- Effect of Passive Solar Design - Credit Rohan Campbell
Light and Shade - the Key to Natural Heating and Cooling
Take a look at these two views of the same house - one is in the middle of winter and the other in the midle of the summer. Which view is which, with no telltale snow or green grass to identify the seasons?
The key lies in the way the house is facing and the resulting pattern of light and shade - the house faces with the majority of its windows towards the equator. In the winter the low lying sun shines in directly, providing enough heat by itself to bring the rooms to a comforable temperature - even when it is thirty degrees Celsius below zero outside. In the summer, the roof overhangs protect against overheating, shading the windows from the high noon sun. You can see this in the view on the right - the windows are shaded almost fully, whereas in the view on the left full sun shines in all the windows.
This idea is part of a high-performance building design system called passive solar design. The method uses informed choices for key building design elements such as building orientation, window placement, overhangs, thermal mass and floor layout to drastically reduce heating, cooling and lighting requirements.
Building Orientation
How should you place your home on the lot to take advantage of this energy, freely provided by the sun? The key idea is to design your house with one of the long walls facing within 10 degrees of true south (or north if you are in the southern hemisphere). This allows the light to enter into as large an area as possible. You should avoid jogs and offsets on the south side since these would shade the windows. On the other walls, porches and garages can provide useful shading and insulation value.
Window Design
So how much window area do you want on the south wall? It is not necessarily true that the more you have, the better. Too much south glass will overheat your house in the winter and you will need heat absorbing materials like concrete, stone or brick to absorb that excess. The excess heat is basically stored over the course of the day and released at night. This moderates the temperature swings in the house.
The amount of glass area is specified as a percentage - take the area of all the glass in one wall and divide it by the total floor area (times 100 to make it a percentage). To maximize the solar heat gain in the winter and minimize thermal losses, window glass should be roughly distributed as follows:
- South-facing Glass: 5% to 12% of the floor area of the house
- East-facing Glass: less than 4% of the floor area
- West-facing Glass: less that 2% of the floor area (to avoid overheating in the summer).
- North-facing Glass: less than 4% of the floor area
Windows on the east and west sides of the house do not receive much solar radiation during the winter months, so they are a source of heat loss rather than heat gain. They are difficult to shade In the summer so can lead to overheating. However, you still want windows on these walls for the views and the light that they let in. Some of them should be opening windows so that you can open them in the evening to naturally cool the house.
Overhangs and Shading
In the summer, overhanging eaves, balconies or awnings are used to keep the sun from shining directly into the home. This protects very effectively against overheating, especially if your home is also well-insulated, and can eliminate the need for air-conditioning. How much overhang do you need?
- Overhangs should not cast shade on south-facing windows on the winter solstice
- Overhangs should completely shade these windows on the summer solstice
The actual length will vary with your latitude. Software tools are available to help you choose the best overhang for where you live.
Thermal Mass
If you have more than 7% south glass, you will need thermal mass such as concrete or tile floors or brick or stone facings to absorb solar heat during the day and reradiate it at night. Some basic guidelines for amount of thermal mass are:
- If the south-facing glass is more than 7% of the floor area thermal mass should be added for the area of glass that is greater than7%.
- In general, the maximum amount of floor mass that should be used is 1.5 times the area of south glass (assuming a thickness of 4” for the thermal mass). If more thermal mass is needed it can be added to the walls.
- The recommended thickness for thermal mass is 2” to 4”.
Building Layout
To fully benefit from passive solar design principles, the home should have an open floor plan so that the heated air can circulate around the home. A plan broken up into many rooms and hallways will probably not benefit from the passive heat in rooms along the north wall of the house.
A Design that Stands Out and Delivers
Energy savings of up to 80% are possible with the use of energy efficient, high solar gain windows, thermal mass and appropriate insulation for the climate conditions. Use the natural heat and light of the sun for a home design that is warm in the winter and cool in the summer, a home that is sunny and open and full of light and energy. Passive solar designs are creative, practical and beautiful.
Angelika Ortlepp - Angelika is a professional engineer specializing in renewable energy, passive solar design and technical writing.
