A Wisconsin Usonian Home: 37 Years of Energy History
Not your average homeowners - this couple has kept energy use data for 37 years.
We chose this house partly for its wooded surroundings, stunning views, and abundance of natural light. However, during our first winter we discovered a great problem. The floor-to-ceiling single-pane windows were repeatedly covered with a 1/8-inch-thick sheet of ice—on the inside—that would melt into puddles in the living room and loft. Comfort considerations, energy bills, and the need to preserve the house’s structural and decorative materials all compelled us to address that problem—right away!
Fortunately, my own academic career was undergoing a transformation from engineering physics into the field of energy systems and policy analysis. I was becoming increasingly involved in state and national issues of energy and environmental policy and planning. Energy efficiency and management was becoming a key part of this policy, nationally and internationally—one to which I eventually devoted much of my professional career. And this severe energy problem in our own home caused me to embark on a 37-year quest to track and optimize energy use throughout our household.
The Original Energy Picture
Our house has many features influenced by Frank Lloyd Wright’s Usonian Home. In designing the Usonian home, Wright aspired to simplicity and economy, using passive solar design, radiant floor heating, and local materials. Simple lines replace expensive ornamentation, the garage is replaced by a carport, and homes are one-story, with radiant-heating. Our house has window walls, which open up rooms to outside vistas; an open floor plan with combined living and dining areas; a cathedral atrium with no attic; a small kitchen; a utility room in place of a basement; recessed lighting; a concrete slab floor; and a large central fireplace. The exterior features include board-and-batten siding, large overhanging eaves, low-sloped roofs and inexpensive concrete block construction of several walls. The southwest wall is dominated by a floor-to-ceiling 8 foot x 11 foot window, to incorporate the natural features of the hillside lot.
Much of the exterior wall is constructed of ₃⁄₄-inch wood siding with 3½-inch cellulose batt insulation between ½-inch plywood sheathing and ½-inch gypsum board. A small part of the exterior wall is 10-inch concrete block—the Usonian favorite—with ½-inch gypsum board on the inside. The original roof, with its cathedral ceiling, has 3½-inch cellulose batt between the sheathing and the gypsum board. All of the original windows were single-pane, most of them set in custom-built frames. The original floor area was a modest 1,617 square feet; three additions have increased it to 2,953 square feet.
I suspect the Taliesin disciples of Wright did pay considerable attention to the use of natural materials and environmentally friendly construction practices, but assigned a lower priority to ongoing energy consumption over the life of the dwelling. We believe that the natural gas-fueled forced-air furnace, which was operating in 1970 when we purchased the house, was the original furnace. It provided heat to continuous-perimeter baseboard registers fed by air ducts embedded in the concrete floor slab. The central fireplace/hearth near the center of the house, with its massive concrete block chimney extending through the cathedral ceiling, is well located for both energy-saving and social purposes. Other natural gas appliances were a range/oven and a 40-gallon domestic water heater. Including the furnace, six gas pilot lights on the three pieces of energy equipment were continuously burning in 1970. A through-the-wall room air conditioner was mounted on the north wall in the second-floor loft. Over the past 37 years, we have used this air conditioner maybe a dozen times, thanks to the fact that the house is shaded by large oak and hickory trees.
Three additions were made to the original house. In 1953, the then-owner added a first-floor utility room (on slab) and above it a large “artist’s studio” with a huge north-facing double-insulated window (see Photo on p. 38), which showed some energy concern. However the east- and south-facing walls of the addition were constructed with single-pane windows and the traditional 10-inch concrete block, uninsulated on both sides. In 1978, we added a solar room. This addition surpassed the residential building codes of the time and was specifically designed to incorporate the solar-thermal domestic hot water (DHW) system we installed that year. In 1994, we built an underground garage. A small stairwell room leading to this garage provided thermal buffering on the north side of the house. Its windows and doors were double pane, and the room envelope met or surpassed the construction codes of the time.
In the winter of 1971, I began a home energy management
process that has extended over almost four decades. It includes
- identifying and analyzing measures for reducing home
- energy use;
- prioritizing and implementing these measures;
- monitoring home energy use, primarily natural gas use; and
- ex post analyzing the technical and financial impact of implementing the measures.
During the period 1971–2007, my wife and I made a steady effort to reduce the use of nonrenewable energy in our home, through a variety of behavioral and technical interventions. There is no doubt that my professional activities played a major role in initiating and sustaining this effort. But it was also greatly stimulated by the external social and political environment, beginning with the environmental movement of the early 1970s and the Arab oil cutoff in 1973, and—perhaps equally important—by Wisconsin state programs designed to encourage energy conservation. New government regulations on electricity and natural gas supply systems, and pioneering work in construction standards, also served to inspire me.
