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How thermal mass affects windows

If designed well, windows can be used to reduce extremes in temperature experienced inside the home making the average internal temperature more comfortable. Regulating inside temperatures can be achieved by understanding and harnessing thermal mass practices in energy efficient window and house design.

Thermal mass is the ability of a material (such as timber, concrete or brick) to store heat - it acts as a battery. During summer it can absorb heat, keeping the house comfortable. In winter the same thermal mass can store the heat from the sun or heaters to release it at night, helping the home stay warm.

Summer: Thermal mass absorbs heat during the day. Cool night breezes pass over the thermal mass, drawing out stored energy.
Winter: Thermal mass stores heat from direct sunlight. It will re-radiate this warmth back into the home at night.

High density materials like concrete, bricks and tiles have a high thermal mass because of their ability to store more heat. This means these materials require more energy and time to change temperature. Homes with higher levels of thermal mass can use larger areas of glass because the inside temperature will not change as rapidly.

Lightweight materials such as timber have low thermal mass because they store less heat and require less energy to change temperature. To balance window size with lower thermal mass materials, it’s important to have smaller windows to reduce rapid and uncomfortable changes in temperature. Alternatively, consider using high performance glazing such as low-e or toned glass.

For example, a home with concrete slab floors can have north-facing windows in individual rooms of up to 25% of the floor area. A home with lower thermal mass, such as timber flooring, needs smaller north-facing windows to combat the quick change in temperature of the low-mass timber. Ideally, windows in individual rooms that have timber floors should have no larger than 20% of the floor area taken up by windows.

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If north-facing windows are too small or sunlight is obstructed, the benefits provided by thermal mass can be reversed. Heavyweight materials will not receive enough sunlight to absorb and store heat increasing the need for artificial heating.

 

Winter and thermal mass

In winter, thermal mass in the floor or walls absorbs heat from the sun through north, east and west-facing windows. During the night, the heat is gradually released back into the room as the air temperature drops. This maintains comfortable temperature for some time, reducing the need for supplementary heating during the early evening.

Tip 
  • For good winter performance thermal mass needs to be exposed to direct sunlight. Locate north-facing windows to allow sunlight to reach thermal mass.

 

Summer and thermal mass

In summer, thermal mass absorbs heat that enters the house as it has a lower initial temperate than the surrounding air. By absorbing heat from the atmosphere the internal air temperature is lowered during the day. During the night, the heat is slowly released to passing cool breezes, extracted by exhaust fans or released back into the room.

Tip 
  • For good summer performance place windows where they will receive cooling summer breezes at night to pass over thermal mass materials. During the day shading and insulation can be used to protect thermal mass from excess summer sun.

More information on using thermal mass to moderate your home’s temperature can be found in in the government’s Your Home Technical Manual.

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