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Everything You Always Wanted To Know About Thermal Mass But Were Afraid To Ask

Building styles designed to minimize energy use by providing plentiful amounts of natural light, passive solar energy for the purposes of heating, and free and easy ventilation to facilitate cooling in summer are the “in” thing these days. But while these “new” construction methods have been receiving a tide of favorable publicity, and leaving “green” builders with a swollen sense of self-importance, what has been going on is actually a classic example of the reinvention of the wheel.

In the past, up to 80 percent of all the funds invested in new home construction were spent on the basic structure itself, as people were smart enough to realize that a livable and functionally-efficient home could not be built on the cheap. The terminology used to describe what was happening back then may have been somewhat different, but the overarching idea was the same: if we want an interior environment that keeps us comfortable, preserves our finances, and provides us with everything we need to make a house feel like a true home, we need to put a real effort into the building process starting right from the original blueprint.

Unfortunately, society no longer prizes intelligent design and craftsmanship the way it used to, as “cheaper and faster” has become the mantra of modern times. Consequently, the percentage of home building funds that are dedicated to the actual structure itself have declined from 80 percent in the past to less than 20 percent on average today, and as a result the landscape is now blighted with an endless sea of pre-fabricated, cookie-cutter houses that can be slapped together in a few days and plopped down anywhere without the slightest thought about how they might fit in with their surroundings, or whether or not they really suit the needs of the families who will be occupying them. So while the movement to design more energy-efficient structures is laudable, it really represents a baby step back toward rational construction practices after decades of foolishness and shortsightedness.

A big proportion of the money being spent on homes now is to pay for the installation of energy-consuming systems that alter the interior atmosphere by heating or cooling the air. At first consideration, this probably seems logical, sensible, and unavoidable. After all, in almost every climate the temperatures can become extreme at least on occasion, and in the wintertime especially it would be very difficult to survive if we did not have a way to keep our living spaces toasty when the mercury decides to take an Arctic plunge.

But there is another alternative, an indirect method for moderating indoor temperatures that works surprisingly well. Rather than heating or cooling the air, we can use environmental sources of heat – available entirely for free – to heat buildings instead. If the right materials are used, walls, floors, ceilings, and certain types of fireplaces can supply a steady and reliable form of radiant heat during the wintertime, and they can even be used to remove unwanted heat from the rooms we inhabit during the scorching summer months. The ability of construction materials to perform such useful and money-saving work is derived from a principle known as thermal mass, and all practicing or prospective off-the-gridders need to begin educating themselves about this exciting concept post haste.

That is, of course, unless they have no interest in constructing new homes or remodeling old ones to take full advantage of the amazing energy- efficiency-promoting and temperature-moderating capacities that building materials with high thermal mass can offer. For those who prefer to continue squandering their money and energy needlessly by relying on the traditional methods for controlling the indoor climate, all the information that follows will be useless and should be completely ignored.

The Science And Logistics Of Thermal Mass

Have you ever felt the outside of a building made from a substance like brick, stone, or concrete, and been surprised to discover how warm it was, even though it was the evening and the sun had actually set hours ago? If you have, then you have experienced the principle of thermal mass in action first-hand, figuratively and literally. Objects with high thermal mass have a sponge-like capacity to absorb copious amounts of heat from nearby or readily accessible radiant sources of energy, and they will release that accumulated warmth gradually over a period of many hours, usually long after the sources that provided the heat originally are no longer present.

When heat energy passes through any physical material, it does so through a process known as conduction, which guarantees that the warmth soaked up by the surface of an object or mass will be transferred into its cooler interior. When the surrounding air temperatures drop below that of the physical object storing the heat, it will then be radiated back into the atmosphere, providing a source of steady heat to any room where the object should happen to be located. The amount of time an object takes to release the heat it soaks up is known as the thermal lag, and the higher the thermal mass the greater the lag time, which is what gives materials their potential value as indoor environmental moderators.

For the purposes of controlled heat release, materials with a high capacity to absorb heat, moderate density and conductivity, and a low rate of reflectivity are most desired. Basically, what is required are materials that can soak up a lot of heat but release it all within a 24 hour period, so once a particular cycle of absorption and emission has finished the process can begin all over again the next day.

Materials with high thermal mass include concrete, masonry (stone work), brick, adobe, ceramic, and water. Glass, steel, and wood, on the other hand, all score very poorly on the thermal mass scale and are not able to contribute anything positive to the heat regulation process. The earth itself also has a high thermal mass, which is why building underground or with earth berms could be a great option for some (earth-sheltered building practices will be discussed more in-depth in a future article).

High thermal mass materials on outer walls contribute little to indoor temperature control in most circumstances. If walls are outside of the insulation envelope, any heat they collect will simply radiate back into the air at some point rather than into the home. Therefore, for the purposes of temperature regulation materials with high thermal mass should be used on the inside of the house, where the principle of thermal lag guarantees that any heat collected during the day time will be released only after the sun has gone down. This is the perfect set-up when the weather is frigid, because it allows for the full exploitation of sunlight entering through southern windows during the daylight hours, and it is also useful in climates where summertime temperatures drop considerably following night fall (in the high-altitude desert southwest, for example, where quite a few off-the-gridders have taken up residence). If summertime temps do not fall off much during the overnight hours, high thermal mass building materials can still help soak up unwanted heat during the day, as long as some kind of a ventilation system, perhaps using fans, has been installed to make sure the hot air released between 9 p.m. and 6 a.m. is mostly blown out of the home.