In this section, I will briefly describe three of the energy-saving measures we implemented. I describe some of the other measures we implemented in the next section, where I discuss the monitoring of natural gas use.
Replacing single-pane picture windows. Our highest priority in 1971 was to solve the problems posed by the two largest single-pane windows. Heat loss from these windows, which measured 8 feet x 11 feet and 10 feet x 5 feet, respectively, accounted for 10% of 1970 total heat load! There was also the moisture problem posed by ice on the inside of the glass. Because large double-pane insulated windows were not readily available (and because we could not have afforded them anyway), we located a carpenter who constructed an interlocking array of storms for each picture window. They have performed beautifully for more than 30 years of silent duty—an immediate payoff of no inside ice and an attractive long-term payoff in Btu and dollars. Since then, we have retrofitted the rest of the house’s 42 windows—the final 3 in 2007.
Installing a solar water heater. In 1978, making use of then-sparse technical literature, and taking advantage of President Carter’s tax credits, we installed a solar water heater. In 2006, we replaced the 80-gallon solar storage tank, which had developed some small leaks and did a modest amount of preventive maintenance on peripheral hardware for the solar-thermal system. One comment on the importance of real-time monitoring: During the warmer months, we wait until the tank temperature reads above 120°F before running the dishwasher or the clothes washer, or taking a shower.
Replacing the furnace. In 1984, when the original (estimated 65% efficient) furnace began to fail, we replaced it with a newly available 90% efficient condensing furnace. It paid for itself in less than six years (see Table 1). Twenty years later, we replaced it with a 94% efficient dual-burner furnace with a variable-speed fan. This fan saves energy by slowing down when the furnace reaches target temperature. The upgrade is also quieter than the old system.
Much remains to be done. For example, some of the walls are not well insulated. Up to now, I have felt that the potential energy savings did not justify the complicated renovation work. However, as climate change becomes a more urgent issue, we may well implement this and other energy-saving measures.
Monitoring the Results
To sustain a long-term energy management process, it is necessary to monitor the results. Our energy and financial records extend back to 1970. To avoid creating a pseudo laboratory environment, with cables and meters in every corner, we have generally relied on monthly utility bills to monitor our project. Under special circumstances, we supplement this data with self-reading of utility meters. The self-readings are done during periods of unusual energy use or severe weather, or periods just prior to or following a change in system operation. In a few cases, such as several years following installation of the solar-thermal system, we have installed additional meters.
We have made an effort to preserve financial records. These include not only utility bills, but also records of the capital cost of installing new systems or upgrading old ones. In general, we have gathered this information contemporaneously. This information includes weather records, energy price records, and energy supply forecasts. The Wisconsin State Energy Office has been a consistent source of local energy, economic, and weather statistics.
Figure 1 shows directly measured annual natural gas use during the heating year (July 1–June 30) from 1970 through 2007. The peak consumption of slightly less than 300 million Btu occurred during our first two years of residency. Over the past few years, consumption has fallen to approximately 110 million Btu. The figure also shows measured gas consumption for DHW and cooking. Actual measured values were obtained only during the heating years 1978–79 through 1985–86, when the local utility lent us gas and water meters that separately monitored hot water use and gas for water heating. Values for other years are indirectly deduced, in part from information obtained during that eight-year monitoring period. Note that nonfurnace gas consumption dropped from 55 million Btu in 1970–71 to 8 million Btu in recent years. The greatest reduction occurred after the solar DHW system was installed; this unit provides most of our hot water from early spring through late autumn. Furthermore, because this unit has electronic ignition, we no longer need the six original gas pilot lights.
External auditors have conducted two energy audits. The first was conducted in the late 1970s by the local utility. Only a cursory audit, it identified some infiltration problems, primarily at doors and fireplace and at masonry/floor boundaries; these were corrected. A very thorough audit was conducted in January 2008 under Wisconsin’s Focus on Energy program by John Viner, a home performance consultant in Madison. It included extensive blower-door and infrared thermographic measurements. The audit identified several infiltration problems—not uncommon in a house of this vintage. The measured air flow at a pressure difference of 50 Pascals (CFM50) was 2,890 cfm, yielding an air changes per hour (ACH50) of 7.68 air changes per hour. The so-called LBL correction factor, based on climate, building height, wind shielding, and leakiness correction, yielded an estimated annual average natural infiltration rate (ACH) of 0.3 air changes per hour. Several of these new recommendations are being implemented; we plan to implement others during future remodeling projects.
