Thermal Mass: where, when and why?
The climate you live in can vary your requirements for high or low thermal mass. along with this, the location within your home where you include thermal mass is also essential if you want to maximise that heat-storing potential.
What was thermal mass again?
Thermal mass is the storage and slow release of heat energy in a material. It’s a property of all materials though some are “heavier” than others, or can hold more heat energy. For more information on thermal mass check out our other article here.
Materials with heavier thermal mass would allow heat energy to be stored longer and slowly released into the space. This can be beneficial in areas where the temperature varies and less so in areas where ventilation is more beneficial.
Climates
Cool Climates
In cooler climates where the temperature can vary throughout the seasons, heavier thermal mass is more favourable. Often brick, concrete or stone is used in these areas as “heavy” materials. These materials have higher thermal mass meaning they charge up energy throughout the sunny time of the day and slowly release this in the cooler parts of the day like the evening.
Tropical Climates
Lighter thermal mass is more valuable in warmer climates where the temperature does not vary significantly (North Queensland and the Top End). Often timber is used for well-ventilated housing in these areas. Timber has a relatively low thermal mass; what is considered “light”. This means that the timber is often at a similar temperature to its surroundings.
If, however, air-conditioning is intended to be used often in the space, then a higher thermal mass is recommended. This reduced the loss of cool air through the walls and floor when the aircon is on indoors.
Where to put it
Thermal mass usually exists in the walls or floors of your home. This is where heavier materials can be used without drastically affecting the structure of the home.
Thermal mass should be located in areas of the house that can receive more direct sunlight in winter and ideally be shaded in summer. For Australia, this is the northern end of the home. This will allow this material to absorb that heat throughout the day. Another way for thermal mass to absorb heat throughout the day is to have it near a heater or fireplace, or other sources of heat. High thermal mass on the south end of your home, or in areas not exposed to direct sunlight, is a waste of this high thermal mass potential. These areas should have good insulation while being constructed of lightweight materials.
Floors
Heavyweight materials for the flooring such as a concrete slab or brick floor connected to the earth below are a highly effective use of thermal mass. There is no draft on the outer side of the floor and thermal radiation from the ground can move straight into the home. Ideally, it will also be insulated underneath and surrounding the slab. Suspended subfloors in north facing rooms can also be effective. The surface of the floor can also influence the thermal mass efficiency, with tiles allowing easy heat absorption and carpet reducing thermal absorption.
Walls
Heavyweight walls such as brick, concrete or rammed earth can be included internally or externally. External walls should only be insulated on the outer side to allow any heat absorbed by the wall to move into the house.
Where thermal mass is included in your home is dependent on the climate and seasons you experience, along with orientation and the surrounding environment of your home. Thermal mass needs to be charged up by the sun or other heat sources and so north-facing walls and floors are ideal locations for it. If you can design your home to optimise the use of heavier materials you can improve the overall comfort and passive heating performance of your home.