Retrofitting thermal mass into walls and floors

Issue 20 Words: Dick Clarke

Retrofitting thermal mass to timber-framed buildings is possible and can make your home that much more comfortable and thermally efficient. Dick Clarke looks at how it’s done.

The benefits of thermal mass in passive design are well known; warmth (and coolth) is stored in the core of a house. However many houses lack any thermal mass where it is most beneficial, such as brick veneer where the thermal mass is on the outside and is subject to the vagaries of the weather. But there are ways to retrofit thermal mass into existing houses, even brand new ones.

Thermal mass comes in a number of physical forms and many conventional building materials – such as rammed earth, bricks and blocks of various kinds, concrete and Hebel AAC – offer useful amounts. Some of these materials can be incorporated into existing buildings quite easily, such as when building new walls, but there are also ways to retrofit thermal mass into existing walls. Another option is phase-change materials, usually waxes or salts that absorb large amounts of energy when changing from solid to liquid, and release that energy again when reverting to solid form. These can be incorporated into the structure, or can stand alone in ‘heat banks’. Water, too, can provide a neat substitute for high-mass materials, most often manifest in hydronic heating.

Internal walls are a very good place to put thermal mass, assuming the building has reasonable solar access, good glazing and is well insulated. Walls are extremely effective in regulating thermal comfort as the vertical thermal mass is exposed near to people inside a room. Retrofitting thermal mass to walls works well both in passive heating and cooling modes. Many project home builders in southern Australia offer floor plans which, with some tweaking, can provide good orientation of living areas to the north, and have internal walls that will benefit from added thermal mass. A few more progressive project home builders provide the option of making these walls brick instead of empty timber frames.

Floors are a more conventional location for thermal mass and concrete slab-on-ground construction is de rigueur in the project home sector. This is good, provided solar access is available to help heat in winter, and that overnight purging of warm air occurs in summer to cool it; otherwise thermal mass will not provide maximum benefit. If it’s not already part of the building structure, there are opportunities for renovators to add thermal mass floors when re-orienting their homes to take advantage of solar access.

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Cover of Issue 20
You can read more about Retrofitting thermal mass into walls and floors in Issue 20 of Sanctuary magazine.

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