Mar 28, 2014

Thermal Mass and Insulation, Local Conditions Dictate the Function

Notes from SouthWoods Advanced Permaculture Series

Thermal Mass and Insulation, Local Conditions Dictate the Function.

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. Speaking of cold climate strategies, principles and solutions still apply; its 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.  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?

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 again 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.

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 surface.

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 constructs.

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.

And another thing:
Phase Change Materials
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.
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 (Figure 63). 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 


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