Thermal mass is the ability of a material to absorb and store heat energy. Building materials that are heavy and dense such as concrete, bricks and tiles are known to have high thermal mass. Used correctly, thermal mass is useful in climates like Canberra’s, where there is often large differences between day and night temperatures.

We use thermal mass in many new projects in the form of fully insulated exposed concrete slabs. When the sun is lower in winter, the sun’s rays can reach deep into north facing rooms, where the slab soaks up the sun and releases the retained heat overnight as temperatures drop outside. It improves comfort levels and reduces the need for mechanical heating, which in turn reduces your energy bills. You can learn more about thermal mass on the Your Home website. You can also hear Jenny chat about thermal mass with Amelia Lee of Under Cover Architect here.

However thermal mass does have its downsides; if used incorrectly it can have disastrous consequences in summer making it very difficult to cool your house down, and in some forms, such as new concrete slabs and new bricks, is very high in embodied energy (take a lot of energy to produce). Learn more about embodied energy here.

Fortunately thermal mass is not essential to achieving an energy efficient, thermally comfortable home. “Light and tight” is another approach that can work really well, particularly for homes with a smaller footprint and volume (ie. no huge high ceilings). This is the approach we take when we renovate any Canberra homes with suspended timber floors (which was the common method of floor construction until the 1980s).

Our Fab-ode project is a fab-ulous example of what can be achieved without thermal mass. The floors in this home are timber insulated underneath with R2 batts. The old home had an EER of 3.6 stars, but after the reno it is now 6.8 stars and uses 75% LESS energy for heating and cooling.
Thorn Place House is another reno project with no thermal mass. This home, which started at 2.5 stars and is now 7.5 stars post-reno, also has suspended timber floors with R2 batts stapled underneath.

What IS essential is a climate-responsive design suited to your site and location. This may or may not include thermal mass, but with Light House it will definitely involve designing for passive solar winter gain and summer shade, insulation, airtightness and ventilation.

Fab-ode: no thermal mass but still fab-ulously comfortable.

Thermal mass - what is it and do I need it?