Thermal Mass

There’s a good deal of information on this out there already – and a good deal of what I’ve read comes from Australia (see http://www.yourhome.gov.au/technical/fs49.html)  – and more from New Zealand and New Mexico.   Can be momentarily disconcerting to read about north-facing windows from here in the UK but it’s no less valuable.  And a relatively recent link from Scotland http://www.historic-scotland.gov.uk/craig_wheatley_thermal_mass_in_traditional_buildings.pdf not that this is a traditional building!

It doesn’t take much energy to heat air up and not that much more to warm up wood.  So in a well insulated, airtight timber-frame building, the energy that pours in through the windows on a summer’s day will rapidly get the house too hot.  OK, so open the windows.  Come the evening, close them again.  Not much energy has to be lost – even though there are triple-glazed windows – for the air in the house to cool down too much to be comfortable, particularly if it’s a cold night.  In other words you need some thermal inertia, something to reduce the temperature extremes – which is where the thermal mass comes in.  Add material inside the insulation of the house which will absorb heat when the sun shines, slowing the rate of rise of temperature, keeping the house from getting too hot during the day – and which will release the heat it has absorbed later, slowing the rate at which the temperature in the house falls at night.

There are some rules of thumb for this.  For short term – diurnal rather than seasonal – considerations – the area of thermal mass should be at least 6 times the area of south-facing glazing and needs to be within line of sight of it.  At least, that’s how I read it.  I assume this means the walls, floors and ceilings of these south facing rooms, not counting areas hidden behind furniture or under carpets.  Porter’s has 21m² of south facing windows, so I’m looking for 126m² of thermal mass.

What material? Well, it could be water, which is the best “material” having the highest specific heat, but lining the walls and floors with water is difficult. Of the commonly available solid materials concrete (though not very “green” – too much embodied energy) compressed earth blocks and some stones are good.  Bricks aren’t bad, though many are not sufficiently dense (and have high embodied energy again).  In Porter’s the foundation slab is concrete, covered with porcelain tiles, there’s an internal wall made partly of high density blocks, covered with concrete render and plaster and partly of exposed second-hand bricks and, as mentioned in the post for weeks 15 and 16, there’s a proprietary material called Energain in the ceilings of two rooms.

The advice I’ve read states that the thermal mass should be 50 – 75mm (2-3 inches) thick – more gives a slower response and presumably less may not store enough heat.  Clearly the 200mm slab is more than enough, the wall is about right and the Energain FAQ says it’s “approximately like 20 – 40 mm of concrete”, so is a little on the light side.  We will just have find out.  It will, I think, be particularly important in the spring and autumn, with sunny days but cool nights.  But the climate is, of course, very different from some considered in the earlier references, which have very hot days and much cooler nights.

We did consider rammed earth floors.  Elsewhere I’ve mentioned that the Zero Carbon House Birmingham, designed by John Christophers, has these.  We thought they were lovely – a bit like walking on leather – but they add 75mm to the floor.  So you either have them throughout the house or, bearing in mind that you want level floors throughout , you have to build up all the other floors to the same height.  And the cost was well outside the budget.  Pity.

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