Understanding passive heating
What is passive heating?
Passive heating uses free heating direct from the sun to dramatically reduce the cost of heating your home. Passive heating is most appropriate in mixed (temperate) or cold climate zones (Australian climate zones 4–8).
Effective passive heating has 5 main elements:
- Letting the sun in to your home - solar radiation travels through glass areas exposed to full sun. Window orientation, shading, frames and glazing type will all affect how well this process works.
- Storing the sun’s heat inside your home - trapped heat is absorbed and stored by materials with high thermal mass inside the home. This heat is released at night when the house starts to cool down.
- Ensuring good heat distribution - heat released from thermal mass is distributed to where it is needed through good design of air flow and convection. Direct re-radiation to areas of need is most effective, but heat can also be conducted through building materials and distributed by air movement. Floor plans should be designed to ensure that the most important rooms (usually living areas used during the day and evening) face north and receive the best winter solar access.
- Ensuring that heat is not lost - appropriate house shape and room layout will reduce heat loss. Heat loss occurs from all parts of the building, but mostly through the roof. In cool and cold climates, compact building shapes that minimise roof and external wall area are more efficient. Heat loss is further minimised with appropriate choice of windows and curtains or blinds, and well-insulated walls, ceilings and floors.
- Ensuring that cold air does not enter your home - infiltration of cold air is minimised with airlocks, draught sealing, airtight construction detailing and high-quality windows and doors.
Why is passive heating important?
Passive heating is the cheapest way to heat your home. It is:
- achievable and low cost when designed into a new home or addition
- appropriate for all climates where winter heating is required
- achievable using all types of Australian construction systems
- achievable when buying an existing house or apartment, by looking for good orientation and shading
- achievable when buying a home off the plan, by paying attention to correct orientation, making slight floor plan changes and choosing appropriate glazing
- possible to retrofit some aspects of passive heating design into an existing home.
Heat is lost through building elements (for example, walls, floors, ceilings, windows) by conduction, particularly at night when the outside temperature is lower than the inside temperature.
Heat flow through any building element is directly proportional to the temperature difference on either side of that element. This is called the temperature differential (also referred to as delta T or ΔT). Temperature differential is like pressure in your garden hose. The greater the pressure, the more water flows through the hose. The greater the temperature differential, the greater the heat flow through the element.
The heat flow through different materials varies depending on their insulation properties (R value). Heat flow through windows is much higher because they usually have the lowest R value of any construction material.
Heat flow also varies depending on temperature stratification. Because hot air rises, air temperatures stratify in a home, with the hottest air in the highest areas. This means that the temperature differential to the outside will be largest in those highest areas, and you may lose more heat through those areas.
For example, if you are trying to keep your main living area at around 22°C on a cold −5°C Canberra night, temperature stratification might lead to 30°C at the highest point in the room and 18°C at the lowest. That means that 33% more heat may be flowing through higher-level building elements than lower ones because the temperature differential is 33% higher.
Heat loss and cold infiltration
Preventing heat loss is an essential component of efficient home design in any climate. It is even more critical in passive heating design because the main heat source is only available during the day. The building fabric must retain energy collected during the day for up to 16 hours and longer in cloudy weather.
Achieving passive heating
Passive heating requires careful application of the following passive design features:
- northerly orientation of daytime living areas
- thermal mass for storing heat
- appropriate areas of glass on northern walls
- passive shading of glass
- floor plan design to address heating needs
- insulation and draught sealing
- attention to air flow and infiltration.
Passive heating maximises winter heat gain, minimises winter heat loss, and concentrates heating where it is most needed.
Many of the features you need for passive heating (for example, orientation, thermal mass) should be designed into the home before it is built. But there are some changes you can make through renovation.
Orientation for passive heating
For best passive heating performance, daytime living areas should face north. Ideal orientation is true north, but orientations of up to 10° west of north and 20° east of north still allow good passive solar gain.
Where solar access is limited (for example, if the sun is blocked by nearby houses or other buildings, as is often the case in urban areas), you can still have an energy-efficient home through careful design. Homes on poorly orientated or narrow blocks with limited solar access can use alternative passive solutions to increase comfort and reduce heating costs.
Alternatively, you can use active solar heating systems that use roof-mounted, solar-exposed panels to collect heat and pump it to where it is needed to achieve energy efficiency on a difficult site. This solution can also be more easily adjusted to adapt to climate change, because the system can be switched off if it is not needed.
Shading for passive heating
Passive heating should be used together with appropriate shading of windows to allow maximum winter solar gain and prevent summer overheating. This is most simply achieved with northerly orientation of appropriate areas of glass and well-designed eave overhangs.
Fixed horizontal shading, such as eaves, blocks summer sun (which is at a high angle) but lets in winter sun (which is at a lower angle). Fixed shading devices can regulate solar access to north-facing glass throughout the year, without requiring any effort. Correctly designed eaves are the simplest and least expensive shading method for northern elevations.
The ‘rule of thumb’ for calculating the width of north-facing eaves is shown in the following diagram. This rule applies to all latitudes south of and including 27.5° (Brisbane, Geraldton). For latitudes further north, the calculation will vary with the climate.
Permanently shaded glass at the top of the window is a significant source of heat loss. To avoid this, the distance between the top of glazing and underside of eaves or other horizontal projection should be 50% of overhang or 30% of window height, where possible.
Use adjustable shading to regulate solar access on other elevations. This is particularly important for variable spring and autumn conditions and allows more flexible responses to climate change
Sealing your home for passive heating
While no home should be completely sealed to air flow, reducing the amount of warm air escaping to the outside can improve energy efficiency.
Air leakage accounts for 15–25% of winter heat loss in buildings (CSIRO 2015). Reduce air leakage in your home by:
- choosing high-quality windows and doors with airtight seals
- using airtight construction detailing, particularly at wall−ceiling and wall−floor junctions
- using tight-fitting floorboards and insulating the underside of timber floors
- avoiding downlights that penetrate ceiling insulation
- avoiding open fires, and fitting dampers to chimneys and flues or blocking them off if unused
- avoiding permanently ventilated skylights
- ducting exhaust fans and installing nonreturn baffles
- sealing gaps between the window and door frames and the wall before fitting architraves in new homes and additions
- improving the performance of existing windows and doors by using draught-proofing strips. Install these between the door and frame, at the door base and between the openable sash of the window and the frame.
- sealing air vents; use windows and doors for ventilation as required. However, this may not be advisable for homes with unflued gas heaters.
- Passive heating, or passive solar heating, means trapping heat from the sun inside your home and using thermal mass, heat flow and insulation effectively to store, distribute, and retain the heat.
- Passive heating can significantly reduce your energy bills. It is useful in nearly all climates, and especially useful in cool or cold climates.
- It is best to use passive heating design principles when building or buying a home, but some aspects of the principles can be used in home renovations.
- The main methods to achieve passive heating are to place living areas on the north side of your home, and include north-facing windows that let sunshine reach areas of thermal mass. You can also locate heaters near areas of thermal mass.
- Good insulation and appropriate glazing will help to ensure that the solar warmth stays inside your home.
- You can also stop cold air from entering your home by including airlock rooms, and sealing around doors, windows and your roof space.