of experts focused on the long-term economic and environmental sustainability of the power and natural gas sectors, “retrofit improvements to the heating and cooling systems of existing homes and their thermal envelope (e.g. by increasing insulation levels and reducing air leakage) present major opportunities for cost-effective investments in efficiency.” In fact, the RAP study maintains that “roughly half of all efficiency and/or carbon emission reduction potential in North American and European buildings is associated with retrofit improvements to existing homes.”

General, fundamental rules on retrofitting a house do exist; however, many different improvements can be applied, with the optimum solution typically predicated on the previous conditions of the house and on the climate zone where the house is located. In cold climates with high heating loads, measures that reduce those loads such as air sealing, adding insulation, and improving the energy performance of windows may be good first steps. In warmer climates, plug loads such as appliances, electronics, and lighting can be relatively more important, and taking action to reduce these will be more appropriate.

Software Base line Scenario Inputs

INPUT PARAMETER	INPUT CHOSEN
Location	West Lafayette, Indiana, USA
Square footage	266 m2
No. of Bedrooms	3
No. of Bathrooms	2
Age	86 years
Heating set point	21° C
Cooling set point	24.4° C
Humidity set point	50%
Walls	Wood stud, uninsulated, 40.6 cm
Exterior finishing	Wood, medium/dark
Unfinished attic	Uninsulated, vented
Finished roof	Uninsulated
Roof material	Asphalt shingles, dark
Finished basement	Uninsulated
Carpet	60% of the floor area
Windows	Single-pane, clear, non-metal frame
Air leakage	10 ACH50
Refrigerator	Energy Factor (EF) = 14.1, top freezer
Cooking range	Electric
Dishwasher	None
Clothes washer	Standard, MEF = 1.41 ft3/kWh-cycle
Clothes dryer	Electric
Lighting	20% fluorescent
Central air conditioner	SEER 10
Furnace	Gas, 80% AFU E
Water heater	Gas standard (EF = 0.59, 151 liters)

Retrofits are generally classified in two categories. Conventional energy retrofits focus on isolated system upgrades such as lighting and appliance replacement and are typically simple and quick. Actions that include improving the building envelope, such as better insulation or fenestration, are considered deep energy retrofits and they target energy savings of at least 30 percent or more. Deep energy retrofits constitute a whole-building analysis and construction process that uses “integrative design” to achieve much larger energy savings than their conventional counterparts. Most useful on buildings with poor efficiency, deep energy retrofits combine a variety of measures from energy-efficient equipment and air sealing to moisture management in order to attain both savings and performance.

By taking an integrative approach, and thus considering how the building functions as a complete system, the design team can make certain design decisions to optimize the overall energy efficiency at the lowest possible cost. To illustrate this point with an example, consider the task of heating and cooling a building. If the design team analyzes the building envelope choices along with the choice of HVAC system, they might decide to increase the insulation levels and upgrade the fenestration in order to reduce the heating and cooling load. This would then allow for a smaller and more efficient heating, ventilation, and air conditioning (HVAC) system to be specified, potentially at an even lower cost because many times HVAC system cost is proportional to size. In the extreme case, a team might make a building so airtight and insulated that only a small emergency heating system would be needed, essentially eliminating the need for a mechanical system, depending on climate region, building orientation, and fenestration arrangement. This extreme case is very difficult in a retrofit scenario as the building orientation and fenestration arrangement is predetermined.

Sound building science practices must be the foundation of deep energy retrofits. For example, installation of insulation must be considered not just in terms of R-value but its susceptibility to moisture infiltration along with how well the wall will dry should that occur. Location of the dewpoint, detailing, flashing, and sealing of windows and other building penetrations become critical concerns, especially in very warm and humid regions such as the Southeastern United States. In some cases, energy savings on the order of 50 percent or more are possible with a deep energy retrofit,³ but a rigorous approach is required, and the relationships among energy, indoor air quality, durability, and thermal comfort must be fully understood and accommodated for throughout the retrofit process.

