A 'Rebound Effect' May Cancel the Energy Benefits of Insulation

Researchers at the University of Cambridge found that insulating attics and walls of existing houses heated with gas don't make much difference in gas consumption. And what little difference there is doesn't last.

In England and Wales, 85% of homes are heated with gas. Most are built with cavity wall construction where there is an inner wythe of concrete block and an outer wythe of brick, with a gap or cavity between them.

After studying data from 55,000 dwellings over 12 years, the researchers found that cavity wall insulation reduced gas consumption by 7% in the first year. But it crept back up: In the second year, the reduction was just 2.7%, and by the fourth year, savings were next to nothing. With loft or attic insulation, the initial fall in gas consumption was 4%, dropping to 1.8% in the first year and becoming insignificant by the second year.

Insulation doesn't usually lose its insulating value, so the research suggests a "rebound effect" where people are using more gas, possibly because they turned up the heat, opened the windows, or added extensions to their homes at the time they added the insulation. Conservatory or greenhouse additions are popular, and in homes with these, there were no gas savings at all.

Study co-author Laura Diaz Anadon said in a statement:

“The recent spotlight on increasing the energy efficiency in U.K. buildings is both welcome and long overdue, and there are very real benefits to households from good insulation, not least in terms of health and comfort. However, home insulation alone is not a magic bullet. High gas prices will reduce the rebound effect in the short term, as homeowners have the need to keep costs down at the front of their minds. In the long term, simply funding more of the same insulation roll-out to meet the U.K.’s carbon reduction and energy security targets may not move the dial as much as is hoped."

My first reaction to this study was that it was shocking and contradicts just about everything I have written about increasing efficiency and reducing the carbon emissions from housing, including our beloved pyramid. It calls into question the billions spent subsidizing retrofit programs. But a closer examination of the study actually confirms what we have been saying all along.

The study, published in Energy Economics, used data from the National Energy Efficiency Data-Framework (NEED), which collects information about gas and electricity consumption, along with information on energy efficiency measures. The researchers did not go out and examine the work itself, but tested the data against their research hypotheses:

1. The installation of EE (energy efficiency) technical improvements in households generates statistically significant reductions in the amount of gas consumed by dwellings in the short term (a year after installation) when compared to similar dwellings that have not adopted them.
2. Any reduction in gas consumption in U.K. households after the installation of an EE technical improvement will not be sustained in the longer term (over 2–5 years) due to mediating factors unrelated to the energy savings potential of the measure adopted, e.g. behavior and purchases of residents. AKA the "rebound effect."
3. Households installing EE technical improvements alongside other renovations in dwellings do not experience a significant reduction in gas consumption in the short or medium term.
4. For the two EE measures investigated, vulnerable households installing EE technical measures exhibit a higher rebound effect that results in no reduction (in the short- or medium-term) to their gas consumption.

The data showed that there were indeed short-term savings, but not much. This is not surprising, We have discussed previously that British houses are notoriously leaky, and they do not usually do blower door tests when they do renovations. As Passivhaus expert Kate de Selincourt told Treehugger in the past, "The U.K. generally has no clue about the value of airtightness, or how to do it, or how to ventilate properly once you've done it."

Building engineer Harold Orr has said, "If you take a look at a pie chart in terms of where the heat goes in a house, you’ll find that roughly 10% of your heat loss goes through the outside walls. About 30 to 40 % of your total heat loss is due to air leakage, another 10% from the ceiling, 10% from the windows and doors, and about 30% from the basement."

Numbers will be different in the U.K. where they do not usually have basements, but the point remains that throwing insulation at the loft or attic isn't going to move the dial very much.

Similarly, the usual method of insulating cavity walls is to drill holes from the outside and pump in styrofoam beads. I am told this is effective, but I wonder how one can possibly know how evenly it is filled or if there is stuff like mortar on the inside that is blocking the flow of beads. This is why gas consumption only went down a measly 7% with the wall insulation and 4% with the attics in the first place.

With respect to the rebound effect, this is a well-known phenomenon. Architect Mark Siddall tells Treehugger: "When people have a limited budget, they can not afford to heat their home to remain comfortable. Instead, they compromise comfort for lower bills. A shallow, fragmented retrofit helps to reduce bills; however, because retrofit has been undertaken in a half-hearted manner, occupants choose better comfort at the same similar cost to their pre-retrofit bills."

Siddall notes that the rebound effect is old news. "Since the 1980s, the rebound effect is often called the Khazzoom-Brookes Postulate," he says. "Daniel Khazzoom and Len Brookes observed that all the efficiency gains of the 1970s were overcompensated by additional consumption, notably after the oil prices came down again."

Passivhaus journalist Kate de Selincourt also told Treehugger that vulnerable households are more likely to turn up the thermostat.

"The phrase 'rebound effect' in itself annoys me, as though people improving their comfort and health was a bad thing and somehow undermining the effectiveness of the policies, when reducing energy poverty is an explicit aim of U.K. retrofit policy—and in fact is the priority for the tax/bill payer-funded elements."

The researchers acknowledged this but pointed out that they are studying energy consumption, not comfort. They noted:

"While energy efficiency policies therefore may be having a positive impact on reducing fuel poverty, the energy efficiency schemes are not effective in this segment of the population in terms of delivering energy savings. This result is relevant for the design of measures targeting different groups and policy goals, e.g., reduction of fuel poverty vs. energy efficiency savings."

They also call for "tighter standards for newly constructed dwellings and for renovations." Blower doors, anyone?

Finally, they called for heat pumps, more clearly in the statement than in the study.

“When trying to get middle income households to conduct energy renovations, as the government are currently doing, it makes sense to further encourage heat pump installation at the same time," said Cristina Penasco, the study’s first author from Cambridge’s Department of Politics and International Studies.

So, in the end, this study isn't so shocking. The rebound effect is not a surprise, shallow retrofits don't make a big difference in energy consumption, and we need both insulation and heatpumpification.

We also need proper regulation and maybe that blower door test at the beginning instead of some guy squirting beads in the wall and fluff in the roof. As Debbie Mauger of the Alliance for Sustainable Building Products tells Treehugger:

“All insulation products reduce heat loss, but can only result in energy savings if the appropriate materials are chosen and installed correctly. Each home needs a whole-house retrofit plan (skilled assessment, design & installation) to ensure a comfortable healthy home—without turning up the thermostat.”

And, our pyramid still stands.