Thermal Mass Defined
When it comes to building an energy efficient home the discussion often rests on how much insulation should be in the walls, in the ceiling, under the slab, etc. But as I’ve written about before, energy efficiency is about more than R-values.
A house also benefits from having well placed windows balanced with a good amount of thermal mass to passively supply heating and cooling requirements.
But how much thermal mass is enough?
Before we really get into it let’s first define what thermal mass is. First off, thermal mass is not insulation.
Insulation restricts the flow of heat while thermal mass freely and easily absorbs and releases heat. They are inversely proportional to one another.
Mass materials are dense and heavy and include things like stone, concrete, earthen floors, adobe blocks, cob, rammed earth, clay sand earthen plasters, brick, etc. In a cold climate, where outside temperatures remain below comfortable levels for long periods of time, massive walls will cool down rapidly and require massive inputs of heat to warm the interior to a comfortable degree. In such climates, a combination of thermal mass and insulation is ideal, especially when the thermal mass is placed on the interior side of a well insulted building.
According to Chris Magwood, this is a winning combination because, as he writes in Making Better Buildings:
The thermal mass is able to absorb and release excess heat (from solar gain, occupant loads, wood-fired heat sources that burn and then go out) while transmitting very little of that heat to the outdoors thanks to the insulation. Hot climates will find this system beneficial in the other direction, with massive walls absorbing daytime heat without transmitting it indoors and releasing that heat during the cooler nights. A lot of North American locations experience both conditions, depending on the season, and will benefit from wall systems that have mass on both sides of an insulated core.
How Much is Enough & Where Should it Go?
Well placed thermal mass, then, is a huge asset, as it both reduces energy use and drastically increases comfort. Knowing this we can now answer the original question: how much thermal mass is enough? And where is it best placed?
To answer these questions we’ll first look at the excellent book The Passive Solar House: The Complete Guide to Heating and Cooling Your Home by James Kachadorian. James writes that the most important detail to get write is the thermal balance, meaning that
“…the overnight loss in heat is equal to the amount of heat that the east-, south – and west – facing glass was able to collect in excess of the amount of heat needed by the home during the day while the sun was out. This excess heat [is] absorbed by and later given by the [thermal mass].”
This makes sense. We first need enough heat to make it comfortable during the day while storing enough heat in the mass materials to carry us through the night until the sun starts shining again. But getting the thermal balance can be difficult and as James writes,
It is difficult to make a general rule that dictates the amount of glass and the amount of thermal mass that a solar home will need to perform optimally throughout the year. Try not to use too much of a good thing. That is, don’t over – glaze. Make sure that the thermal mass is sized to allow no more than a 8-degree [F] temperature swing from its warmest to coolest state.
Attempts have been made to produce ratios that will dictate the ideal relationship of glass to mass, or glass to wall area, or glass to floor area. Again, considering the wide variations in regional climatic conditions and in the specific characteristics of local building sites, general rules are difficult to create and apply. There really is no substitute for good solar design and good judgement.
Some Numbers to Follow
While James’ book is full of wonderful principles and lays the step-by-step process for calculating the perfect balance using R-Values, solar heat gain factors, shade coefficients, etc he’s reluctant to give any rules of thumb on how to approach this balance. For this we’ll look to Dan Chiras, author of The Solar House: Passive Heating and Cooling.
Here we learn that thermal mass is best evenly distributed around the house at a thickness of 4 to 6”. Further, its effectiveness increases proportionally up to four inches, then it falls victim to the law of diminishing returns — a 6” mass wall is only 8% more effective than a 4” wall). Surface area to mass is also very important. According to Ron Judkoff, director of the Buildings and Thermal Systems Center at the National Renewable Energy Lab in Golden, Colorado, “two hundred square feet of two-inch-thick mass is more effective than one hundred square feet of four-inch-thick mass.”
While adding 4” of thermal evenly distributed around the house is a good starting point, it only tells part of the story. Mass is sized in relation to solar glazing. Once you determine how much solar glazing you have you can calculate how much mass you need. An under-massed and over-glazed house is just as disastrous as and under insulated and leaky home. Window placement and glazing allocations should be calculated as a percentage of total home square footage. Dan Chiras gives the following guidelines:
North: Less than 4%
East: No more than 4%
West: No more than 2%
South: 7 to 12%
Of course, these guidelines depend on proper home orientation, with the true south (in the Northern Hemisphere) aligned on an east-west axis, and a desirable house shape, as volume to surface area plays an important role in getting the right balance. Eves must also be properly positioned such that the glazing is shaded from the hot summer sun, allowing the low winter sun to enter the building and warm the thermal mass. A good online tool to help with this can be found at: www.susdesign.com/overhang/index.html
In a cold climate the south glazing is most important and is generally on the high end of the spectrum (e.g. 12%), even approaching 20% if a thermal storage wall (a trombe wall) is made part of the design. But to make this work you need the right amount of thermal mass to buffer against thermal variance from any increase in glazing. Another important rule to remember is that the incidental thermal mass of the house – the furniture, drywall, framing, floors – will buffer the first 7% of glazing (as a percentage of total square footage). Any additional glazing will require further mass to buffer it. To determine how much we can use these ratios from The Solar House:
Ratio 1: Each square foot of solar glazing over the 7% mark requires 5.5 square feet of uncovered sunlit (directly illuminated) floor mass. Uncovered means no carpets, couches, or tables in the way of solar radiation on the floor.
Ratio 2: This ratio relates solar glazing to floor mass not in direct contact with the incoming solar radiation, but in the same room. In this case, 40 square feet of uncovered and “unlit” mass accommodate one square foot of solar glazing. The efficiency of floor mass falls of dramatically when it is covered by carpeting).
Ratio 3: This ratio relates solar glazing to wall mass. In a room being warmed directly by sunlight, you will need 8.3 square feet of wall mass for each square foot of solar glazing over the 7% mark.
Keep in mind that you can use one or a combination of these strategies to get the proper balance of glass to mass. You only need to make sure that for every incremental amount of glazing you match the appropriate thermal mass to balance it. As noted above, both floor mass and wall mass should be four to six inches thick.
It’s also interesting to note that while thermal mass absorbs heat from air it is far less efficient at doing so than if it is struck by sunlight. According to Steven Winter Associates, “storage mass that is heated only indirectly by warm air from the living space requires four times as much area as the same mass in direct sun to provide the same thermal effect.” It follows that the more mass you can locate in direct sunlight the better.
In the end thermal mass is part of a larger energy efficiency picture. Efficient homes not only use the right amount of insulation, but have proper orientation and shape, and windows (and overhangs/shading) balanced with internal thermal mass to supply most of the heating and cooling requirements.
Get these things right and you may cut your heating and cooling loads by as much as 50 to 80%. A little design work up front can have big savings in the long term, not to mention a high degree of comfort throughout the seasons.