The ReNEWW House—A Casefor Net-Zero Energy

Purdue University researchers along with several industry partners are involved in a multi-year project to transform a 1920s Craftsman-style home in West Lafayette, Indiana, into an ultra energy-efficient, net-zero energy residence.⁴ The so-called ReNEWW House (Retrofitted Net-zero Energy, Water and Waste) is being retrofitted to generate as much energy as it consumes over a year, creating a net-zero energy, water, and zero waste to landfill dwelling. Later phases will focus on the water system and waste streams in the home.

In its pre-study state, the home required about 12,000 kilowatt hours (kWh) of electricity and 50,000 kWh of thermal energy per year, costing some $3,000 in annual utility bills. The nearly 90-year-old structure earned a Home Energy Rating System (or HERS index) rating of 177. The HERS index, a standard of energy efficiency not unlike a miles-per-gallon rating for a car, was developed by the Residential Energy Service Network (RESNET) and is the nationally recognized system for evaluating a home's energy performance. Certified RESNET home energy raters conduct inspections to verify the specific features of a home that impact energy performance and use software tools to model the home energy use and generate the HERS index rating. The lower the HERS number, the more energy efficient the home. The DOE has determined that a typical resale home scores 130 on the HERS index, while a standard new home is awarded a rating of 100. For additional perspective, RESNET deems a house with a 140 HERS score “near the very top of the HERS index, a position a homeowner definitely doesn't want to be in,” and maintains that it is “performing 40 percent worse than a home adhering to the basic building code requirements. This is probably one of the major reasons for its high energy costs, less than ideal comfort level and, though one might not be directly aware of it, its negative impact on the environment.”⁵

At a HERS rating of 177, the ReNEWW House is 77 percent less efficient than a typical new build adhering to code. The objective of the retrofit project was not just to get that HERS rating to equal that of a new home, but to target a HERS index rating of 0, meaning that the home would produce at least as much energy than it uses on a yearly basis.

The Model—Simulating Usage and Retrofits

In selecting the proper suite of retrofits, the researchers' aim was to use an energy simulation tool to create a model, verify the model by matching the results with real-time energy usage data before the retrofit, perform an optimization analysis on the home to inform the retrofit actions required, and then use the model to predict the energy consumption post retrofit. The first step was to establish a baseline of the home's energy profile, including measurements of the structure and how much energy it currently consumes—information required to determine the best way to upgrade the home, whether it be window replacement, insulation, the addition of solar equipment, or other measures. After the house was taken over in July 2013, the baseline measurement system was installed and operational in November.

In the recent past, scores of increasingly sophisticated software solutions have been developed to provide energy modeling of residential buildings. Researchers at the ReNEWW House chose the software known as BEOpt (Building Energy Optimization), developed by the National Renewable Energy Laboratory (NREL), in support of the DOE Building America program goal to develop market-ready energy solutions for new and existing homes. BEOpt uses existing, established simulation engines (currently DOE-2.2 and EnergyPlus), and is able to run optimization analyses and recommend the most cost-effective improvements that can be applied. The software produces detailed simulation-based analysis and design optimization predicated on such house features as size, occupancy, age, location, and other factors. The objective of the optimization analysis is to minimize the annualized energy-related cost over a 30-year analysis period. The annualized energy-related cost (AERC) measure accounts for four major household cash flows which are loan costs (principal + interest) for performing the retrofit work, utility bills, replacement costs for when equipment such as a water heater wears out within the analysis period, and residual value of all depreciable equipment in the house at year 30 of the analysis period.