Electricity usage is not the focus of this article, but we have monitored it over the past 37 years. Electricity use has decreased slightly over this period, despite a 25% increase in floor area and increased use of electrical devices.
Technical Analysis of Energy Interventions
We carried out technical analysis for most of the energy interventions, to determine how much energy the intervention would save. Most assessments of window and wall modifications were made with straightforward heat loss methods, based on conventional ASHRAE formulations. In the 1970s and early ’80s, use was made of the University of Wisconsin F-Load and F-Chart models for home heat load and solar design calculations. Over the past two decades, many sophisticated engineering-based models have become available. Ease of use varies; one user-friendly model is Lawrence Berkeley Laboratory’s Home Energy Saver. Rather than wed myself to one of these models, I constructed a very simple spreadsheet model. This model incorporates and calculates
- exterior surface areas and house volume;
- U-values for the exterior surfaces of 7 wall types, 3 ceiling types, and 12 window types;
- heating degree-day data from 1970 to date for Madison;
- conductive heat loss through all exterior surfaces;
- heat loss due to infiltration based on estimated annual average ACH, determined from blower door measurements;
- internal heat gain from electric and nonelectric appliances and heat generated by human activity;
- furnace gas consumption based on heat loss and internal heat gain; and
- gas consumption of nonheating appliances: range, DHW heater, and pilot lights.
With this simple, transparent load model, it is a straightforward task to estimate energy savings and relative importance of each actual or potential energy intervention. The calculated 37-year results are shown in Figure 1 where they are compared with the measured values taken directly from meter readings (see “Determination of Balance Point Temperature”). The calculated magnitudes and the trends are both reasonably consistent with the measured values. Most importantly for me, this model makes it easy to estimate the savings and relative importance of potential interventions, and to list them in order of priority.
Financial Analysis of Energy Interventions
Financial considerations played a major role in our energy efficiency projects. During the early 1970s, we conducted back-of-the-envelope calculations with then-current energy prices, using simple payback as figure of merit. As energy prices rose dramatically after the 1973 oil crisis, we took a longer-term approach, using conventional wisdom to construct scenarios with increasing energy prices. The financial, as well as the technical, analysis for our 1978 solar DHW heating system was based on the well-known Wisconsin F-Chart, as described in the preceding section. In the 1980s, with the advent of personal computers and spreadsheets, we began to rely more on computerized financial analysis. However, in hindsight I can say that one of the strongest influences in our project decision-making process was the firm conviction that energy conservation and striving for efficient energy use is important.
I recently conducted a retrospective analysis of several of our more important projects, using actual historical prices and Microsoft Excel’s macro financial functions. Table 1 shows key inputs and results for the analysis of two major energy interventions. The first is the 1971 installation of interlocking storms over our 8 foot x 11 foot single-pane picture window. The second is the 1984 installation of a high-efficiency condensing furnace.
Annual energy savings from installing the storms are 8.9 million Btu. Historical natural gas prices rose at an annual average rate of approximately 6.5% over the past 37 years. As shown in the table, lifetime dollar savings total more than $1,800 from 1971 through 2007. For us, the most relevant quantitative results are simple payback and internal rate of return: 14 years and 8.3%, respectively. These are attractive values from our particular social perspective.
The 1984 furnace investment paid off handsomely. The initial investment of $1,950 was recouped by 1990, yielding a simple payback of six years, and a 17% internal rate of return. By 2003, when we replaced the 1984 furnace with a new variable-fan furnace, its cumulative energy savings were 1,100 million Btu, corresponding to a reduction in operating cost of $7,230.
The payback period for the solar domestic water heater, originally calculated in 1978, was too optimistic. This was because I incorrectly assumed that natural gas prices would continue to rise throughout the ’80s and ’90s. Taking into account all energy savings and maintenance costs through 2006, retrospective analysis shows that the cumulative net cash flow was still slightly negative. However, in 1978 our greatest motivation for installing a solar unit was to obtain insight into an emerging technology, learn how the system worked, and help stimulate interest in alternative energy sources. We more than achieved these goals, and after 30 years, the system is still operating as originally designed.
The Wisconsin Context
Because our house is not a typical Wisconsin residence, it is interesting to put our 37 years of energy experience into the overall context of Wisconsin home energy use. Scott Pigg and colleagues at the Energy Center of Wisconsin (ECW) have made this easy for me with their excellent report Energy and Housing in Wisconsin: A Study of Single-Family Owner-Occupied Homes.
