Australian Passive Solar Home

During the research for our house and my book I was quite surprised to come across a lot of misinformation about passive solar design. Equally surprising is that it often didn’t seem to be being challenged. Much of the confusion related to thermal mass.

Let’s look at some examples.

Comment: “I call thermal mass thermal mess. After a few days of cloudy weather in winter, it doesn’t work.”
Reply: Adequate thermal mass will continue to ‘work’ (i.e. provide warmth to the house) in winter, even after several days of cloudy weather. For example, in our house, the measured temperature of the concrete slab shows that it decreases in temperature only by 0.5 – 1°C per cloudy day in mid-winter. In fact, because cloudy weather typically has higher overnight minima (the cloud reducing radiative heat loss from the earth’s surface), the reduction in the temperature of thermal mass in cloudy winter weather is less than you might expect. If – say – a week of cold, overcast weather is predicted, mechanical heating can be used to warm the air in the house.

Comment: “Thermal mass was proved in the 1970s to be ineffective.”
Reply: I am not sure why you say that – thermal mass is highly effective in moderating variations in internal temperature. All Australian Government authorities recommend the use of thermal mass in energy-efficient home designs in southern Australia.

Comment: “If you try to design passive solar homes for optimising solar heat gain in the winter then you run the risk of summertime overheating. Design against the sun in the summer (heavy shading) and you pay some energy penalty in the winter for not getting the sun’s free energy.”
Reply: Any effective passive solar design will be optimised for the climate in which the house is located. In a cold climate (one dominated by the requirement for heating), winter sunshine will be optimised. In a warm climate (one that mostly needs cooling), summer shading will be optimised. Taking this approach means that when measured over a full year, mechanical heating and cooling energy requirements will be much reduced over a conventional home that doesn’t use passive solar techniques. Software modelling of the house design will quickly show the best passive solar approaches to give minimum annual mechanical heating and cooling energy use.

Comment: “Passive solar design made a lot of sense when homes had no insulation and energy was cheap like cutting down a few trees per year. The sun was a free heater and in summer, all the windows and doors were wide open and it was a Utopian paradise. In the winter, provided you were gaining as much heat as you were losing then you could stay above death.”
Reply: Because some people may take these comments seriously, let’s look at them in turn. (1) Passive solar homes have always used insulation as part of the approach. (2) Passive solar homes have always been designed to reduce the use of energy, irrespective of its cost. (3) No passive solar home is operated with all the doors and windows open in summer. The door and windows are open at night to cool the thermal mass of its accumulated heat. During daytime in hot conditions, the windows and doors are closed. (4) In winter, during daytime, sunshine the house will gain more energy than it loses – that’s why the thermal mass rises in temperature. At night, it will lose more energy than it gains, the thermal mass keeping the house warm.

Comment: “Passive solar design was once a smart concept but is now incredibly stupid. It never really worked but kind of did enough that it survived as long as it did. If you really knew how to operate the house, and didn’t mind periods of being way too hot, way too cold or both at the same time, then you could make it reasonably comfortable but that’s kind of an ‘Active Solar Design’.”
Reply: Yes, for passive solar design to be effective, the occupants need to be involved with the house. That includes opening and closing window curtains, and at night in summer, opening windows. Any adequately designed passive solar house will not be overly hot or overly cold, and it’s rather difficult to be both of those at the same time! Nothing has changed to make passive solar design once a smart approach but now stupid. It remains a smart design – working with the climate rather than attempting to seal the occupants from the climate; giving a natural variation in internal temperature rather than holding the inside temperature at one level like in an office; providing adequate natural ventilation; and most importantly, able to be implemented at very little extra cost over a conventional Australian home and yet save a massive amount of mechanical heating/cooling energy. And of course, with its lovely internal environment, it is a wonderful place to live.

Comment: “Heat gains and losses of a house are complex. You need complicated software and skills to predict the energy use of a building and even then, you’re probably going to be wrong. This is why people still think passive solar design is a good idea – it’s too complex to understand.”
Reply: Yes, heat gains and losses from a house are complex, but luckily we now have well-proven software that will effectively and accurately model the way a proposed house design will work in a specific climate. This software is widely available and getting an expert to do the modelling is quite cheap. Passive solar design is actually very easy to understand, and optimising a proposed passive solar design using the existing, widely-available software is straightforward.

Comment: “Thermal mass like the water tank in your lounge room will suck all the heat out of the air. It will be like a block of ice sitting there. Just put your hand on it and I bet it feels cold in winter.”
Reply: Thermal mass inside the insulated envelope of the house stays quite close to room temperature e.g. typically less than 2°C above or below room air temperature. If it makes it easier to understand, don’t think of thermal mass as cooling or heating the air; instead think of it as resisting room temperature change. Yes, it does this by slowly changing in its temperature as it absorbs or gives out heat, but it is never hot like an electric radiator or cold like an air conditioner. Regarding thermal mass feeling cold to the touch, that’s not a good guide to temperature as at room temperature, anything that is conductive (like a metal tank) will feel cold when you touch it.

Comment: “If you have a lot of thermal mass, your home will be hard to heat in winter.”
Reply: There are two points to say to that. The first is that the greater the thermal mass in most well-designed passive solar homes, the less mechanical heating that will be required. (But this should always be checked by software modelling of the proposed house before construction.) The second point is that when heating is needed in a house with high thermal mass, it is better to heat the people within the house rather than the house fabric itself. This can be achieved by radiant and personal heating.

Comment: “Thermal mass should only be placed in locations where it gets the winter sun.”
Reply: This is not the case. Thermal mass in winter is heated not only by direct sunshine, but also by the warmed air within the house. In summer, thermal mass anywhere in the house will help absorb heat from the inside air. Therefore any additional thermal mass, irrespective of its position within the insulated envelope, will reduce interior temperature fluctuations.

Comment: “The way the house faces (orientation) is of only minor importance. For example, I have heard that it’s worth only about 0.5 stars energy rating.”
Reply: That’s an interesting point because the answer is yes – and no.
If a house is designed not as a passive solar house but as one reliant solely on insulation, sealing and artificial heating and cooling, it will probably not have a ‘correct’ orientation. So in that case, orientation will indeed make little difference to energy efficiency. On the other hand, a passive solar house depends heavily on orientation for best results. With our house, changing from the correct orientation to the worst makes 1.4 stars difference – a change in modelled annual heating/cooling energy of 63 per cent! Also note that strictly speaking, orientation doesn’t refer to the way the house faces, but to the walls in which there are most windows. A house may have a front door that faces west, but have most window area on the north – this is therefore a house with a northerly orientation.

Comment: “Why do so many people pay so much for their energy-efficient houses?”
Reply: I am not sure, but my guess is that those who benefit from consulting fees are a large part of the problem. Making things mysterious, esoteric and complex is always a good earner!