Energy modeling software is widely used to ascertain a home's energy performance, determine retrofit effectiveness, and size HVAC systems. Many experts agree that energy modeling is a solid investment and leads to good decision making. However, residential energy modeling, particularly of older homes, is not without drawbacks, and researchers at the ReNEWW House did acknowledge the limitations of the modeling process. This type of modeling is a complex process, with many required inputs that are often difficult to measure. Further complicating the process, each dwelling is different, increasing the difficulty of using standardized measures, and there is a wide discrepancy in retrofit costs among various markets and time periods. In addition, a key driver of energy costs is occupant behavior, which is extremely challenging to quantify. Even within a family, there can be a significant difference as to how a particular occupant sets comfort criteria, uses lighting, and regulates heating or cooling systems. Energy philosophies of occupants may differ widely, ranging from avid energy savers to ordinary consumers to those who are even wasteful or oblivious to energy usage. Occupant behavior in residential energy modeling, including BEOpt, typically follows the Building America Simulation Protocol which dictates usage patterns such as length of showers, temperature set points, appliance usage, etc. As a result, researchers did not use the model to dictate the outcome; rather, the energy models were used to inform decisions which were ultimately finalized by leveraging intuition. The model was not intended to be a pure retrofit case study.

In developing the baseline, researchers created a 3D model and selected the inputs that closely matched the dwelling's structural characteristics. Inputs that were selected were related to the geometry of the home, the envelope characteristics, the HVAC system, and any other device that uses energy, such as lighting fixtures and appliances.

There are four main factors that affect the energy consumption of a household: the building envelope itself; the HVAC system and hot water heater (collectively the mechanicals of the house); the end use devices, such as lighting and appliances; and human behavior. Inputs for the first three factors were relatively easy to select and represented fairly accurate parameters of the real condition of the existing home. Such inputs can be selected from a large library of predefined options embedded in the software. The fourth factor is human behavior, such as length of a shower, or appliance usage. Because these factors are so difficult to define, the software simulates human behavior from generally accepted assumptions based on NREL studies that sought to describe the average American family energy consumption. As previously mentioned, these behavioral assumptions are documented in the Building America Simulation Protocol. The table on page 154 highlights the main parameters chosen to simulate the pre-retrofit conditions of the test house.

As the ReNEWW house is located in a cold climate zone, understandably, heating demand represented the largest use of energy, with the baseline model predicting 70 percent of energy consumption was due to heating. The effects of the cold climate were exacerbated by the dwelling's lack of insulation and poorly sealed envelope compared to an average home in Illinois. Because recent data related to Indiana were not easily accessible, baseline model data was compared to a similar geographic and climate zone in the neighboring state of Illinois. The most noticeable difference between the two is the dissimilarity between the percentages of energy consumption due to the heating—72 percent for the test house and 51 percent for an average home in Illinois. This analysis showed early on that most of the energy savings would come from focusing on the building envelope. It is important to note, however, that in newer homes which typically have better building envelopes, space conditioning is becoming a less dominant factor in relative energy use; more attention might be better directed in a retrofit to other aspects of energy consumption, such as appliances, plug loads, and lighting.

Comparison of Real and Simulated Data

The baseline energy simulation results were then compared with real data collected by the instrumentation system that had been installed in the house. That monitoring system was installed with the aim of collecting real energy and water consumption data before and after the retrofit. Because retrofit options were predicated on the goal that living in a net-zero energy home requires no sacrifice in comfort or convenience, temperature and relative humidity were also monitored in many rooms.

To generate useful energy consumption data, the Building America Simulation Protocol was replicated for one week in March, and then extrapolated for a full year to determine a proxy for measured data. There was no measured cooling load due to retrofit work occurring during the cooling season. The accompanying table compares the baseline simulation and the real data; given the inputs that best match the test house, the simulation engine calculated the energy consumption of the household within 10 percent of the various measured end uses. For a residential energy simulation this is considered to be fairly accurate.⁷ The larger difference between the measured and simulated data for the appliance category could be due to a lack of specificity in available inputs in the BEOpt software as well as difficulty in replicating the exact usage schedule.

Optimization Results

Through software optimization, researchers can identify strategies and systems that will minimize the home's annualized energy-related cost (AERC). As previously mentioned, the AERC is calculated by annualizing the energy related cash flow over the 30-year analysis period. From the multitude of available retrofit actions, software can pinpoint the retrofit package that offers the lowest AERC with the best energy savings. BEOpt, in fact, uses a sequential search algorithm, which essentially runs all combinations of inputs to determine energy savings and annualized costs. In other words, in the optimization mode, the software can rate the best combination of all input characteristics, e.g., multiple wall heat transfer resistances in the range between R-11 and R-19, etc. Several factors affect the optimum retrofit solution in BEOpt of a single-family home, notably weather conditions, product cost, local labor rates, and energy and financing costs.

