All About Thermal Mass
Interior thermal mass can sometimes help lower energy costs — but in cold climates, it’s won’t help much
UPDATED on December 4, 2013 with a citation of recent research findings.
What’s the deal with thermal mass? Since manufacturers of materials that incorporate concrete often exaggerate the benefits of thermal mass, it’s easy to get cynical and conclude that the buzz around thermal mass is all hype. But in many climates, it’s actually useful to have a lot of thermal mass inside your house. Just keep in mind that thermal mass may not be as beneficial as its boosters pretend.
Thermal mass is a solid or liquid material that can store heat. Most of the objects inside your house can be considered thermal mass, including plaster, furniture, books, and canned tomato soup.
The specific heat capacity of building materials varies. In general, denser building materials have a higher specific heat capacity per unit of volume than less dense materials, which is why concrete, stone, and gypsum wallboard are more likely to be used to provide extra thermal mass than wood.
Three analogies: cistern, frying pan, truck
A building with lots of thermal mass on the interior side of the insulation may have lower energy bills than one without as much thermal mass, for reasons I’ll explain soon. But it’s important to point out that thermal mass can’t heat or cool your house. It’s just plain old concrete. To heat and cool your house, you still need HVAC equipment.
Here are some analogies:
Early high-mass walls were made of adobe and stone
Traditional homes in hot climates often have thick walls made of stone or adobe. If daytime temperatures are often above 80° or 85°F and nighttime temperatures are usually below 65°F, this method of wall construction — one that incorporates a lot of thermal mass — makes sense. At dawn, the wall is cool. When the sun begins to heat the wall during the day, it takes a long time for the heat to penetrate the thick wall. At 1:00 in the afternoon, the interior surface of the wall is still cool, and so is the interior of the house. In the evening, just as the sun is setting, the heat has fully penetrated the thick wall. At night, the warm walls begin to cool, giving off some of their heat to the interior, keeping the occupants warm even if the weather is chilly outdoors. At the same time, the exterior surface of the wall cools, releasing the stored heat to the outdoor air. The next day, the cycle repeats.
As long as the diurnal temperature cycle sticks to this pattern — with the temperature uncomfortably hot during the day and uncomfortably cool at night — these walls work well. In these circumstances, the 12-hour time lag between when the sunlight warms the wall and when the wall gives off heat proves very useful.
However, if the weather doesn’t follow this pattern, the walls stop working well. During the winter, if the weather is uncomfortably cool for 24 hours, the walls never get warm. And during the hottest months of summer, the walls never cool off. During those months, the occupants will probably wish the walls included a little bit of insulation.
Here’s an important thing to remember about these traditional walls from Syria and Arizona: when these walls were commonly built, the builders didn’t have anything else to use for wall construction except mud and stone. They soon learned that there were advantages to making their walls thick, and the houses performed pretty well. But they didn’t have access to insulation, central heating systems, or air conditioners. Had these materials and appliances been available and affordable, they might have chosen to use them.
Thermal mass is most useful in hot climates
Modern homes with high-mass walls or floors usually locate the mass on the interior side of the insulation. A high-mass wall might consist of concrete masonry units (CMUs) insulated on the exterior with a layer of rigid foam. A high-mass floor might consist of a 4-inch-thick concrete slab placed on a continuous horizontal layer of rigid foam.
Researchers have found that hot-climate homes with high-mass exterior walls require less energy for air conditioning than low-mass wood-framed homes with similar levels of wall insulation. According to Alex Wilson, the editor of Environmental Building News, “Nearly all areas with significant cooling loads can benefit from thermal mass in exterior walls.”
There really aren’t any benefits to thermal mass if temperatures stay hot all night long. In an article called “Mass Confusion,” Charles Wardell reported, “Greg Kallio, a professor of mechanical engineering at California State University in Chico who specializes in heat transfer, recently … model[ed] ‘the whole gamut’ of wall systems, from stick-built to SIPs to insulated concrete, using industry standard energy analysis programs like EnergyPlus, as well as his own custom software. His conclusion? ‘The effectiveness of thermal mass is very dependent on diurnal temperature variation. You want nighttime temperatures that get at least 10 degrees cooler than the thermostat set point.’”
There are two reasons why high-mass walls can lower cooling bills:
- On days when the outdoor temperature ranges above and below the indoor temperature set-point during a a 24-hour period, the direction of heat flow through the wall reverses. During the hours when it is hot outdoors and cool indoors, the heat flows inward. At night, when it is cool outdoors and relatively warm indoors, the heat flows outward. This reversal of the direction of the heat flow reduces the overall heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. during a 24-hour period, because the heat flow reversal permits heat recovery. This heat recovery from the wall's thermal mass reduces the overall need for energy input from the HVAC equipment.
