Temperature loggers can help us build more energy-efficient homes.
One billion (yep, 1,000,000,000) new homes will be built worldwide by 2025. How do we make homes that will stay cool in the summer? Stay warm in the winter? How much does insulation help? What about passive cooling systems? Window shading?
Temperature loggers can help answer these questions. This LogTag Trix-8, for example, costs about $25 and can record hourly temperatures readings for almost a year.
Note: The LogTag Trix-8.
We used hundreds of LogTags in a new project testing energy-efficient homes in Mexico. The LogTags worked flawlessly, generating rich high-frequency data about thermal comfort and facilitating statistical tests comparing homes with different features. I see great potential for using temperature loggers like these in similar projects. If we are going to make homes as energy-efficient as possible, we need to test what works and doesn’t work, and loggers can help.
Temperature loggers have been around for a while. But, like most digital technologies, they keep getting better and cheaper. For our project we tested seven different models of loggers made by five different companies (Rotronic, LogTag, HOBO, Newsteo, and iButton). We selected the LogTag because it was particularly easy to use but our overall experience was positive with all models.
Note: Here are four of the other loggers we tested. It is hard to get a sense of scale from these images but the iButton temperature logger, far right, is much smaller than the others, about the size of a thick U.S. dime.
To access the data, you use a cable to connect the LogTag to a laptop. We tested other models that use radio and Bluetooth, but decided against the wireless approach because it costs more and because, in our case, real-time feedback was not particularly important. That said, my colleague Catherine Wolfram is working on a new industrial energy-efficiency project with wireless meters, and the ability to do continuous monitoring and real-time feedback is very compelling in many settings.
We conducted the field experiment near Monterrey, Mexico. We provided 230 new homes with free wall and roof insulation and other energy efficiency upgrades, and we compare them to another 230 homes without upgrades. For both the upgraded and non-upgraded homes, we installed temperature loggers in the homes’ living rooms, and collected data over 16 months. The complete results are available in an Energy Institute working paper (available here coauthored by Sebastian Martinez and Bibiana Taboada).
Note: Long rows of nearly identical homes, with a mix of upgraded and non-upgraded homes.
Before I get to results, I want to step back for a minute and say why I think this is important. Buildings are responsible for more than one-fifth of global energy consumption and energy use from buildings is expected to increase 32% by 2040. We’ve learned a lot about how to build energy-efficient buildings but, as always, it is important to continue validating technologies using real data on outcomes.
Focusing on new buildings also makes a lot of sense. We know from Energy Institute research that it is very expensive to retrofit existing homes, so wouldn’t it be cheaper and easier to make homes energy-efficient the first-time around? In our study there are long rows of nearly identical homes, allowing for significant economies of scale in construction.
The figure below plots indoor temperature as a function of outdoor temperature. We plot averages for homes with and without upgrades in three-degree temperature bins. As expected, there is a strong positive correlation. The thermal mass of the homes protects households from the most extreme temperatures, but these homes still get very warm and very cold. Only 12% of these homes have air conditioning, and few have heaters, so there is little active heating or cooling.
Source: Davis, Martinez, and Taboada (2018). Brackets indicate 95% confidence intervals.
Surprisingly, indoor temperature is essentially identical in homes with and without upgrades. That’s why the orange and black bars are virtually indistinguishable in the graph above. Upgraded homes are not cooler in the summer, nor are they warmer in the winter. In the paper we also examine energy consumption, finding no evidence of savings. These results stand in sharp contrast to the ex ante predictions from a simulation model which predicted a 26% decline in electricity consumption as well as significant improvements in thermal comfort.
The lack of evidence of impact is not because of noisy data (i.e., wide confidence intervals). With such rich data from the temperature loggers, we can rule out a 1% improvement in average temperature. This precision reflects the large sample size as well as the otherwise nearly identical homes.
Throwing Energy Savings out the Window?
What happened? Several mechanisms contributed, but open windows played a key role. In the paper we document that most households have their windows open on hot days. We find, for example, that 96% of households had at least one window open on a hot day in June 2017 and that households report having windows open for an average of 16.7 hours per day. Even households with air conditioning tend to have their windows open a lot during the summer.
An open window provides air flow on hot and humid days, but it also largely nullifies the thermal benefits of building insulation and the other energy-efficiency upgrades. We thus conclude in the paper that the benefits from these energy-efficiency investments are less than the costs, which added several hundred dollars to the cost of each home.
Why I’m Nonetheless Optimistic
Still, I’m optimistic about the broader mission. Our paper shows how data from temperature loggers can be used to make comparisons across hours-of-day, months-of-year, and across climatological conditions. We also show how temperature data can be a valuable complement to energy consumption analyses, facilitating a direct test of the “rebound effect”. There is an inherent tradeoff between energy use and thermal comfort, so it is important to monitor both in order to really know what is going on.
Much is at stake. Understanding how to reduce energy consumption from buildings is enormously important, and rigorous testing is an essential part of this. If energy efficiency investments are going to play a significant role in improving thermal comfort, reducing energy consumption and lowering carbon dioxide emissions, then we need to keep optimizing these investments.
Note: The housing development in Northeast Mexico.
Lucas Davis is an Associate Professor of Economic Analysis and Policy at the Haas School of Business at the University of California, Berkeley. His research focuses on energy and environmental markets, and in particular, on electricity and natural gas regulation, pricing in competitive and non-competitive markets, and the economic and business impacts of environmental policy.