Naturally, where to spend retrofit dollars for maximum results is the linchpin of a sound plan, both in terms of the homeowner's return on investment and highest possible energy savings. For the ReNEWW House, the previous analyses demonstrated that the envelope needed particular attention due to the dwelling's high heating demand. While variations of the program inputs were considered for all the characteristics, the procedure was first used to focus on the optimal building envelope upgrades since it had already been determined that a ground source heat pump and solar system would be installed. The HVAC system variation was not considered in this simulation due to a limitation of the software that prevents comparing different types of HVAC equipment in the same optimization analysis (e.g. geothermal heat pump versus natural gas furnace). However, a separate simulation containing the new geothermal HVAC system was conducted in order to isolate the impact of the envelope's improvements.

As the final goal of the project was net-zero status, the HVAC system selected was a geothermal heat pump, due to its high efficiency use of electricity for heating and cooling. In use since the late 1940s, geothermal heat pumps utilize the earth's constant temperature as the exchange medium instead of the outside air temperature. According to the DOE, this allows the system “to reach fairly high efficiencies of 300 percent to 600 percent on the coldest winter nights, compared to 175 percent to 250 percent for air-source heat pumps on cool days.”⁸ As with any heat pump, the geothermal pump can heat, cool, and, in certain cases, supply hot water, and models with two-speed compressors and variable fans offer added comfort and energy savings. The DOE maintains that geothermal heat pumps offer advantages relative to air-source heat pumps, in terms of quiet operation, durability, and maintainability.

Once the options were selected, the energy simulation engine was able to compare all the different combinations of options to generate a trade-off curve. The trade-off curve shows the energy savings (x-axis below) and AERC (y-axis below) related to any single combination of options chosen, and enables the user to identify the best solution. The accompanying graph shows the trade-off curve. Each point in the graph represents a different combination of building parameters and their associated investment and AERC.

Note the point on the left side of the graph—it represents the baseline configuration. Therefore, the AERC reported in this case does not account for any loan costs, which are only required for retrofit work. For the retrofit solution, however, the AERC also includes the initial capital cost (principle + interest) related to the retrofit solution annualized over a time period of 30 years. It is important to note that most of the retrofit solutions result in an AERC lower than the baseline solution, such suggests that the baseline home is a prime candidate for a deep energy retrofit.

The curve created by the points in black represents the optimization front (or trade off curve). The source energy savings that can be obtained are in the range of 45 percent to 55 percent, but the optimum solution—the highest source energy savings for the lowest cost—is obtained with AERC of $2,360 a year, and source energy savings of above 50 percent. In other words, optimization results for the ReNEWW house showed that the retrofit solution with the lowest annualized energy-related cost results in source energy savings of more than 50 percent, under the assumptions used in BEOpt.

The results of the energy simulation were used to inform actual retrofit actions for the ReNEWW House that were implemented during the summer of 2014, notably installing triple pane windows, insulating to R-13 below grade, upgrading all the appliances, and replacing all the lighting with LED. The table below presents a comparison between what the simulation recommended as optimal and what was actually implemented in the retrofit. Most importantly, the energy simulation can predict the post energy retrofit consumption of the house, which in turn, can provide important information on sizing the solar energy system that will be required to achieve net-zero energy.

Retrofit Savings

The total capital cost of the optimum solution according to the assumptions considered by the BEOpt is $32,037, which enables the household energy consumption to be reduced by 50 percent. Cost data was obtained from the National Renewable Energy Laboratory's Retrofit Cost Database. The accompanying figure shows the modeled energy consumption of the ReNEWW House both before and after the retrofit in kWh. As can be seen, the total site energy consumption of the house decreased from 59,136 kWh/year to 24,855 kWh/year for a total energy saving of 34,281 kWh/year, a 58 percent savings. As expected, the largest reduction was in the heating demand, which dropped from 42,322 kWh/year to 24,855 kWh/year, a consequent energy savings of 30,356 kWh/year, or a 71 percent savings.

