Climate Design Issues
CLIMATE SENSITIVE DESIGN - The importance of climate sensitive design can not be overrated. Homeowners pay hundreds of dollars more than they need to each year in heating and/or cooling bills because they are not taking advantage of free heating and cooling from passive design.
DESIGN FOR CLIMATE CHANGE
- Scientific evidence suggests that climate change (global warming) is influenced by greenhouse gas emissions.
- Some changes are already becoming apparent. These include:
- Freak storms and atypical weather patterns, temperature increases, more frequent droughts and floods.
- As homes are designed with a 50 year life expectancy (the best ones last for hundreds), it makes sense to choose and design homes that make allowance for climate change.
General principles include:
- Build well above historic flood levels.
- Design stormwater controls for more intense rainfall.
- Plant gardens that will survive longer dry periods.
- Generally design or choose homes appropriate for warmer and more extreme weather conditions.
REMEMBER: transitions between climate zones are gradual. Every site will have specific micro-climate variations and these should have a significant impact on any design.
House performance varies across a spectrum, from very good to very bad. Incorporating any element of sustainable building practice will make a difference. Simply specifying the optimal eave widths on a project home or renovation may prevent unwanted sunshine overheating your home in summer. This step on its own will improve your thermal comfort and reduce your energy bills. All home design includes compromises, but try to do what you can to incorporate good design features.
HEATING AND COOLING
- Draught sealing around doors and windows can save up to 25 percent of heat losses and gains in many climate zones.
- Myth: Roof ventilators will keep your house significantly cooler.
Roof ventilators do not make an appreciable difference to house temperatures if the roof is insulated, particularly if reflective insulation is installed. If your ceiling is uninsulated a ventilator might make a small difference, but insulation is a better investment. There may, however, be other valid reasons for installing roof ventilators such as moisture removal.
- An oversized air conditioner not only costs more to buy but cannot dehumidify air properly. It will only run for short periods and not have time to remove much moisture from the air. Consequently, the occupants feel sticky even when the air conditioner is running. Most air conditioners are less efficient when running at part load, and frequent cycling on and off may shorten their life.
- Oversized heating systems cost more, and will give bursts of heat, followed by long periods when no heating is occurring. Occupants are subjected to varying levels of heat and cold, especially when sitting near a window, where the temperature falls faster than it does near the thermostat.
- Appropriate sizing of heating and cooling equipment can save on purchase and running costs.
- In winter turn the thermostat down a few degrees, and in summer up a few degrees. Each degree can reduce energy consumption by up to ten percent.
The main factors influencing human comfort are:
- Temperature.
- Humidity.
- Air movement (breeze or draught).
- Exposure to radiant heat sources.
- Cool surfaces to radiate to for cooling.
- Sound building envelope design will moderate all of these factors except humidity.
- To do this effectively, envelope design must be varied to suit the climate. It can significantly improve comfort levels while reducing heating and cooling bills.
- Evaporation rates are also influenced by air movement. Generally, a breeze provides comfort by increasing evaporation and making us feel cooler.
- We also lose heat by radiating to surfaces cooler than our body temperature. The greater the temperature difference, the more we radiate. While not our main means of losing heat, radiation rates are very important to our perception of comfort.
Solar
- Even with advanced glazing and framing systems, glass areas remains the single greatest source of heat loss and heat gain in a home.
- The use of clear glass allows maximum winter sun penetration and "free" solar heating. Additionally, the glass significantly reduces the penetration of ultra violet light and reduces fading of furnishings.
- The windows have light transmission in excess of 60 percent, minimising use of artificial lighting and maximizing outlook to the excellent views.
Shading
- Shading of the building and outdoor spaces reduces summer temperatures, improves comfort and saves energy. Direct sun can generate the same heat as a single bar radiator over each square metre of a surface. Shading can block up to 90 percent of this heat.
- Shading of glass to reduce unwanted heat gain is critical. Unprotected glass is often the greatest source of unwanted heat gain in a home.
- Radiant heat from the sun passes through glass and is absorbed by building elements and furnishings, which then re-radiate it. Re-radiated heat has a different wavelength and cannot pass back out through the glass as easily. In most climates, 'trapping' radiant heat is desirable for winter heating but must be avoided in summer.
- Shading of wall and roof surfaces is important to reduce summer heat gain, particularly if they are dark coloured and/or heavyweight.
- Shading requirements vary according to climate and house orientation.Use external shading devices over openings. Lighter-coloured shading devices reflect more heat. Internal shading will not prevent heat gain unless it is reflective.
- Use plants to shade the building, particularly windows, to reduce unwanted glare and heat gain. Evergreen plants are recommended for hot humid and some hot dry climates. For all other climates use deciduous vines or trees to the south and deciduous or evergreen trees to the east and west.
- With ideal south orientation sun can be excluded in summer and admitted in winter using simple horizontal devices, including eaves. For situations where ideal orientation cannot be achieved (eg existing house, challenging site) it is still possible to find effective shading solutions.
- South facing openings receive higher angle sun and therefore require narrower overhead shading devices than east or west facing openings. Fixed horizontal shading is often adequate above south facing glazing. Examples include eaves, awnings, and pergolas with louvres set to the correct angle.
- East and west facing openings require a different approach, as low morning and afternoon sun from these directions is more difficult to shade. Keep the area of glazing on east and west elevations to a minimum where possible, or use appropriate shading devices. Adjustable shading is the optimum solution for these elevations.
- Deep verandahs, balconies or pergolas can be used to shade east and west elevations, but may still admit very low angle summer sun. Use in combination with planting to filter unwanted sun.
- Pergolas covered with deciduous vines provide self adjusting seasonal shading. A 500mm gap between the wall and planted screens should be left for ventilation and cooling. Vines on walls (where appropriate) can also provide summer insulation to all orientations. Evergreen vines block winter sun and should only be used in tropical climates.
- Use drought tolerant ground-cover plants instead of paving where possible, to keep the temperature of the ground and surrounding surfaces lower in summer.
- Protect skylights and roof glazing with external blinds or louvres. This is crucial as roof glazing receives almost twice as much heat as an unprotected west facing window.
- Double glaze skylights in cooler climates to prevent excessive heat loss.
- Advanced glazing solutions such as solar films and tinted glass may be appropriate as a secondary measure on east and west elevations. They can exclude up to 60 percent of the heat compared to plain glass.
- Avoid using tinted glass on south facing windows designed to let in winter sun.
FIXED SHADING FOR PASSIVE SOLAR ACCESS
- Fixed shading devices can regulate solar access on southern elevations throughout the year, without requiring any user effort.
- Summer sun from the south is at a high angle and is easily excluded by fixed horizontal devices over openings. Winter sun from the south is at a lower angle and will penetrate beneath correctly designed fixed horizontal devices.
- For hot humid climates and hot dry climates with no passive heating requirements, shade the whole building at all times. For hot dry climates with passive heating requirements allow some low angle winter sun to reach walls, concrete floors and especially windows.
- Permanently shaded glass at the top of the window is a significant source of heat loss, especially in cool and cold climates.
- Adjustable shading appropriate for northern elevations includes adjustable awnings or horizontal louvre systems above glazing, and removable shadecloth over pergolas or sails. Shadecloth is a particularly flexible and low cost solution.