Thermal Mass and Insulation Strategy

Notes from SouthWoods Professional Permaculture Series

Thermal Mass and Insulation, Local Conditions Dictate the Mix.

Building strategies in changing climates vary based on the biomic tendencies of the region.  Cold or hot regions also have sub-regions of precipitation, wind, and landforms. In either hot or cold climate strategies, principles and solutions still apply; it’s the various materials and conditions on site that dictate the implementation. In mountainous and higher latitudes, the cold can be relentless. Unlike a Desert, the daytime and nighttime temperatures (Diurnal Temperature Variations) may not fluctuate enough to use thermal mass to buffer the changes, such as with adobe or earth bag homes.   

In terms of building and construction, a thermal mass is essentially a solid that absorbs heat from the sun during the day and slowly radiates the heat at night. It may take the form of a thick wall or floor slab, made of either stone, concrete, clay, adobe, brick, or

even a volume of water. A thermal mass offers a much more energy efficient alternative to using a standard, forced-air heating system.   Heat is transferred from a thermal mass by radiation, convection and conduction. In the winter, the heat moves from the wall to the interior space, and in the summer, it works in the opposite direction and is expelled outdoors. Therefore, a consistent and comfortable temperature may be maintained throughout the day. The right orientation for a thermal mass depends on the climate in which a structure is built. In a cold climate, it should face the winter sun, whereas in a warm climate, it should not be exposed to direct sunlight. In general, the larger the mass, the more effectively it performs.

The continuous lower temperatures quickly transfer into the thermal mass, and in time, begin to absorb the heat within the structure.  Large log or stone homes, which keep cool in summer as the temperatures cool at night and rise during the day, lose their benefit in a cold climate. This relationship between structure and function needs serious consideration when planning a natural building. Should the thermal mass be inside? Such as with a rocket stove fireplace and insolation in the exterior walls. Or, should the thermal mass be on the outside to moderate the diurnal temperature differential of hot days and cold nights? There would seem to be hybrid combinations for every climate. How dense and how thick should the walls be in each situation? What is the strategy of isolation and mass density to achieve the best efficiency? Each side of a building may have a different material. Highly insulate on the side against prevailing winds and appropriate glazing on the sunny side.

 Insulation is used much more in colder climates to abate the infusion of extreme temperatures into a structure. Homes have thick walls and roofs filled with millions of airlocks cells that moderate the temperature change between the two sides. Double and triple pane glass is used in the windows to add a barrier to temperature change between the solid materials. Air and gases are used between the panes of glass,

much like a thermos bottle, which uses a vacuum chamber between the outer wall and inner container filled with hot liquid. Reducing the energy transfer on the edges increases the energy storage capacity of the space.

While working in Haiti I found that the dense block buildings a cool place in the day, but frigid each night. As a result, I lay on top of my 0ºF Polarguard sleeping bag in the sweltering temps of the late evening and around 2 AM, when the cold woke me up, I crawled inside the warmed bag. Part of the solution might be integrating our own habits with the dynamics of the built environment, just as we do the natural environment. Inside our homes we can insulate ourselves with fingerless gloves, and a sweater, or stocking cap. This is much easier than another cord of firewood or gallons of fuel oil. Start with zone “0”. Cultural adaptation is much easier than mechanical solutions. – Dan Halsey

So we have two principles working in a cold climate. There is the thermal mass of the structure including the air or water within the structure and the degree of insulation, which buffers the temperature changes across the layers of the edge or exterior surface.

 High tunnels used in agriculture have translucent sides that allow light to enter and heat the air inside. The thermal mass of the air in the high tunnel buffers the temperature changes at night. Using row covers in a high tunnel stratifies the air (insulates) and again slows the heat transfer from the cooling exterior to the plants beneath. Double plastic on the exterior is many times inflated by a fan or even bubble to increase the insolation factor.

Organisms use respiration and change their insulation depending on conditions. The dynamic relationship between thermal mass and insulation is optimized when managed as needed (warm blooded), rather than being a static and passive system (cold blooded).  Low temperatures extract energy from sources of heat. Cold climate systems slow that heat energy loss and minimize calories used to replace it. Using sector maps can help in this too.

In the end, think of your construction model in polyculture terms. Is it a homogenous “monoculture” of materials regardless of the external aspect, or a mix of materials used in conjunction with each walls external exposure?  Is your building static or does it (and you) respond dynamically and creatively to change in the seasons? Within the climate and the changing seasons, the modality you choose (cordwood, rammed earth, earth sheltered, earth bag, straw bale, conventional wood framed, concrete, adobe, or a hybrid), the building materials will set up a cycle of benefits and/or disadvantages. Knowing the dynamics of the final structure within a climate should be well understood before a design is implemented. You will be living in it.

New Technology: Phase Change Materials

(Source: http://www.new4old.eu/guidelines/D3_Part2_H2.html)

Phase change materials (PCM) are special materials for increasing the heat storage capacity without adding extra weight to the structure. Here the phase change is used for energy storage. As the temperature increases, the material changes phase from solid to liquid and during the chemical process energy is absorbed. This way the room temperature will be lower. Later when the temperature decreases, the material changes phase from liquid to solid and dissipates the heat.

This energy is expended and removed from the room through night-time warming of the air. Ventilation can be increased with a fan and the air blown directly on the phase change mass. The temperature of the PCM itself remains constant during the reactions.

For building applications, the phase change should take place near the comfort temperatures, between 18 and 25°C. The phase change point depends on the type of material applied. If this temperature is too low, the heat storage capacity is exhausted too early, if it is high, starts too late and the influence is small. -Hegger, Auch-Schwelk, Fuchs and Rosenkranz. Construction Materials Manual, Birkhaeuser, 2006

Microencapsulated PCM (e.g. paraffin) can be mixed to interior plaster, wallboard panels or aerated cement blocks and applied in the building without any special measures, just like conventional materials. Encapsulation is important, as the PCM must not be in direct contact with other materials to avoid damages due to the “melting” process. 30 mm plaster coating with 30 % PCM has a heat storage capacity equivalent to 180 mm concrete. -Fraunhofer ISE, Germany, www.pcm-storage.info