Sun shade reduces solar gain in Mexico (C) Daniel Friedman Thermal Mass Design Advice for Building Heating, Cooling, Insulation

  • THERMAL MASS in BUILDINGS - CONTENTS: What is thermal mass? How does thermal mass really work in buildings? Rules of thumb for sizing and using thermal mass for heating, cooling, insulation. Use of thermal mass in passive solar homes and other energy efficient buildings. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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Thermal mass in buildings:

This article, an update to Remember Thermal Mass?, discusses thermal mass in buildings, its effects in buildings, how to use thermal mass for passive solar houses, and using thermal mass for both heating and cooling as well as for insulation.

References to texts and guidelines for sizing thermal mass and using thermal mass are included. Here we explain what thermal mass is, how thermal mass works in buildings, reviews a number of thermal mass claims, thermal mass "rules of thumb", and it discusses use of thermal mass in passive solar homes, use of thermal mass for heating and cooling, and the effects of thermal mass as "insulation" in buildings.

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How to Understand Thermal Mass in buildings

This material is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. The text below paraphrases, quotes-from, updates, and comments an original article, "Remember Thermal Mass?" (see links just above) from Solar Age Magazine and written by Steven Bliss.

What is Thermal Mass?

Here are a number of thermal mass claims that I have heard over the years and tried to evaluate:

Thermal mass isn't easy to understand, and its effects are difficult to measure and predict. No wonder builders resist adding thermal mass to their buildings except where high-mass materials are standard. For the most part, it's just as well.

Thermal Mass in the Mind - Simple Analogies Explain Thermal Mass

Because the dynamics of thermal mass are pretty complex, I find it helpful to think about simple analogies. My favorites are the beer cooler, King Tut's tomb, and the refrigerator. The beer cooler has a low-R shell (1/2" Styrofoam) with a high mass interior (cold beer) that keeps the contents cool for several hours. Once it warms up, though, the mass does you no good -- unless you drink it.

In King Tut's tomb, on the other hand, the thermal mass is on the tomb exterior. Because there's so much of it, it will stifle almost all thermal swings on the interior of the structure, keeping it close to the annual average temperature. The un crowded pyramids, I'm told, are cool and comfortable. But those with big crowds (causing high internal heat gains) are hot, smelly, and stuffy.

The refrigerator combines a fairly high-R shell with a low- or high-mass interior,depending on the refrigerator's contents.

Question: Does a full refrigerator use less energy than an empty one, as is sometimes claimed?

Answer: The empty refrigerator will cycle on and off more often, because air cannot store much heat (or coolness). But no one I spoke with could tell me whether this saves energy. [It may consume more energy to cycle any compressor motor on and off frequently than to have a compressor run with longer on cycles because of extra energy that is used to start the motor --DF] Like the cooler, the full refrigerator will stay cold longer in a power outage.

What Does Thermal Mass Actually Do in a Building?

In a building, as in a refrigerator, mass really does only one thing. It delays the flow of heat. Delaying the flow of heat (say from indoors to outside, or from outside to indoors) can save fuel (and energy) in two ways. It can store energy in the form of heat (or "coolness") that you've bought cheaply (solar energy source or off-peak electricity, for example) for use when that same energy is more expensive.

In climates where daily temperatures swing above and below the human comfort zone, thermal mass can save energy by applying the day's heat (gained during warm hours) against the night's coolness. Thermal mass also improves the comfort of building occupants, particularly in solar homes, by reducing the range of temperature swings.

Thermal Mass for Passive Solar buildings

There's no argument that in passive solar houses you need to add thermal mass when the glazing area exceeds 6 or 7 percent of the floor area. There is less agreement, though, on where and how to place the thermal mass in the building.

The More Thermal Mass the Better

Many early passive solar houses were under massed, said solar pioneer Doug Balcomb. "You can't have too much mass in a direct-gain house." Balcomb said in a Solar Age Magazine interview [10/1985]. In a well-insulated shell, he said, the more mass there is, the more stable temperatures will be - and this, he emphasized, increases comfort.

A Thermal Mass Example from Mexico

This is exactly our experience [DJF] of living part time in a masonry structure in the mountains of central Mexico. Outdoor temperatures range from the upper 40's F on the chilliest nights to the high 80's on the warmest days. Because the building's walls and roof are constructed of thick adobe or concrete, the building's thermal mass keeps indoor temperatures much more even, typically ranging between 64 degF. and 72 degF. if we do nothing particular to affect temperature. The home is located in a high (6200 ft) arid area where days are hot and sunny and nights are clear and chilly.

Sun shade reduces solar gain in Mexico (C) Daniel Friedman

If we open windows in the cool morning to further cool down the building interior, closing them by mid-day, the home stays another 5-8 degF. cooler during the hottest part of the afternoon, a period typically between 1PM and 4PM daily.

Strategic placement of short sun-shades over windows receiving the most afternoon sun (photo at left) made a palpable difference in the comfort of those indoor spaces by reducing the time period over which direct sun shines against those windows. Only a small shade roof was necessary as we're simply reducing the amount of direct sunlight shining in those windows for a few hours during the hottest part of the afternoon.

In our photo you can see the shade just beginning to cover the upper part of the windows and doors of this room as the angle of the sun is shifting.

This Mexican home has no central heating and relies on gas-fireplaces (not a great idea) and small electric heaters for supplemental heat only on the very coldest mornings. At no time have we ever had to run these heaters throughout the day.

Diminishing Returns of Thermal Mass

Beyond some point, though, adding more thermal mass won't help performance. Once you put in enough thermal mass to prevent solar overheating, piling a few tons of brick in the living room won't reduce heating bills any further. It can even be a liability if night time thermostat temperature setbacks are planned.

