Thermal mass is the ability of a material to absorb and store heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Lightweight materials such as timber have low thermal mass. Appropriate use of thermal mass throughout your home can make a big difference to comfort and heating and cooling bills.
Thermal mass can store solar energy during the day and re-radiate it at night. (8h delay)
Correctly used, the thermal mass, moderates internal temperatures by averaging out diurnal (day−night) extremes. This increases comfort and reduces energy costs.
Poor use of thermal mass can exacerbate the worst extremes of the climate and can be a huge energy and comfort liability. It can radiate heat to you all night as you attempt to sleep during a summer heatwave or absorb all the heat you produce on a winter night.
Allow thermal mass to absorb heat during the day from direct sunlight or from radiant heaters. It re-radiates this warmth back into the home throughout the night.
Allow cool night breezes and/or convection currents to pass over the thermal mass, drawing out all the stored energy. During the day protect the thermal mass from excess summer sun with shading and insulation if required.
To be effective, thermal mass must be integrated with sound passive design techniques. This means having appropriate areas of glazing facing appropriate directions with appropriate levels of shading, ventilation, insulation and thermal mass.
Locating thermal mass
As a first priority, locate thermal mass where it is exposed to direct solar radiation or radiant heat sources. Insulated or internal suspended slabs that are not earth-coupled make ideal thermal mass storage for solar heat gains, as do masonry walls, water filled containers and phase change materials. They should receive direct solar radiation.
Thermal mass walls between northern living areas and southern sleeping areas are ideally located as thermal lag radiates daytime solar gains into sleeping areas at night and provides acoustic separation.
Thermal mass should be situated on the interior face of the building envelope and must be thermally separated from the outside via insulative materials
Direct sun can generate the same heat as a single bar radiator over each square metre of a surface, but effective shading can block up to 90% of this heat. By shading a building and its outdoor spaces we can reduce summer temperatures, improve comfort and save energy. A variety of shading techniques can help, from fixed or adjustable shades to trees and vegetation, depending on the building’s orientation as well as climate and latitude.
Eaves or other fixed overhangs are the simplest way to provide protection against solar gain. They must be sized correctly to exclude summer sun but still admit winter sun.
Eaves depth recommended for New Zealand homes:
> North Island minimum 600mm
> South Island minimum 700mm
1 – If you have a lot of full height doors, you will need to increase the eaves depth up to 900mm
2 – Eaves depth are calculated from the outside of the cladding. (Not from the wall studs)