Have you ever walked in an evening, and passed by a brick or stone wall that has been in the sun during the day, and felt its stored heat still radiating out, heating the air around it?  Or have you ever gone into a old double brick house on a hot day, and felt the relief of the still cool interior?  This is the power of heavy weight building materials, known in the design literature as 'thermal mass'.  

Objects with thermal mass take a long time to heat up and a long time to cool down, so houses built with high thermal mass will have a lot more stable temperature, compared to lightweight structures which will be much more reactionary to daily temperature fluctuations.  Using mass is not as easy as simply building a structure with brick (or block, or rammed earth, concrete, mud brick, etc.), and that's because thermally massive materials are heavy and dense and typically very poor insulators. The solution of course is to combine mass with good insulation on the outside of the mass, leaving the mass internal to the living space to do its job.   
(That is, in standard brick veneer construction, whilst there is a lot of mass, because it is on the outside of the insulation, it doesn't  interact with the internal airspace, so has minimal effect on house performance.)

When thermal mass is added to the inside of a house, inside the insulative skin, it increases the energy storage potential of the house. This is a good thing when combined with good passive solar design.  In winter, when the sun shines in the northern windows, instead of being frittered away, the heat can be absorbed in the slab and internal mass walls, so that when the sun goes down, that energy will re-radiate out and keep the temperatures up well into the evening.  Conversely, during summer, proper eaves design should keep direct sunlight out, but even with good insulation some heat will still make its way into the structure.  The slab & mass walls will then be able to absorb and disappate this heat, keeping the structure cool longer than it would be if made from lightweight materials.  Sometimes in long hot weeks of summer, even a thermally massive structure would have captured so much heat that it has become too warm internally, which is why breeze paths and passive cooling strategies are important.

The general rule of thumb for houses in Melbourne with good northern light, is the more mass the better, though there will be diminishing returns as extra mass is added.  The easiest and cheapest way to spread mass evenly throughout a home is with an insulated concrete slab.  A House Energy Rating on the design will then allow you to see if further mass, in the form of internal heavy weight walls, or reverse brick veneer is warranted by quantifying the benefit in the various rooms of the house. 



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