So it becomes the designer's job to decide on the right amount of thermal mass for a building.

Thermal Mass Rule of Thumb

Mass surface area 6-7 x South-Facing Glass area

Balcomb's simplest rule of thumb is that a direct-gain space should have a heavy mass surface six or seven times the area of the south facing glass (windows and doors), or 15 times that area for drywall construction. This is for mass in the direct-gain space. This so-called radiatively coupled thermal mass is any mass in the line of sight of the mass where the sun first strikes.

The thicker the better for solar mass.

Insightful passive solar designers recognized early on that only the first 3 or 5 inches of thermal mass did any good. Nonetheless, many passive solar house designs still feature huge fireplaces or accent walls as the home's thermal mass.

Solar mass design thinking (as of the mid 1980's) is that, pound for pound, thin mass works better than thick mass. The reason, said Balcomb is simply that heat moves into and out of thin mass more readily. There is more surface area.

Thickening the thermal mass, though, is still an effective strategy up to a point. A dense material like concrete performs better up to 4-inches.

The walls of the Mexican home discussed above range from 17 cm to 20 cm (the age of various parts of the home vary and the materials vary among cement-stucco covered adobe and poured concrete). The indoor temperatures of the home do not vary significantly from the indoor temperatures of a 300-year old stone structure nearby even though the older building has much thicker walls. -DJF

A not-so-dense material like drywall probably levels off at a couple of inches. So doubling the drywall or plaster thickness is a good way to gain thermal mass. It tends to double the thermal benefit of the drywall. This thin-mass approach has won converts such as California architect David Wright, who has used an extra-thick finish of high-density plaster.

Thermal mass has to be in direct sun?

Early texts on solar energy and thermal mass said that thermal mass had to be in direct sunlight. Studies have shown, said Balcomb, that thermal mass alone is about 30-percent more effective in direct sun than in reflected sunlight. But it's still more important, he added, to spread the mass around the occupied space.

How about convectively coupled mass - materials in rooms not exposed to solar gains, but heated by airflow? Heavy mass that is convectively coupled, said Balcomb, is about one quarter as effective as mass in south-facing rooms. But thin thermal mass, such as drywall or plaster up to about an inch thick, is equally effective whether it's convectively or radiatively coupled.

With a south-facing room, the floor and back wall are good thermal mass locations. But the ceiling too, is an excellent spot that is often overlooked. The ceiling gets reflected light and reradiated heat off the floor and walls.

The darker the thermal mass the better?

The darker thermal mass is another area where solar gospel has been updated. Early solar designers called for dark massive materials. Current thinking, said Balcomb, is that only floors should be dark. The object here is to keep heat close to the floor to counteract stratification. Walls and ceilings, he said, should be light colored to bounce the light around and get maximum use of a broad surface area of thermal mass.

Another reason for light colored interior walls and ceilings is their daylighting value, particularly with clerestories. Dark colors placed against bright spaces cause uncomfortable contrast glare and bring less light into the building's interior.

Using Thermal Mass for Building Cooling

Thermal mass can store coolness. This is helpful when coolness can be obtained for little or nothing during part of the day and used when it is scarce or less expensive. Our Mexican home example above described simply opening windows to admit cool outside air in the early morning to provide additional coolness inside to "prepare" the home for the hot afternoon temperatures. Night flushing and photovoltaic-powered cooling are two additional ways to take advantage of this strategy.

Thermal mass can also delay and reduce peak cooling loads. For example, a west wall in Florida gets brutal sun on a summer afternoon. A high-mass wall will absorb that heat and delay its transfer indoors until the evening, when electricity is cheaper. If nights are cool, those peaks will be reduced as well as delayed.

Using Thermal Mass for Building Insulation

Brick lined wall (C) Daniel Friedman

Promoters of envelope thermal mass as insulation have fought an uphill battle, but this is by no means a new idea. Brick-lined walls in pre-1900 homes used brick and mortar both to stop through-wall drafts and to provide enormous thermal mass in buildings.

See BRICK LINED WALLS for details. The efforts of promoters of envelope thermal mass to assign R-values or correction factors to thermal mass have been largely unsuccessful. Their main problem is that the effects of thermal mass depend on too many factors, such as its location in the wall (inside the insulation is generally considered best) and the climate.

In general, thermal mass in exterior walls only helps when outside temperatures swing daily above and below the comfort zone. Thermal mass as building insulation has the biggest effect where the swing is great and equally balanced above and below the house's temperature setpoint. High, arid areas in the Southwest and our Mexican home example above (also in a high, arid area) are a good example: days are hot and sunny; nights are clear and cold.

Under these conditions, thermal mass can reduce the overall heating/cooling load, theoretically to zero if daytime gains equal night time losses, and if the mass is thick enough and sufficient in total quantity to provide a half-day lag for the building interior.

In very mild climates, or in swing seasons, this thermal mass effect also works. But although percentage savings may be great in these periods, overall savings will be small since there's not much energy to save.

During periods when the temperature is always too high or too low, however, envelope mass has no demonstrated effect on heating or cooling loads.

Original Thermal Building Mass Article in PDF form

"Remember Thermal Mass? The tools for understanding thermal mass may lie in the beer cooler, King Tut's tomb, and the refrigerator" - this article is provided in original form (the PDF links below) and in expanded, updated text in the web article that is just below the links to the original version in PDF form.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Readers should also see the passive solar design articles organized at SOLAR ENERGY SYSTEMS.

Thermal Properties & Thermal Mass Articles


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