Sustainable Home Design - Passive Heat Control

Passive Heat Control

Winter and Summer Sun | Airflow, Plants and Water | Heat Exchanges – Passive | Solar Heating | Passive Heat Control Further Reading

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Winter and Summer Sun

The best way to gain warmth in winter and be cool is summer is to control how the sun shines on your house. Unfortunately few new homes in SA are built to take this into account. The rules are fairly simple:

Face the house exactly north (not magnetic north). Thus your house will face the arc that the sun travels across the sky. If you don't face north, still do as much of the below as makes sense for the sun you do get.
Design your roof or stoep roof to be deep enough to keep summer sun from hitting your windows (when the sun travels high in the sky), but shallow enough to allow the winter sun to hit your windows (when the sun is lower in the sky).
You can probably have roof/window overhangs in SA between 400mm and 600mm, however exact measurements should be made based upon orientation of house, height of overhangs above windows etc. Worst case scenario, you should be able to plan for 400mm to 600mm
If you have tall windows that extend to the floor such that summer sun will enter even with such a stoep as described above, you can use a roof/stoep with depth to control most of the sun, and then extend it with a seasonal plant such as a vine growing along a pergola (the vine will create shade in summer when it has leaves, but allow sun through in winter when it is bare).
If you want to calculate this yourself accurately a simple (10 min learning) app to use is SketchUp from Google (http://www.sketchup.com $500, but you can use the download for 8 hours before you have to purchase). More software is also available (eg http://squ1.com/ecotect/features/shadows US$700 and Sombrero at http://nesa1.uni-siegen.de/ - click the English flag to get the English version, then click on products and scroll down – Euro220). Actual calculations on paper get quite complex.
You can also use exterior shutters to keep the sun from hitting your windows in summer, like most Mediterranean countries do (interior shutters don't do the job nearly as well – the sun has already hit the window and heated the air up – however they will still help – there are some products such as honeycomb shades that do well on the interior - http://symphonyshades.com/energysmart.html from the US)
The next trick is to have interior floors and walls be built out of materials that absorb heat when it's radiated onto the material (daytime in winter), and release it when its cool (night time in winter). Concrete and stone are the best options. Which means that in the extreme, wooden floors are out. However, if a wooden floor is insulated properly (which can only be done when it's put in – see under Insulation), you will still gain something with a wooden floor.

Airflow, Plants and Water

Airflow around a house is important in summer for cooling, and winter for warming. In summer try to open low windows on the south side, and high windows on the north side. This works best in a multi-storey house.

Plants around the exterior of the house help absorb the suns rays that would otherwise be entering the house. Grow creepers on walls, and have bushes below and around windows.

If you can (and the water and electricity conservation need considering), have a fountain in-front of entrances that face north. The water will cool the air entering the house. Turn it off in winter.

Heat Exchanges – Passive

Your car gets hot in the sun right? That's because there's an enclosed space behind glass being warmed up. This concept can be built into a house design – have a green house type design – it doesn't have to be large, and can be used to grow veggies, north facing, with a thick wall behind it that is part of the house.

Have vents in the top and bottom of the wall. In winter, open the vents – warm air from the glassed in area will rise and move into the top vents, and cold air from the house will move into the glassed area. Thus all the heat from the greenhouse will enter the house. If there is a thick wall as part of this, it will collect heat and let it off during the night too (this is known as a Trombone wall, after the inventor). In summer, close the vents, and open roof windows in the green house to let air out, and even cover the green house with shutter, blinds etc. See pg 72 The Natural House Book.

A heat exchange can also be used to cool a house down – by using the same system, but opening low south facing windows in the house in summer together with the sunspace roof windows, you'll create a flow of cooler air into the house. This can be taken further by building vents into the top of the house to capture wind. Of course in SA we have to compromise and design for both – as this site describes. More on cooling is also in The Natural House Book, pg 77

Solar Heating

Solar heating can take several forms. You can install a solar geyser to heat water – see Geysers. You can install solar panels to provide electricity to heat under floor electrical heating etc – see Electricity.

You can also install simple black piping systems on your roof that then either heat your pool, or if you're clever, your floor. The latter needs to be done at building time – laying pipes in the concrete, or in insulation below your floor:

Passive Heat Control Further Reading

The Good House Book, pg 68 onwards