Thermal Mass and Multiple Functions

The concept of multi-tasking is familiar to most of us, but the concept of multiple functions less so, although we encounter many examples of it every day. For instance, with a modern mobile phone you can: ring your mate, text your partner, take a photo, tell the time, store your friends’ contact details, light up a dark night, and in some cases check your email or tweet. There are probably another 326 functions that I cannot figure out because I’m over 40.

Eco-thrifty design embraces the concept of multiple functions. A prime example involves the vintage Shacklock 501 multi-fuel cooker we installed during our renovation. The most obvious functions it serves are cooking and heating, but these only scratch the surface. (We did not hook up the wetback because we have so much solar hot water already.)

A naked Shacklock. 

 Jonah the Magnificent. 

More significantly, the Shacklock plays a key role in our passive solar home, which could be easily overlooked by those not familiar with eco-design. That role is ‘thermal mass’, or something very heavy (mass) that absorbs and releases heat (thermal).

Traditionally, thermal mass has been the neglected member of the passive solar trio: solar gain, thermal mass, and insulation. The term ‘passive’ indicates that the design harnesses solar energy effortlessly. (Active solar is another story.) Like a car parked on a sunny day, a passive solar home absorbs the sun’s energy by being in the right place at the right time. But a car on a sunny day gets too hot during the day and then cold at night. This was the case with many solar structures built in the late 1960s and early 1970s – too much glazing and not enough mass and insulation.

 Tiling the 70 mm thick concrete hearth.

Tiled hearth with beveled oak frame to match the oak floor.

Similarly, in late July I was invited by a homeowner to look at a TV room that had been added to an older home. The addition was recent, but before the present family bought the property. At 2 pm on that sunny winter afternoon the temperature was 27 degrees C. But she also complained of the room being uncomfortably cold at night.

This is a classic case of poor design I see over and over in Wanganui. It represents a lost opportunity, and detracts from the comfort and health of the human beings occupying the space. Not eco and not thrifty.

My suggestion for the volatile TV room was this: cut out the middle 2/3 of the timber floor and replace it with an insulated concrete slab. This would decrease the high daytime temperature and increase the low nighttime temperature. Problems solved.

We insulated our stove foundation with pumice.  

Due to expense, it is unlikely the family will take this advice. Sadly, it is equally unlikely that the architect or builder considered passive solar design ten or so years ago when the addition was built.

Morning winter sun enters through a northeast window. 

For the passive solar renovation of our old villa, we didn’t need an insulated slab because of the strategic placement of our Shacklock 501. The 700 kilogram beast is centrally located between our kitchen, dining room and lounge, and receives direct winter sunlight three times a day: morning, midday, and afternoon. 

Midday winter sun enters through French doors.

The range, brick and hearth absorb the sun’s heat during the day and release it at night. The process is passive because it simply does it. There are no moving parts. As mentioned earlier, it would be overlooked as a ‘massive element’ in our design by the casual observer because it simply looks like an old coal range. It is that, but so much more. Other functions served by the Shacklock are: focal point of the kitchen, conversation piece, and, most notably, wedding present from me to my wife. BTW, Happy Anniversary, Dani.

Learn more: “Ask a Solar Question”

Thursday, 5^th September, 7-9 pm.

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