The ECW report found that houses built in the 1950s have today the highest heating energy intensity (HEI) of all existing Wisconsin houses—more than 9 Btu per square foot per heating degree-day (Btu/ft2/HDD). Heating energy intensity is often referred to in the literature as the Home Heating Index (HHI). Our home’s HEI in 1970 was 15.8 Btu/ft2/HDD. In contrast, its HEI in 2007 was 6.1 Btu/ft2/HDD. The average Wisconsin home has an HEI of 7.5. New construction (built in 1994 or later) has an HEI of 5.8.
|Determination of Balance Point Temperature
In carrying out the calculations described in this article, I discovered an uncertainty in the estimate of our house’s average balance point temperature (BPT). BPT is the outdoor temperature at which house heat loss equals the sum of intrinsic heat gain (from appliances and occupants) and solar heat gain. BPT varies constantly with changes in the amount of intrinsic and solar heat. It is also dependent on the house’s heat loss coefficient and the indoor air temperature. Heating degree-days reported by utilities and newspapers are traditionally based on an average BPT of 65°F.
In an alternative approach to my heat loss calculations described earlier (BPT=65°F), I used our house’s average BPT and the corresponding adjusted heating degree days for each year. Based on suggestions of Scott Pigg of the Energy Center of Wisconsin, this average BPT was determined from statistical analysis of my monthly monitoring data, using the PRISM model developed at Princeton University.
From this analysis, I estimated that the average BPT increased gradually from 59°F in the 1970s to almost 62°F in recent years. This is probably due to lower intrinsic heat gains and to higher daytime thermostat settings, as our increasing age (is it thinner blood?) calls for warmer rooms, despite our admiring adherence to the Jimmy Carter sweater philosophy. We used a different average BPT each year to obtain an adjusted heating degree-day value for each year. Internal heat gains were then ignored in the spreadsheet model since in principle the adjusted average BPT accounted for them directly. This alternative approach gave slightly higher calculated gas consumption values than those shown in Figure 1.
Monitor and Save Energy
In 1970, we moved into a 1950s architect-built house with high natural gas usage, unusually large window area, and ice-and-internal moisture problems in winter. During the next 37 years, we made a steady effort to reduce nonrenewable energy use through a variety of behavioral and technical interventions. We paid particular attention to conducting a technical and financial analysis for each intervention, and to monitoring energy use before and after each intervention. We collected energy consumption data for 37 years, with modest exceptions for intermittent years away from home. Here are the technical and financial highlights of that experience:
Both personal comfort and preservation of structural materials were significantly increased over the time period. Total home natural gas energy decreased from 290 million Btu in 1970 to 110 million Btu in 2006, despite a 25% increase in house floor area. Nonheating gas energy decreased from 55 million Btu in 1970 to 8 million Btu in 2006.
The total gas energy intensity (TEI) of the home dropped from 20 in 1970 to 6.6 Btu/ft2/HDD, more than a factor of 3. Although the house was constructed with 1950 building practices, and has an extremely high window fraction in the exterior envelope (18% in 2006), the HEI is less than 80% of the average value for all Wisconsin homes.
Total natural gas energy used over the 37-year period was 5,730 million Btu. Without the energy efficiency measures, energy used would have been 10,440 million Btu, a savings of 4,710 million Btu over that period. At today’s natural gas prices, this would be the equivalent of $55,000. Without these savings, our gas bill in 2007 heating would have been approximately $3,000 instead of $1,100.
But equally important is the realization that the process we began in 1970 is just as relevant today. Although the dollar is worth less today than it was in 1970, a Btu of energy saved in 1970 achieved the same reduction in climate change as a Btu saved in 2007. We feel that we have learned from this process and are happy to have stuck with it. What have we learned?
Monitoring is important—you should know what is happening and where you are in the process. Utility bills and Web sites that inform customers about their energy use are a step in the right direction. To take the next step, you will need to install more meters and more indicators in your home, Microelectronics and smart metering are probably the key to future breakthroughs in this area.
Economics count—pay attention to cost and financial analysis. But also take a long-term view, one that matches your vision of a sustainable energy future.
Finally, establish and hold to an energy savings principle. Keep at it. Our experience shows that small things add up!
Wes Foell is an energy educator, researcher and consultant. Formerly a faculty member at the University of Wisconsin, he founded Resource Management Associates, an international energy/environment consulting firm in Madison, Wisconsin (firstname.lastname@example.org).
>> For more information:
To download the Home Energy Saver simulation model, go to http://hes.lbl.gov/hes/about.html.
To learn more about Wisconsin residential energy, see Pigg, S., and M. Nevius. Energy and Housing in Wisconsin: A Study of Single-Family Owner-Occupied Homes, Research Report 199-1. Madison: Energy Center of Wisconsin, 2000. To download a copy go to: www.ecw.org/resource_detail.php?resultid=293.
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