Achieving Net-Zero Energy

The net-zero energy goal was achieved for the test house by converting all end use devices to a 100-percent electric system and installing on-site renewable energy generation equipment to satisfy the amount of electrical energy consumed by the house over the course of the year. Natural gas could not be considered as it is not a renewable resource and it cannot be produced on site, so the entire heating and cooling system as well as the water heater (furnace and boiler, respectively) had to be replaced by electrically driven systems.

Modeling was again employed in this stage, with the same software used to complete another simulation to evaluate the total energy consumption of the test house post retrofit, including the effect of a geothermal heat pump. All other inputs remained the same as in the optimum retrofit case scenario. Not surprisingly, total site energy usage was lower than without a geothermal system, on the order of 12,424 kWh/year versus 24,855 kWh/year—an achievement that resulted from the heat pump's high coefficient of performance (COP). The COP of a heat pump is a ratio of heating or cooling provided to electrical energy consumed. Higher COPs mean lower operating costs.

The annual site energy consumed by the geothermal heat pump to cover the heating and cooling demand is 4,080 kWh—that contrasts favorably with the retrofit case which required 13,387 kWh/year for the natural gas energy required for the furnace for heating and the electricity required for the air conditioner. In short, the use of a geothermal heat pump was able to deliver a further HVAC system savings of 9,307 kWh/year on top of the retrofit simulation which focused on the envelope, appliances, and lighting.

For the main renewable energy system, researchers selected a solar photovoltaic system that will be placed on the roof to generate electricity. While the actual sizing of the solar system was outside the researchers' scope, online software PVWatts was utilized to arrive at some basic conclusions. Analysis parameters were the solar irradiance in West Lafayette and the test house roof pitch of 25°; however, the model did not consider shading. The simulation was conducted to place the solar system in the exact configuration of test house roof, with 1.33 kWp installed on the south side—a position that corresponds to the total available space there—and 10 kWp on the west side. Results showed that it is possible to have a total electricity production from this system configuration of 12,313 kWh/year. From researchers' experience, the online software can “under predict” actual solar array production and it is believed that a 11.3 kWp system, or even smaller, is more than enough to fulfill the net-zero energy requirement for the ReNEWW House.

Net-Zero Energy In Older Homes

With rising energy costs and global warming concerns, energy efficiency is crucial. The existing housing stock in the United States, much of which was constructed prior to the time when energy-efficient construction was envisioned much less implemented, represents a sizeable source of energy savings through retrofits combined with on-site renewable energy. Through the ReNEWW House in Indiana, Purdue University and its industry partners have demonstrated the ability of older homes to achieve retrofitted net-zero energy status. Using a simulation model whose baseline prediction was within 10 percent of actual consumption, researchers used an optimization tool to select the best retrofit actions, demonstrating that 50 percent of energy savings can be obtained. The model further showed that the net-zero energy goal can be achieved for the test house by replacing the HVAC system with a geothermal heat pump, generating hot water with an air source heat pump hot water heater, and installing a 11.3 kWp solar photovoltaic system.

The ReNEWW House is both a valuable research lab and a sustainable living showcase dedicated to highlighting technologies which are at market or just coming to market—some of which might be applicable in retrofit scenarios and others which may be more appropriate for new construction. As such, it provides useful insight for millions of Americans as a way to not only reduce their own utility bills but to contribute to the worldwide effort to reduce energy use, reduce the emission of greenhouse gases, and put an end to climate change.

Whirlpool Corporation, the leading manufacturer of major home appliances, brings you Inside Advantage®—innovative products providing efficiency and performance; market insights helping you to create more appealing designs; and targeted services that make doing business with them easier and more profitable. www.insideadvantage.com