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Thermal Mass

Sustainable Architects Toronto

Rammed earth, masonry, straw bale and other high mass materials can be used as thermal mass in buildings.  In climates with large changes between day and night temperatures and in buildings deeply connected to the ground, thermal mass can play a large role in moderating both humidity and temperature. 

Heavy mass wants to remain at a constant temperature and humidity level.  It has the ability to absorb excess heat and moisture from the air.  Both will slowly travel to all parts of the wall until it is homogeneous.  The change in temperature from day to night and within seasons ensures this is a continuous process moving in alternating directions.


Insulation in the center of thermal mass unlinks the exterior and the interior mass and reduces the temperature differences between them and therefore slows the transfer of heat.  The thermal lag effect uses the warmth from an earlier point in the day and releases it later when the temperature is lower and the sun has set.

Research has shown that lower insulation levels can, in combination with thermal mass have more insulative effect than higher insulation levels alone.  The home is warmer with higher humidity levels in winter and cooler with lower humidity levels in summer.

Thermal mass requires only the presence or absence of the sun to function.

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