Passive Solar Design – Building an Eco Home

Bigger Picture

In Aotearoa New Zealand, the majority of homes perform to a very poor standard. They are cold, damp, expensive to run, and making us sick. Each year there are 1600 deaths caused or contributed to by poor housing conditions.

New homes are usually also poorly designed; constructed to minimum Building Code standards.

‘The worst house you are legally allowed to build’

In addition, we are building these poor quality homes on some of the world’s best farming land. And not with diverse people and households in mind:

BRANZ is an independent and impartial research, testing, consulting and information company providing services and resources for the building industry – They have created a resource ‘Measuring our sustainability progress: Benchmarking New Zealand’s New Detached Residential Housing Stock’.

According to the World Health Organisation, the following temperatures lead to health implications:

  • Less than 16oC affects respiratory system
  • Less than 12oC affects cardiovascular system

(These values are for healthy adults. Children, elderly and people with illness or disabilities require at least 2 degrees warmer than these values.)

Living areas should not fall below 18 degrees and bedrooms should not fall below 16 degrees. Yet in New Zealand, 17.9% of living rooms are colder than 18 degrees during winter months (despite active heating), and 13.6% of bedrooms are colder than 12 degrees. In addition, 55% of our homes have visible mould.


This is making people sick, but we can do a lot to avoid it.

For example, it has been calculated that for every $1 you invest into insulating your home, you save $6-10 through direct cost savings (e.g. power bill) and indirect savings (e.g. lower medical bills, better school attendance and education, etc.).

When designing or retrofitting a home, it is important to consider the three factors:


If you try to cut on one, you will end up cutting on the others as well. We spend 55% of our lives in our home, so it is important to make sure we make it good.

pexels-photo-280221.jpegPassive Solar Design


There are 5 key principles to Passive Solar Design:

* Orientation

* Insulation

* Thermal Mass

* Airtightness

* Shading and venting

Orientation (inverse North and South if you are in the Northern Hemisphere!)

By getting the shape and size of the building right, and by facing it to Solar North, we are able to harvest ‘free heat’. This is because windows are energy loss and gain points. Some windows lose more than they gain. Too much glazing leads to too much heat loss. Most glazing should be to the North, with moderate amounts to the East and West. There should be as little glazing as possible to the South.


Surface area is also a factor to consider. The greater the surface area, the greater the heat loss; the more compact the shape, the less heat will be lost. We can think about this in terms of volume: perimeter ratio.

surface area

It is also important to be aware of corners, as they are difficult to insulate. Every external corner is a weak point in the structure, and the more corners, the greater the thermal weakness of the building.


energy use

There are 3 sources of heat in the home:

  1. Solar heat that is harvested
  2. Occupant load (heat generated through people living, e.g. bodies, appliances, etc.)
  3. Added heat (e.g. heat pump, etc.)

In New Zealand, our main aim (for most of the year) is to keep warm air inside. To do this, we need to insulate.

In an uninsulated house, heat can be expected to be lost in roughly the following ways:


Insulating different places (e.g. ceiling, underfloor, etc.), reduces the overall heat loss speed. However, it is important to think about the whole thermal envelope of the house. It does not make sense to just insulate one area of the house as the heat will then just escape somewhere else. It can be likened to being naked in the cold: it helps if you put on a jacket, but to get truly warm you will also need trousers, socks, hat, etc. 3 jackets but no trousers/socks/hat is not as useful as one of each.

When insulating, another important factor to consider are thermal bridges. Thermal bridges are materials that connect from the inside to the outside of the house. It is very easy to lose heat through thermal bridges. As with house design, edges are often the weak point of insulation, as well.

Curtaining windows is important because windows allow a lot of heat loss. There are 4 rules to good curtaining:

  • Curtain all glass (or cover it in some way if it is not possible to put a curtain on)
  • Have 2 (or more) layers
  • Stop convection currents through pelmets and having curtains to the floor
  • Good curtain behaviour – close curtains before heat is lost, around 4pm.

The diagram below illustrates the difference between a well and poorly performing curtain:


Shading and Venting (inverse North and South if you are in the Northern Hemisphere!)

shadingNorth facing side needs to have shading that is moveable and non-permanent, because it has to be able to let sun in in winter and shade from sun in summer à otherwise it will be too cold in winter and too hot in summer.

Deciduous trees are not the answer to shading in this climate, as even with no leaves they still shade at least 50% of light and heat.

The ways to keep the home cool in summer are:

  • Insulation (keeps home cool in summer, as well as warm in winter)
  • Shading
  • Ventilation


Generally, we over ventilate New Zealand homes, plus many homes are draughty. Draughts are uncontrolled air exchanges, while ventilation is when it is controlled (i.e. we have taken control over when and how the air change takes place).

We need to change air in the house every day to ensure good health. We want to do it quickly, not trickling (e.g. through draughts and by leaving a window open a little bit). By opening a window in one part of the house and another on the other side, we can create a path of air movement so that the air exchange is quick. This is especially good in winter, as fast air exchange means thermal mass will not cool down, and so the home will remain warm.

Thermal Mass


Further Resources

“Remember, we don’t have to do everything; we just need to do as much as we can. And if more of us do more, that will be enough; we don’t all have to do it all.” – Ian Mayes

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