What is Solar Passive Design?

The EIGHT main elements of Solar Passive Design   

The main aim of solar passive design is to get the sun to do as much as possible to both heat and (believe it or not) cool your building, keeping it at a comfortable temperature year round. 

So, in order not to use anything else – like oil or gas for heating, or aircon for cooling, there are a number of design elements that we need to be balance in order to get the right amount of heat from the sun – but not so much as to overheat the property. Likewise when the weather gets cold one has to try to maximise the amount of heat the sun can provide, and at the same time stop its heat from escaping. 

This very brief guide pertains to the simplest and most easily used form of solar passive energy called ‘direct gain’. Direct gain is where the sun is encouraged to enter directly into the space where solar heating is wanted – hence the name! There other forms of solar passive heating such as ‘indirect gain’ but for the purposes of this guide we are addressing direct gain. 

Here are the main balancing elements to a passive solar gain building design: 

1. Orientation: The general rule is this: If you can’t face exactly the solar midday direction (South for property on the Northern hemisphere, North for property in the Southern hemisphere) then try to orient the building 30 degrees or less off the solar midday direction. This will ensure that you can still receive at least 90% of the solar radiation available. See Ed Mazria’s* diagam below which shows the solar passive ‘sweet spot’. 

(* Edward Mazria The Passive Solar Energy Book. Rodale Press 1979) 

Design Note 1: Get the building to face the sun. Or as close to the solar noon direction (where you are) as you can get it. 

2. Building Form:  If you want to gain the most from the sun you should stretch out your building across the path of the sun to catch as much solar radiation as possible. That means, if you have the land available, making your building oblong with its longest side facing the sun. 

Above: Maximising solar exposure with the buildings form and orientation. 

Design Note 2: Stretch the building across the site to maximise solar exposure if there is space.

Prioritising: If you don’t have land available then you will need to give those areas of your building that are most used top priority in terms of exposure to the sun. In a house the most used spaces are normally the Living room, Kitchen and Dining areas.  Where space is limited the living areas should ideally be prioritised over bedrooms, bathrooms and circulation areas like stairs. So, this aspect affects internal planning.. In the diagram below there are three scenarios shown for maximizing solar exposure to the living areas of the house, and what this means to the disposition of spaces within the house for different site sizes.   

Scenario 1: The site is relatively wide, and there is space to avail all of the rooms of south exposure. A note of caution here, however. Bedrooms are seldom used during the day and so solar exposure is less important in these rooms than giving the fullest solar exposure to the main living areas. Also, the more stretched out a building is the greater its external surface area – which could increase heat losses to more than the heat gained by the solar input. Stretching out the accommodation as in scenario 1 would be justified if there were a great view in the solar direction, if the house is in any case highly insulated, or if bedrooms are used regularly during the day (say, for studying, or if they doubled for some other use such as an art or music studio). 

Scenario 2: The site is not wide enough for all of the accommodation to have direct solar exposure. So here the living accommodation fronts the bedroom accommodation as it is higher priority. 

Scenario 3: The site is only wide enough for a single block of accommodation and here the living areas are placed at the front where there is maximum exposure to the sun, and the ancillary accommodation is lined-up behind. 

Design Note 2A: Where a site has limited scope for stretching the building, prioritise the accommodation to give the living areas the maximum exposure to the sun. 

3. Open Planning:  Some rooms will get a lot of solar-passive heat while others might not. Open planning makes it possible to share the heat generated by solar radiation between the main spaces of the building. In a house the areas that would be most advantageously open-planned would be the main living areas – Lounge, Dining and Kitchen – where most of the time is spent during the day. 

In the plan of Solar Leaf No. 5 below by Solarity, the main living areas including the main circulation areas, have been open-planned to allow migration of solar-heated air. Here, in addition, the entire house has been bowed to maximise solar exposure (like a bay window in a house). South is directly down on the drawing.

Design Note 3: Open plan the most used areas of the house (usually the Living and Kitchen/Dining areas) to promote circulation of solar-heated air. 

4. Glazing, and the Solar Collector Wall: The ‘solar wall’ refers to the largely glazed wall facing the sun, usually. This is a wall that is used to gather solar radiation in direct gain solar passive design. The solar wall is commonly made of glass which has the effect of magnifying the sun’s radiation whilst also enclosing the space and preventing the heat produced from escaping easily. 