- A high-mass wall introduces a thermal lag or time delay in the flow of heat from the exterior to the interior. If the effects of strong afternoon sunlight only penetrate to the interior surface of the wall in the middle of the night, the air conditioner is more likely to be operating when outdoor temperatures are cooler. Since air conditioners operate more efficiently at night, when the outdoor temperature is low, than they do during the heat of the day, energy savings can result from the thermal lag introduced by a high-mass wall.
Can thermal mass lower my heating bills?
In the classic thermal mass scenario — a hot-climate house with uninsulated adobe walls — a high-mass wall can provide thermal benefits. But what happens in a cold climate during the winter?
If daytime highs are 50°F or less for months at a time — as they are in colder areas of the U.S. during the winter — thermal mass won’t help much. After all, heat is flowing through your walls in just one direction: from the interior to the exterior. Under these condition of steady-state heat flow, you need insulation more than you need thermal mass.
Studies have shown that thermal mass can provide heating energy savings in only a few areas of the country. “The sunny Southwest, particularly high-elevation areas of Arizona, New Mexico and Colorado, benefit the most from the mass effect for heating,” Alex Wilson has written. “In northern climates, when the temperature during a 24-hour period in winter is always well below the indoor temperature, the mass effect offers almost no benefit, and the mass-enhanced R-value is nearly identical to the steady-state R-value.”
According to a document posted on the Oak Ridge National Laboratory (ORNL) web site, “The most favorable climate for application of the massive wall systems is in Phoenix. Relatively worst location for these systems is in Minneapolis (especially for less insulating walls).”
That said, there are some circumstances that call for extra interior thermal mass. The classic example is a passive solar house with lots of south-facing glass.
Passive solar design principles
The more south-facing glass a home has, the warmer the house gets on a sunny day in winter. (Of course, much of the gathered heat escapes through the windows each night, so the south-facing glass is a double-edged sword.) If the home’s roof overhangs are properly designed, and the weather isn’t cloudy, south-facing windows will probably get sun from late September until late March.
If there are only a few small windows on the south side of the home, there’s no reason for the house to overheat. However, if the area of south-facing glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. is large, the house will be flooded with so much solar gain that it risks overheating.
That’s where thermal mass can help. If the sunlight streaming in the south-facing windows strikes a concrete floor or a concrete wall, the concrete will soak up some of the heat, preventing the house from overheating — or at least delaying the event.
Here are some rules of thumb for thermal mass in a passive solar building:
- The area of the thermal mass should be about three to six times the area of south-facing glazing.
- The maximum thickness of the thermal mass (usually concrete) should be about 4 inches. Thicker concrete won’t absorb heat quickly enough for the extra thickness to be useful.
- Dark-colored concrete floors work better than light-colored floors.
- Concrete floors should be bare — not covered with carpets.
The net result of including plenty of thermal mass in a passive solar building is to reduce the amount of energy needed for space heating. However, it’s important to remember that the reduced energy bills may come with a comfort penalty. Like the tractor-trailer that speeds down hills at 70 mph and struggles up hills at 60 mph, a passive solar home gets the most benefit from “free” solar heat if the indoor air temperature is allowed to climb up to 80°F on sunny afternoons and allowed to drop to 60°F or 65°F in the early morning.
Some homeowners are happy with these indoor temperature swings, while others find them uncomfortable. GBA’s technical director, Peter Yost, reported, “When I have been in well-designed passive solar homes with plenty of thermal mass in Santa Fe, New Mexico (a climate with nearly ideal diurnal swings) during the winter, I’ve concluded that it is best to expand your thermal comfort zone quite a bit in the early mornings until the sun catches up on that mass.”
Finally, it's important to remember that there is a simpler solution to the problem of overheating caused by too much south-facing glass: just reduce the size of your south windows. There is another benefit to this approach: the house won't lose as much heat on winter nights.
Disadvantages of high-mass buildings
In addition to their advantages, high-mass buildings have a few disadvantages:
- If the building is used as a vacation home or weekend retreat, it will take a lot longer than a low-mass building to heat the building up when you arrive on Friday night.
- In a high-mass building, nighttime thermostat setbacks won’t save as much energy as in a low-mass building. This effect is small but real.
- In some climates, high-mass buildings use more energy than low-mass buildings — but only if the insulation is installed on the wrong side of the wall. According to ORNL researcher Jan Kosny, “For buildings located in Minneapolis and Miami that have low R-value massive walls with the insulation material located on the interior side, total building loads can be higher with thermal mass than with the equivalent lightweight wall of the same steady-state R-value.”
