The Insulation That Melts in Your Walls — and Could Reduce Your Energy Costs

Ever since the first caveman got the idea to keep himself warm in a pelt of woolly mammoth fur, thermal insulation has been an important part of human comfort and survival.

Hidden in the walls of our homes, in the sides of our coffee cups, and even in the International Space Station, wherever humans go, we take some form of insulation with us to keep us at just the right temperature.

But just because the idea is old doesn’t mean that there haven’t been innovations. Over the past few years, an entirely new category of insulation has made its way to the market. It’s called “phase change materials” (PCMs), and they control the temperature inside a room by melting and freezing over the course of a day.

How Do Phase Change Materials Work?

To understand how PCMs work, first, it’s important to remember that matter can be in various “phases” such as solid, liquid or gas. When something is changing phase, its temperature stays the same. 

Take ice water for example. A pot of ice water is a mixture of solid and liquid H₂O. As long as there’s still ice in the pot, the temperature will stay at about the freezing point (0 degrees Celsius or 32 degrees Fahrenheit) even if you put it on a hot stove. The heat from the stove will cause the ice to melt faster, but the temperature won’t increase until all the ice is gone. 

All the energy from the stove goes into melting the solid ice first. Then the water’s temperature will rise until it starts to boil, changing phase again from a liquid to a gas.

Different materials melt at specific temperatures, and the melting point can be fine-tuned by combining various materials. For example, adding salt to ice lowers the melting point, which is why people salt their driveways in the winter.

The key idea behind PCMs is to find a material that freezes at a temperature that’s comfortable for people, say 21 degrees C (70 F). If the outside temperature rises above 21 C, this material starts to melt. Below 21 degrees, it starts to freeze. Without using any electricity or gas, this material would naturally try to keep a building’s temperature around 21 C, as long as it’s partly liquid and partly solid (in mixed phases). It’s the same way a cooler uses melting ice to keep the inside cold.

You can buy panels of PCM material today. They come in a few different shapes to keep the liquid-solid mixture contained. One is a rigid panel that looks like a segmented chocolate bar. Each square segment contains some of the special phase-change material. Another looks like a flexible mat of connected pouches, like large bubble wrap. If you pinch one of the bubbles, the material inside feels thick like honey, and little solid crystals crunch between your fingertips.

Even though these materials are available, they haven't been widely adopted. One major barrier is the gap in information about how well the materials perform and how to best use them. That makes it hard for homebuilders and engineers to know if PCMs are worth the cost for their particular building. 

Finding ways to measure quantities, including unusual ones,  is what NIST does best. NIST mechanical engineer Jae Hyun Kim studies all types of insulation and is working on ways to measure PCMs effectively. 

Kim first warmed up to the study of insulation while researching jackets for an athletic clothing company. “There are a lot of similarities between insulating a house and insulating a person,” says Kim. “A jacket and a house both need layers to manage moisture, temperature and reflecting sunshine.” 

How Do You Measure the Effectiveness of Insulation?

At NIST, Kim plays a key role in helping labs and private industry get better measurements of how well insulation works.

“One of the key missions of my lab is to measure the thermal resistance of insulation as accurately as possible,” he says. “Other labs and industry use our work to make sure their own measurements are accurate.” 

If you buy insulation at a home improvement store, you’ll see that it’s labeled with an R-value. The higher the R-value, the better it is at stopping heat from moving from one side of it to another. In the same way that nutrition facts allow you to easily compare food at the grocery store, the R-value lets you compare different kinds of insulation.

The History of the Insulation Lab

R-value labeling is commonplace today, but it didn’t exist until the early 1900s. Air conditioning and refrigeration were new technologies at the time, and they both needed really good insulation to be effective.

In 1912, Kim’s predecessor, Hobart Dickinson, pioneered a way to accurately measure the effectiveness of insulation. The basic principle is straightforward: You make one side of the insulation hot and the other side cold. Then, you measure the temperature difference between both sides.

The tricky part is making sure the heat goes exactly where you want it and nowhere else. An ordinary hot plate won’t work because it’s impossible to know how much heat is going through the insulation and how much is escaping from the sides of the hot plate. 

Dickinson’s solution was to surround the hot plate with a second hot plate at exactly the same temperature, separated by a small gap. Heat always moves from hot to cold, so if two objects are the same temperature, then no heat can move between them at all. This second hot plate ensures that heat can’t escape around the edges, and all the heat from the first hot plate goes through the insulation. The whole apparatus is called a “guarded hot plate.” It is still the gold standard for measuring the effectiveness of insulation. 

NIST has two guarded hot plates. The older one was built in the 1980s and is still regularly used today in its original configuration. It’s a large machine and can measure thick insulations. The newer guarded hot plate measures smaller samples, but it can handle more extreme temperatures and was specially built inside a vacuum chamber to test insulation in different environments. 

Air is an important part of many types of insulation. Spongy materials like Styrofoam and fluffy materials like down feathers are good insulators because they make lots of tiny pockets of air. But not all air is the same.

Thin mountain air may be less insulating than air at sea level. So by putting a guarded hot plate in a vacuum chamber, NIST can test how well insulation works at different altitudes and in extreme environments like airplanes or satellites.

How to Measure Phase Change Materials (PCMs)

This guarded hot plate approach has worked well for traditional insulation, but measuring PCMs needs a different approach. When he first encountered a prototype PCM panel in 2022, Kim wasn’t sure how to measure it. The unusual shape made it difficult to fit into existing measurement machines. He also wasn’t sure which numbers would be important. For example, is it better to measure how much heat a PCM panel can hold instead of how fast heat can travel through it?

In a real house, PCMs wouldn’t work in isolation. They are part of a whole system of insulation, roofing, outside weather and air conditioning. So he decided to start by building small-scale models.  

In Kim’s latest experiment, he and his team put together little plywood houses. They are simple cubes about a meter (or yard) on each side that can be lined with many combinations of roofing material, insulation and phase change materials. Their first experiment will adjust the amount of sunlight that reflects off the roofs of the boxes to see how that changes the performance of the PCMs. 

“It isn’t clear yet where PCMs would be most effective or how best to combine them with traditional insulation,” says Kim. “But we’ll never know until we test them."

In the best-case scenarios, PCMs could have a big impact on the energy grid. In addition to reducing the power needed for air conditioning, they could also “flatten the curve” of energy consumption. On a typical summer day, energy usage spikes during the hottest part of the day when air conditioners are working their hardest. That extra power tends to come from fossil fuels, which are easier to burn on demand. Power during that part of the day is also at its most expensive. PCMs could help spread that spike in power consumption to later in the day. This would allow people to reduce their energy costs and the use of fossil fuels. 

But the industry is still early in its development. It will need a clear, standard, reliable and reproducible way to measure PCMs. 

“Even though we’ve been studying insulation at NIST for more than 100 years, there are still new problems to solve, and we can make it better by using new materials and ideas,” Kim says.