“I’ve worked on insulating my home and conserving energy. How can I tell if my work has reduced my household energy consumption?”
Homes of all ages can benefit from higher levels of insulation, higher-performance windows, and weather stripping—but measuring the actual benefits can be difficult because fuel costs and the weather vary from year to year. This article describes one way to answer the effectiveness question, by using information available to most homeowners at no cost.
My wife Penny and I live in a small house in Reno, Nevada. Our 1952 home was built with minimal insulation—as we discovered when we started renovations in 1995. There was no insulation in the wall cavities, and only about 1 inch of deteriorated fiberglass batts in the attic.
Over the past 15 years, we added layers of R-21 fiberglass batts in the attic; they now total about 9 inches in depth. We inserted R-21 fiberglass batts into the previously empty outer wall cavities, with 3/8-inch rigid-foam insulation (R-2) over the fiberglass batts and the wall studs to reduce thermal bridging. As of this time, we have insulated about one-third of the perimeter wall area.
We also replaced the old steel-frame, single-pane windows and the old doors with double-pane vinyl units (U-value approximately 0.5). About 70% of the window replacement took place in 1995, before this study began, and the remaining 30% in 2002.
Our winter solar exposure is very poor, so our focus has been on incremental, yearly do-it-yourself insulation projects. For us, January is “attic insulation month.”
Most homeowners can measure the amount of energy used for home heating in the winter months. We can also factor in colder or warmer weather conditions by using appropriate temperature data. We can then calculate our winter heating energy use using a “weather factor” with one caveat: the “human factor”—individual consumption patterns that may be uncontrollable. For example, you may not be able to take into account unusual adjustments to your home thermostat—like if a guest stays for a lengthy visit or if someone gets sick. We have a programmable thermostat and we don’t override its settings often. So, in our case, it’s typically weather that drives heating fuel consumption.
Electricity, natural gas, and oil are commonly used in home heating; you can determine your energy consumption for whichever of these fuels you use. The consumption of electrical energy, whether generated by your own renewable energy system or the local power company, is measured in kilowatt-hours (kWh). Natural gas is typically measured in “therms,” the energy content of the gas consumed. If you buy these fuels from a utility, you’ll find monthly consumption numbers reported on your bills. Fuel-oil consumption is measured in gallons. Oil consumption from a tank can be read using a dipstick, but the stick needs to be calibrated for the specific tank.
In any case, you want to measure your actual energy consumption, not use a dollar energy cost that’s subject to the ups and downs of market forces.
During the main heating months of November through February (2005–2006 and 2009–2010), we used 80 to 136 therms of natural gas per month. (During that same period, the cost per therm varied from $0.98 to $1.22, which demonstrates why you cannot base your energy use calculations on billing dollar amounts.)
Reno, Nevada, is a high-desert environment subject to moderately cold winters. Our average temperatures for November, December, January, and February are 40.9, 33.6, 33.6, and 38.5°F respectively. But these are averages derived over decades; what we wanted was the monthly average for specific months—say December of 2008 and 2009—since we wanted to be able to take colder or warmer months into account as we examined our energy use.
Heating and ventilating engineers often use “heating degree-days” (HDD) as an index of the amount of heat needed to maintain comfortable surroundings for people occupying homes or businesses. HDD are defined as 65°F minus the average temperature for one or more days. In Reno last December, with an average temperature of 26.9°F, our HDD total for the month was (65°F - 26.9°F) times 31 days, or about 1,181 HDD. (December 2008 was milder, with 952 HDD.)
Average temperature data for specific months in specific years is available for various cities through the National Oceanographic & Atmospheric Administration’s (NOAA) Regional Climate Centers (see Access). If you live close enough to one of these cities, you can probably use NOAA’s HDD data. However, you will need to judge whether or not the temperature variations at the official measurement site are similar to those at your house, since microclimate variations can be significant depending on heat-island effects (i.e., the prevalence of buildings and pavement); elevation differences; and proximity to water. It’s important that the official site’s temperature rises and falls roughly in proportion to what you observe at home, but absolute accuracy isn’t necessary. You just want reliable and consistent HDD data to take into account warmer and cooler months as you examine your energy consumption.
If you can’t find HDD data from an official site close enough to your home, you can probably find monthly average temperature data for a nearby site and calculate your own HDD numbers. You can also install your own thermometer and measure maximum, minimum, and average temperatures at your home (see “Taking Temperatures” sidebar). Automatic digital systems will record the data even if you’re away, but the siting of the temperature sensor is somewhat critical.
Using HDD allows comparing, for example, my energy use in December 2008 to that of December 2009—taking warmer or cooler weather conditions into account.
A Carrier 132,000 Btu, 80% efficient natural gas furnace replaced the original oil burner and it has heated our home since 1995. We have monthly data showing the amount of therms consumed since 1999. We concluded that National Weather Service monthly average temperature data from Reno International Airport (about 4 miles away) would serve our purpose as an index of major monthly temperature changes.
The “Results” graph shows the amount of natural gas used to heat our house, divided by the number of HDDs, averaged over our two coldest months, December and January. The downward trend reflects our year-after-year insulation and efficiency efforts, equaling about 40% over 12 years..
Of course, 12 years aren’t needed for this kind of study. For example, if you’re interested in examining your energy consumption before and after an insulation retrofit, collect your fuel-usage numbers for a couple years before and after the work, and then obtain or calculate HDD for the winter months for those years. Plot your data to see if there’s a significant reduction following your insulation project, then you’ll be able to clearly visualize the benefits of your work.
C.F. (Fred) Rogers, Ph.D., is a retired atmospheric scientist. His current interests include looking for good science education materials for his grandchildren and composing a Web site honoring a pioneering cloud physicist.
HDD data for selected cities • www.cpc.noaa.gov/products/analysis_monitoring/cdus/degree_days
National Oceanic & Atmospheric Administration (NOAA) regional climate centers • www.ncdc.noaa.gov/oa/climate/regionalclimatecenters.html
Many thanks to Jim Ashby, Michelle Breckner, and Kelly Redmond at the Western Regional Climate Center for their help in accessing the wealth of temperature data on the WRCC web site. Thanks also to Adrienne Furman at Nevada Energy, who kindly helped me recover missing natural gas consumption data from the early years of this study.