The solar wall: In some early solar passive buildings (in the 1970’s) the solar wall was large and tipped up to face the sun, which had the effect of gathering in too much radiation and overheating those buildings. Lessons were learned. The area of the solar wall is an important balancing factor in solar passive design but it is modified by whether, for example, it is tipped up to face the sun in the sky or whether it is shaded by trees or deliberate solar shading devices (of which more later..). But in simple terms the area of glazing doesn’t need to be more than about 15% to 30% of the internal floor area of the area to be passively heated. If the shading can be varied in any way – for example with movable louvres or awnings then an excessive area of glazing can be shut down so as not to overheat the building, and opened up again in cooler weather. 

Windows facing other ways: In direct gain solar passive design windows facing the solar direction are usually net gain sources – in other words over an entire year the net effect of them, taking into account heat gains through solar radiation against heat losses through losing heat when it is night time or when the sun is not shining through them – is positive. Windows facing any other direction other than the solar direction (or within, say, 45 degrees of it) will likely be net heat loss windows. So, normally in a solar passive design windows in directions other than the solar direction should be kept as small as possible. However such windows should not be omitted entirely as they have great value in promoting natural cross ventilation – for natural cooling (of which more anon). 

Design Note 4: The solar wall area is a difficult calculation. One way to ensure that enough solar heat is produced is to slightly oversize the solar wall and use variable solar shading devices to reduce its area if overheating occurs. 

Design Note 4A: Windows not facing the solar direction (or within approx.. 45 degrees of it) should be kept as small as possible notwithstanding the need for them to be openable in order to promote cross ventilation when needed. 

5. Thermal Mass:  Thermal mass is a crucial balancing element in solar passive design, and it has very useful properties. High thermal mass is the property of a material to retain heat. So a building material of high thermal mass might be any dense material such as stone, brick, block or even thick ceramic or clay tiles. Building materials with low thermal mass are such as timber, plasterboard, insulation, and carpet. 

What happens when the sun’s rays hit a high thermal mass material is that they are turned into heat and stored in that material for a while, before being given off into the surrounding air. This is why a clay tiled floor will feel warm to the touch on a sunny day while a carpeted floor will feel much less so. So high thermal mass materials are used inside a building to gather in and hold the sun’s heat. Without this the sun’s heating effect would quickly dissipate leaving the space feeling cooler as soon as the sun disappears. 

Thermal mass materials are used to hold the sun’s heat inside a building and give it off gradually into the building after the sun has gone. Thermal mass has another useful effect. When it is in the shade it acts to cool a building. Imagine going into a big old church or old stone cottage on a hot summer day – the cooling effect of high thermal mass in shade is palpable. So high thermal mass can be used to effect cooling too, where this is part of the balancing act. In such cases it is used away from the solar wall. For this reason buildings of low thermal mass such a kit houses do not usually make very effective solar passive designs – because as they tend not to be made of high thermal mass materials they don’t retain solar heat, and in hot weather they can be rather harder to cool naturally. 

Design Note 5: Use a high thermal mass material to the floor and walls of the main living areas if possible. Don’t cover the floor with carpet – use rugs if really necessary. Use more high thermal mass materials in hotter climes for cooling. 

6. Solar Shading:  It is somewhat strange that nearly half of the balancing elements of a solar passive design (from here to the end of this article) are to do with cooling. But paradoxically cooling is possibly the hardest to achieve naturally and possibly the most important element in the  solar passive equation. And in a rapidly warming world natural cooling is bound to become more important still.

Covered briefly in the section on the solar wall, solar shading is an important balancing factor to make sure that the right amount of sun gets in to the building, but not so much as to overheat it. Solar shading devices can take many forms. Trees and act as very effective solar shading devices, and deciduous trees are favoured for this. The reason for this is that they have leaves in summer when shading is important to prevent overheating but then lose their leaves in winter when the extra solar heating is most useful. Other solar shading devices are louvres over the solar wall, shutters, trellises outside the solar wall, sails, awnings, thickened window or door frames that cast shadows over the glass, or simply extended roof overhangs over the solar wall. The image below shows an example of extended roof overhangs used to the South side of a Solarity-designed extension being used for solar shading. Here the roof overhang is perforated with trellice openings.

Design Note 6: Solar shading is essential and needs to be an integral part of the building so that it isn’t removable. However, having some form of variable shading is an advantage. Sails and awnings, or a place to fix these, might be an option. 

7. Insulation and adequate draught-sealing: A high level of insulation is an essential component of solar passive design. Without it all of the heat produced by the sun would simply leak away. So the rule is the more the better. 

An important component of the insulation package is how to insulate the solar wall at night. During the day the solar wall is actively collecting solar heat for the property, but at night this situation is reversed, if the solar wall isn’t insulated. The most obvious form of insulation would be having insulating curtains that are drawn across the solar wall at night. Other options might be, depending upon circumstances and the design of the house – insulated shutters or doors that are closed over at night. 