Where should the insulation go?
If you want to use thermal mass to help stabilize indoor temperatures, the thermal mass should be inside the insulation, and there shouldn’t be any insulation between the thermal mass and the interior of the home.
Although manufacturers of insulated concrete forms (ICFs) sometimes brag about the benefits of thermal mass, one of the insulation layers in an ICF wall is in the wrong place. ICF walls sandwich the concrete core between two layers of rigid foam. The interior layer of insulation prevents the thermal mass from easily absorbing heat from, or releasing heat to, the building’s interior.
In a study conducted at the Oak Ridge National Laboratory, researcher Jan Kosny validated this common-sense analysis. “The most effective wall assemblies are those in which thermal mass (concrete) remains in good contact with the interior of the building,” Kosny wrote.
Several researchers have investigated whether the high thermal mass in ICF walls provides any reduction in energy use in cold climates. One of these studies was particularly thorough; it was performed in 2007 by Duncan Hill of Enermodal Engineering. Hill looked at the performance of ICF walls in a seven-story multi-unit residential building in Waterloo, Ontario.
I described Hill’s findings in an article (“ICFs Provide No Thermal-Mass Benefit In Canada”) published in the July 2008 issue of Energy Design Update. I reported, “At the time of construction, temperature sensors were placed at nine locations in the building; at each location, four sensors were inserted in the ICF wall. The researchers also installed data loggers to record indoor air temperatures. … While ICF construction creates a wall with very low levels of air leakage — ‘the concrete is a poured-in-place air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.,’ says Hill — the concrete had no thermal-mass benefit in Canada. The researchers wrote, ‘No thermal-mass impact or higher effective insulation value was observed.’ ... The lack of any benefit from the thermal mass in ICF walls was confirmed by computer modeling: ‘Comparing the simulation results of Models 1 and 3 shows that the increased thermal mass offered by the ICF wall construction in this building showed an insignificant improvement when compared to a low-mass wall assembly having the same thermal resistance and infiltration.’ The researchers examined the data carefully for signs of a thermal-mass effect. ‘We couldn’t detect one in the work that we did,’ Hill told EDU. ‘We weren’t able to tease anything from the monitoring data to show anything left over that might be seen as a benefit from thermal mass.’”
Oak Ridge National Laboratory quantifies the effects of thermal mass
In the 1980s and 1990s, researchers at the Oak Ridge National Laboratory (ORNL) undertook several studies designed to better understand the performance of homes with higher than average amounts of thermal mass.
Using data from hot-box experiments and energy modeling, four ORNL researchers — Jeffrey Christian, Jan Kosny, Andre Desjarlais, and Phillip Childs — quantified the advantages (and occasional disadvantages) of high-mass walls in a variety of climates. The team came up with a new metric (or “matrix”) called dynamic benefit for massive systems (DBMS). Kosny wrote, “The thermal mass benefit is a function of the material configuration, building type, and climate conditions, since high-mass walls are of greatest benefit in climates with large diurnal swings in temperature. DBMS values are obtained by comparing the energy performance of a one-story ranch house built with lightweight wood frame walls to the energy performance of the same house built with exterior massive walls. The product of DBMS and steady-state R-value is called an R-value equivalent for massive systems. This R-value equivalent does not have a physical meaning. It should be understood only as an answer to the question, ‘What wall R-value should a house with wood frame walls have to obtain the same space-heating and -cooling loads as a similar house containing massive walls?’”
This new metric was both useful and easy to misinterpret. Ever since ORNL researchers started writing about the concept of “an R-value equivalent,” advertising copywriters have been having a field day with the idea.
Is there such a thing as “effective R-value”?
While R-value is defined by federal statute, the term “effective R-value” has no agreed-upon definition — other than the narrow one created by ORNL researchers when they defined the “dynamic benefit for massive systems.” As noted by consulting engineer Maribeth Bradfield in her article, “The Effectiveness of Effective R-value,” “A quick online search for effective or equivalent R-values reveals a wide range of results. Depending on the industry and the building assembly being marketed (or marketed against), effective R-values can mean: the combination of standard R-value and air leakage (or lack thereof); R-value of insulation adjusted to account for thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. ; R-value plus thermal mass effects; R-value plus thermal mass plus air infiltration; and probably other combinations as well.”
Alex Wilson has echoed Bradfield’s observation. Wilson wrote, “All sorts of claims are being made about mass-enhanced R-value (usually called ‘effective R-value’) with little standardization.” Because the term is all but meaningless, “effective R-value” claims in advertising brochures should be ignored.