Harness the power of the sun for your energy needs…

High Thermal Mass Building Features: The Options

For climate control purposes, high thermal mass materials can be used to construct floors, interior walls, or a special type of fireplace called a masonry heater, and they can even be placed on the roof so that interior heat levels can be regulated from the top down. Whatever choices are made, high thermal mass walls or floors should never exceed six inches in thickness, otherwise they will be too dense to absorb and release heat at a consistent and reliable rate.

Concrete slab floors can meet as much as 30 to 40 percent of the effective thermal mass heating requirements of a home, as long as there is sufficient glazing on the southern façade to allow an adequate quantity of sunlight to enter at the ground floor level. To efficiently harvest the southern sun in winter, the standard recommendation is that the total square footage of the windows on the south side of a home should be 7 to 12 percent of the total square footage of the floor area, and if the percentage reaches 12 percent, at least 60 percent of the adjacent floor area should be left open and uncovered by furniture or rugs. Furthermore, to guarantee maximum thermal mass efficiency, when the slab floor is installed it should be poured into forms that have been placed directly on top of the (dry) underlying earth, as the soil’s high thermal mass will allow it to pass on heat from below when the weather is cold, or absorb heat from above when the floor becomes hot in summer.

Ceramic or vinyl tiles, slate, or thin layers of masonry can be used as decorative covering for a concrete slab floor without affecting its capacity to absorb heat, since these materials also have high thermal mass. If the concrete floor is left exposed to the penetrating light directly, an un-polished texture should be maintained, and it should be colored in a medium hue such as brown or dark red so that it will not reflect the beams of sunlight that hit it.

Interior walls made of brick, stone, or concrete, along with masonry heaters if they are installed, will furnish the remaining 60 to 70 percent of the thermal mass temperature regulation effect in most homes designed to maximize this effect. Rough textures and darker hues also work the best here, since they will absorb more heat, but of course darker colors will also reflect light poorly, which means they may increase the need for indoor lamps. Sunlight should shine directly on high thermal mass walls if at all possible, although if they are located near a wood stove, masonry heater, or other heat-generating device, they will also be able to soak up significant amounts of heat from these other sources. The most important rule for interior walls intended to provide climate control is that they must be kept enclosed inside the insulation envelope, regardless of where they might be located in relationship to the exterior of the house.

One neat innovation that can work quite superbly in a number of settings is the trombe wall. Rather than comprising the outside wall of a house, a trombe wall is located directly inside of a southern façade made primarily from glass, disconnected from the outside of the home but still functioning as an inner wall. Made of masonry or concrete, the trombe wall will absorb the heat collected in the open hot-box space that separates it from the outer glazing, and then gradually pass that warmth on to the interior rooms as the day turns into night. Additionally, if extra heat is desired during the daytime, trombe walls usually have vents installed that will allow this, but the general idea behind the trombe wall is to trap as much of the incoming sunlight as possible inside its high thermal mass so it can all be released at night when it is really needed, or when it will create the least havoc if we are talking about keeping things cooler in the summertime. Ideally, a trombe wall will have retractable night insulation, which can be used to cover the backside of the structure to keep heat from escaping once the sun has drifted below the horizon.

Trombe walls can also be filled with water, which has a higher thermal mass than any type of building material. The wall could be made transparent in this case, but while this will allow light through, it will also somewhat lessen the effectiveness of the wall as a heat sink. Nevertheless, water’s ability to absorb and release heat slowly over the course of the day is without parallel, and any way that can be devised to take advantage of this property is highly recommended. Sometimes, rather than a fully functional trombe wall, people choose to install water tanks directly beneath south-facing windows, which can leave the interior of the home open to the sunlight even as much of the heat it produces is being absorbed for later re-radiation.

Roof ponds are another way to exploit the high thermal mass of water. Alternately left exposed to the elements or covered by a sheet of insulation, roof ponds are placed directly on the roof (hence the name), usually on a strong steel deck designed to support their weight. If the pond is left open in the daytime, it will absorb heat from the sun and pass it by convection through the ceiling of the house during the night when the insulation cover is in place; or conversely, it can be left open at night so its water shed any heat it contains, and then when it is covered during the day it will cool the roof and ceiling and thereby cool the house below as well.

Finally, a centrally located masonry heater is another excellent way to use high thermal mass materials to keep interior spaces warm overnight in the wintertime. This stone fireplace/wood stove hybrid, which steadily releases the heat from its interior fires into the surrounding room through its thick walls over the course of the day, is specifically built to exploit the high thermal mass of its constituent materials, and unlike a normal fireplace, it will not suck warm air out of the house through its chimney.  Of course, even when a masonry heater is not being used it can still absorb heat from the atmosphere, meaning it will help regulate internal temperatures constantly just like any other part of a home that has been made of materials with high thermal mass.

Going With The Flow

The concept of thermal mass is derived from some very basic principles of physics. Heat always follows the path of least resistance, flowing from where it is warm to where it is cool, and by regulating the pace of this flow, it is possible for ingenious builders and home designers to cut down on the artificial energy consumption requirements of home owners considerably.

High thermal mass and off-the-grid living are the proverbial match made in heaven, and all who are currently pursuing or planning to soon pursue their homesteading dreams would be well advised to learn everything there is to know about thermal mass and its many possible applications to home construction and remodeling.

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