Design Note 7: The more insulation the better. Provide some form of insulation across the solar collecting wall at night. Keep the solar heat that you have generated – don’t let it escape! 

8. Natural ventilation:  It is counter-intuitive to suggest that natural cooling is important in a building designed specifically to harvest heat. 

But the goal of solar passive design is to provide a comfortable environment, not just a hot one. So sometimes cooling is needed rather than heating and just as solar passive buildings harvest heat from the sun without recourse to carbon energy forms so it is that we seek to find ways of cooling that don’t use carbon either. And, as it happens, history is brim-full of techniques and building examples that do just this. 

Cross ventilation: The provision of ventilation openings on opposite sides of the building and a plan that allows them to connect (i.e. the spaces with windows that are not closed off by doors) is a cooling balancing factor in the solar passive equation. This is  not surprisingly called ‘cross ventilation’. 

On hot summer days a combination of high thermal mass (in the shade) and cross ventilation may be enough to cool a building naturally. 

In order to be truly effective the ventilation openings need to be controllable and variable – so you can have anything from a small to a large area openable. This might point to the desirability of different sized openings. 

There are some forms of cross ventilation that enhance its cooling effectiveness. This can occur when for example, the air entering into the building comes in over shaded ground or across water – both of which act to cool air passing through or over them. Imagine the difference between air entering a building over a shaded pond versus air entering the building over a concrete hard-standing exposed to the sun. 

Above: the cooling effect of shade, and air over shaded water. 

Stack ventilation: Added to cross ventilation there is another form of natural ventilation that can be used to cool a building – stack ventilation. This is ventilation that uses the property of heated air to rise. So, on a hot day if one were to open up a hole in the roof much of the heated air collecting under the opening would start to move towards it and exit the building. What happens with both forms of natural ventilation – cross and stack – is that either cool breezes are introduced, or in the case of stack ventilation cooler air is pulled in at lower level to replace the warmer air exiting upwards through the stack opening. 

The stack effect can be supercharged by using the sun to heat the exiting air in what is called a ‘thermal chimney’. In this case the sun is actually used to cool a building. See the diagram below. 

Above: a thermal chimney supercharging stack effect ventilation.

It is worth taking some time to study the architecture of the ancient world. The ancients, particularly Islamic builders, were masters of natural cooling and used many of the strategies described here to cool buildings in hot climates naturally – without any use of carbon power forms. At the Alhambra Palace in Granada, Spain, for example, natural cooling by use of air over water and air through the sprinkled water of fountains was used in addition to high thermal mass in shade, cross and stack ventilation, the cooling effect of the shade of plants and solar shading. All were combined to cool the inhabitants on hot days, with the additional effect of delighting them too (the sound of water fountains, and the reflections off the water). And no carbon was used at all. 

 Design Note 8: Natural ventilation is essential to provide cooling to solar passive buildings. Use solar shading, high thermal mass inside (in the shade) and cross ventilation as a start point. Consider whether additional natural cooling methods might be tried also.

Don’t be persuaded to use mechanical ventilation – it isn’t as effective, and it is immensely damaging to the environment. Learn to use nature as the ancients did. It is not what the ancients did that is causing global warming and climate change – it is what we are doing, using mechanical anything for everything. 

Summary and Conclusion: The forgoing describes strategies that can be adopted to design a building that heats and cools itself naturally. 

Nearly all buildings that exist in the western world today don’t even face the sun. Or if they do it is mostly by accident. This is because the sun wasn’t any part of their heat input strategy – carbon supplied all their energy, and this is still the case with 99% of buildings being built now. They are so heavily dependent upon carbon heating and cooling that most are uninhabitable if carbon is withdrawn from them. As a test of this, turn off the heating in your home and see how long you can live there. In this sense most all modern buildings old and new today are ‘carbon-junkies’. They are a wasteful product of a civilization that, according to overwhelming scientific evidence, is headed for extinction in short order.

Solar passive buildings take lessons from the distant past, from a time before high carbon heating and cooling methods using coal, oil and gas were devised. The ancients have shown us by living on this planet for many thousands of years before coal, oil and gas that it is perfectly feasible to live without them. The bonus to us of living without these is that our children get to live on a living breathing planet that isn’t dying. 

From particular to the general, and back again. Using the sun to heat and cool our homes is part of a future that would allow humans to thrive on a living planet. If we are to have a future at all on planet Earth then we really must turn and face the sun. As you have seen from the above – solar passive design isn’t rocket science. If it were the ancients wouldn’t have used it! The above guide goes some way, I hope, in showing how you can turn to face the sun.


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