“The mass effect is real,” Alex Wilson wrote. “High-mass walls really can significantly outperform low-mass walls of comparable steady-state R-value — i.e., they can achieve a higher ‘mass-enhanced R-value.’ But (and this is an important ‘but’) this mass-enhanced R-value is only significant when the outdoor temperatures cycle above and below indoor temperatures within a 24-hour period.”
Facts rarely deter those aiming to profit from exaggeration. For example, consider claims made for Rastra walls. (Rastra is a manufacturer of ICFs made from recycled polystyrene. According to tests performed at Oak Ridge National Laboratories, a 10-inch-thick Rastra wall has an R-value of R-8.2 or less.) A Rastra brochure boasts, “A typical ‘advertised’ R-value for new wood frame construction ranges from R-13 to R-19. A Rastra wall provides a much higher Effective R-value of up to R-46.” Notice that the copywriter capitalized “Effective,” to make the adjective sound more official.
Or consider a claim made by Massachusetts Building Products of Warren, Massachusetts. The company boasts that the “actual R-value” of its concrete-filled iForm is R-24, while the “effective R-value” of the wall is R-32+. While that might be true in Phoenix, it isn’t true in New England. The company notes its area of distribution proudly on its website: “Serving Massachusetts, Connecticut and Vermont.” It would be hard to choose three states in the lower 48 where thermal mass is less useful.
What the building code says
The 2009 International Residential Code includes a definition for “mass walls.” (All walls have mass, so the phrase “mass walls” is awkward. For that matter, all walls have thermal mass, too. But people who write code books are immune to logic.)
According to section N1102.2.4 of the 2009 IRC, “Mass walls, for the purposes of this chapter, shall be considered above-grade walls of concrete block, concrete, insulated concrete form (ICF), masonry cavity, brick (other than brick veneer), earth (adobe, compressed earth block, rammed earth) and solid timber/logs.”
If you choose to build a wall out of one of these materials, the code allows you to include less insulation than would be required for a wood-framed wall — even in colder climate zones. For example, wood-framed walls in climate zone 5 need to be insulated to R-20, but concrete block walls can get away with R-13 (as long as at least half of the insulation is on the exterior of the wall).
It’s hard to know whether the code provision that allows “mass walls” to have less insulation than wood-framed walls is logical or not. A modeling study by Cheryl Saldanha and Joseph Piñon (“Influence of Building Design on Energy Benefit of Thermal Mass Compared to Prescriptive U-Factors,” 2013) shows that a three-story office building in climate zone 2 or climate zone 5 with high-mass walls will use less energy than a comparable building with lightweight walls, even when the high-mass walls have a lower R-value than the lightweight walls. The study supports the conclusion that prescriptive tables in the code that allow lower R-values for high-mass are logical.
In many cases, however, the improved performance of concrete block walls or ICF walls may be due to factors other than the wall’s thermal mass:
- Most walls that are built of concrete or adobe have lower rates of air leakage than most wood-framed walls. (This wouldn't be true, of course, if air-sealing requirements in the building code were consistently enforced.)
- Most concrete walls are insulated with continuous insulation (for example, rigid foam) rather than ribbons of insulation interrupted by thermal breaks (otherwise known as studs).
Here's my advice: if you want an R-20 wall, build a wall that includes (at a minimum) R-20 insulation. (Of course, continuous insulation performs better than insulation installed between studs.) It doesn't make any sense to build your wall with only R-13 insulation and hope that “R-13 will perform just as well as R-20 because the wall includes lots of thermal mass.”
One thing is for sure: the code provision that allows log homes to get a break when it comes to minimum R-value requirements is nuts, since most log homes are leaky.
Does your new home need interior concrete?
Should you include some concrete on the interior side of your home’s insulation to increase your home’s thermal mass? Maybe — but only if you are planning to include lots of south facing glass or your house is located in a hot climate.
If the thermal mass is part of a passive solar design strategy, the best type of thermal mass is probably a concrete floor on the south side of your house.
If you live in a hot climate, and want the thermal mass to help lower your air conditioning bill, your thermal mass should be located in your exterior walls. Remember, though: your walls still need plenty of insulation.
Even though interior concrete sometimes has thermal benefits, it also has drawbacks — including its high cost. In most climates, you can get the same benefit that concrete might provide by simply installing more insulation — and in most cases, the added insulation will cost less than the concrete.
Remember: the better insulated your house, the less thermal mass matters.
Martin Holladay’s previous blog: “Choosing HVAC Equipment for an Energy-Efficient Home.”
- Martin Holladay
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