Is thermal mass over rated?

Are concrete slabs the best floor for an efficient home?

By Richard Keech 2018-06-16

Thermal mass

Figure 1: Thermal mass is a two-edged sword

Thermal mass equals efficiency?

The use of concrete slabs on ground is ubiquitous in contemporary Australian home construction. Conventional wisdom says this is a great thing for energy efficiency because of increased thermal mass.  There’s some truth to this.  However, I think the reality is not quite so simple.

Responsiveness

All other things being equal, increased thermal mass slows down the thermal response to extremes of temperature. This is obviously a good thing when the inside is comfortable and it suddenly gets very hot or cold outside.  However, what if your starting point is a home that’s uncomfortable and you need to get it comfortable? In this situation you need time and lots of energy to get things to where you want them. So thermal mass is two-edged sword – it can work against you as well as for you.

Thermal response time (T) varies with both thermal mass and insulation in the same way.   T is a function of mass times insulation.  So halving thermal mass and doubling insulation leaves thermal response unchanged.  This means that improving insulation can also be used, like thermal mass, to slow down thermal response time.

Thermal responsiveness

Figure 2: Responsiveness to hypothetical rapid change in ambient temperature

Activated thermal mass

In a temperate climate it’s important to configure things so that:

  • the winter sun can shine directly onto the thermal mass, and
  • the summer sun is always kept off the thermal mass

Ground coupling

A conventional slab on the ground is thermally coupled to the ground. In other words, there’s a very tight thermal connection.  As a result, the slab will gain or lose heat to the ground and tend towards the stable ground temperature.  When the stable ground temperature is within the comfort range of 21C – 26C then the slab is going to be good year-round.  However in most of temperate southern Australia the stable ground temperatures are much lower than this.

For example, in Melbourne the stable ground temperature is about 15C.  In summer this works to favour thermal comfort. However in winter it means that the slab is generally losing heat energy to the ground.  Given that homes in southern Australia tend to use much more energy heating than cooling, it’s easy to see that maybe slabs are working against our interest.

Insulating the slab

Slabs can be insulated underneath. This will de-couple the slab from the temperature of the ground.  The effect, in Southern Australia, will be to benefit the winter-time energy situation. However it will make summer worse because of the risk of overheating.

Slabs lose heat to the ground both through their edges and through their underside.  The ground adjoining the edges of the slab is much more variable in temperature.  So it turns out that insulating the edges of a slab can make an appreciable beneficial difference in both summer and winter by isolating the thermal mass from the big daily temperature swings of the ground very close to the surface.

Insulating the edge of slab is not often done. However there is a well-considered technique for doing it that’s been described here by Dick Clarke.

Slab-edge insulation

Figure 2: Insulation fitted on edge of slab (courtesty Peter Reefman)

Other slab issues

Some other issues with slabs:

  • To work best slabs should be exposed (ie not covered with carpet etc). Aesthetics normally dictates a polished surface which is expensive;
  • Concrete is an unforgiving surface – things (including people) that fall on it are prone to break;

  • Concrete brings maintainability issues – pipes are entombed. Access in and under is very difficult for inspection, modification, repair;

  • Embodied energy is reasonably high, even with most eco concrete mixes;

  • In a world where flooding and flash flooding is an increasing risk, it makes sense to get up off the ground, which is harder with a slab.

Conclusion

If you want a home that can work with no mechanical heating or cooling then high thermal mass is definitely the way to go, making sure that you get the winter sun shining onto the slab. However, in temperate Southern Australia, and where a source of heat/coolth is available, then a light-weight, highly insulated structure might actually be better than a conventional slab-on-ground build.

If you do build on a slab, then insulate the edges if possible.

If you insulate under the slab then modelling should be done to find the best balance between summer and winter benefit.


Further reading

Matthew Cutler-Welsh, October 2016, “Thermal mass is not that useful”, Home Style Green, http://homestylegreen.com/136-thermal-mass-is-not-that-useful/


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