Passive solar heating is a design approach that uses the sun’s energy to heat buildings without the need for mechanical or electrical equipment. The principles of passive solar heating design can be applied to both new and existing buildings, and can significantly reduce energy costs and greenhouse gas emissions.
- Passive solar heating uses free heating direct from the sun to dramatically reduce the estimated 40% of energy consumed in the average home for space heating and cooling.
- Most climates require both passive heating and cooling.
- Many heating and cooling design objectives overlap but different emphasis is required depending on your climate needs.
1) What is passive solar heating?
- Passive solar heating is the least expensive way to heat your home.
- Put simply, design for passive solar heating aims to keep out summer sun and let in winter sun while ensuring the building’s overall thermal performance retains that heat in winter but excludes it and allows it to escape in summer.
- Passive solar design also depends on informed, active occupants who remember to open and close windows and isolate zone spaces, for example, each day.
It is also:
- Low cost when designed into a new home or addition
- Achievable using all types of Australian construction systems
- Appropriate for all climates where winter heating is required
- Achievable when buying a project home through attention to correct orientation, slight floor plan changes and appropriate glazing selection
- Achievable when choosing an existing house, villa or apartment through looking for good orientation and shading.
Passive solar heating requires careful application of the following passive design principles:
- Northerly orientation of daytime living areas (see Orientation)
- Passive shading of glass (see Shading) and selection of appropriate glazing
- Appropriate areas of glass on northern façades
- Thermal mass for storing heat
- Insulation and draught sealing
- Floor plan design to address heating needs including zoning
- Climate appropriate glazing solutions
This maximises winter heat gain, minimises winter heat loss and concentrates heating where it is most needed.
Passive solar heating is low cost when designed into a new home. Passive solar houses can look like other homes but cost less to run and are more comfortable to live in.
2) How passive solar heating works
- Solar radiation is trapped by the greenhouse action of correctly oriented (north-facing) glass areas exposed to full sun.
- Window orientation, shading, frames and glazing type have a significant effect on the efficiency of this process (see Orientation; Shading; Glazing).
- Trapped heat is absorbed and stored by materials with high thermal mass (usually masonry) inside the house.
- It is re-released at night when it is needed to offset heat losses to lower outdoor temperatures (see Thermal mass).
- Passive solar heating is used in conjunction with passive shading, which allows maximum winter solar gain and prevents summer overheating.
- This is most simply achieved with northerly orientation of appropriate areas of glass and well-designed eaves overhangs (see Orientation; Shading).
Passive shading features can control the entry of sunlight and wind
- Re-radiated heat is distributed to where it is needed through good design of air flow and convection.
- Direct re-radiation is most effective but heat is also conducted through building materials and distributed by air movement.
- Floor plans should be designed to ensure that the most important rooms (usually day-use living areas) face north and receive the best winter solar access.
- Heat loss is minimised with appropriate window treatments and well-insulated walls, ceilings and raised floors.
- Thermal mass (the storage system) must be insulated to be effective. Slab-on-ground edges should be insulated in colder climates, or when in-slab heating or cooling is installed within the slab (see Insulation; Thermal mass).
- Air infiltration is minimised with airlocks, draught sealing, airtight construction detailing and quality windows and doors.
- Appropriate house shape and room layout is important to minimise heat loss, which takes place from all parts of the building, but mostly through the roof. In cool and cold climates, compact shapes that minimise roof and external wall area are more efficient. As the climate gets warmer more external wall area is appropriate, to allow for better cross-ventilation.
3) Passive solar design principles
i) Greenhouse (glasshouse) principles
Passive design relies on greenhouse principles to trap solar radiation.
- Heat is gained when short wave radiation passes through glass, where it is absorbed by building elements and furnishings and re-radiated as long wave radiation.
- Long wave radiation cannot pass back through glass as easily.
Solar heat gain through standard 3mm glazing
- Heat is lost through glass (and other building materials) by conduction, particularly at night. Conductive loss can be controlled by window insulation treatments such as close fitting heavy drapes with snug pelmets, double glazing and other advanced glazing technology.
ii) Heat flow through building elements
- Heat flow through any building element (e.g., wall, floor ceiling, window) 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).
- The greater the temperature differential, the greater the heat flow through the element.
- Think about temperature differential as pressure in your garden hose.
- The greater the pressure, the more water flows through the same hose. While the heat flow through different materials varies depending on their insulation properties (R-value), heat flow through each element with a similar R-value is directly proportional to temperature differential. Heat flow through windows is much higher because they typically have the lowest R-value of any construction material.
- Because hot air rises convectively, air temperatures stratify in a home with the hottest air at the highest point. For example, if you, on a cold −5°C Canberra night, are trying to keep your main living area at around 22°C (although most acclimatized Canberra residents find 19−20°C quite comfortable), temperature stratification might lead to 30°C (ΔT 35°C) at the highest point in the room and 18°C (ΔT 23°C) at the lowest.
- That means that 33% more heat is flowing through higher level building elements than lower ones because the temperature differential is 33% higher.
- Again, windows are the weakest point.
iii) Orientation for passive solar heating
- For best passive heating performance, daytime living areas should face north.
- Ideal orientation is true north but orientations of up to 20° west of north and 30° east of north still allow good passive sun control (see Orientation).
- Where solar access is limited, as is often the case in urban areas, energy efficiency can still be achieved with careful design.
- Homes on poorly orientated or narrow blocks with limited solar access can employ alternative passive solutions to increase comfort and reduce heating costs (see Challenging sites; Shading; Insulation; Thermal mass; Glazing).
- Active solar heating systems that use roof mounted, solar exposed panels to collect heat and pump it to where it is needed are a viable solution where solar exposure of glass for passive heating can’t be achieved.
- This provides a more flexible solution that is more easily adjusted to adapt to climate change warming (see Heating and cooling).
iv) Passive solar shading
- Fixed horizontal shading devices can maximise solar access to north-facing glass throughout the year, without requiring any user effort.
- Good orientation is essential for effective passive shading.
- Fixed shading above openings excludes high angle summer sun but admits lower angle winter sun. Correctly designed eaves are the simplest and least expensive shading method for northern elevations.
- 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.
- The ‘rule of thumb’ for calculating the width of eaves is given below. This rule applies to all latitudes south of and including 27.5° (Brisbane, Geraldton).
- For latitudes further north, the response varies with climate (see Shading).
- Permanently shaded glass at the top of the window is a significant source of heat loss with no solar gains to offset it.
- 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 (see Shading).
Rule of thumb for calculating the width of eaves.
- Heat loss through glass (and walls) is proportional to the difference between internal and external temperatures.
- Because the hottest air rises to the ceiling, the greatest temperature difference occurs at the top of the window.
v) Thermal mass and thermal lag
- Thermal mass is used to store heat from the sun during the day and re-release it when it is required, to offset heat loss to colder night-time temperatures.
- It effectively ‘evens out’ day and night (diurnal) temperature variations (see Thermal mass).
- Thermal mass can significantly increase comfort and reduce energy consumption.
- When used correctly, thermal mass can significantly increase comfort and reduce energy consumption.
- Thermal mass is very useful for some climates but can be a liability if used incorrectly.
- Thermal mass must be externally insulated (so the stored heat is not lost) and internally exposed (so solar heat can flow easily into the material).
Thermal mass must be externally insulated and internally exposed
- Adequate levels of exposed internal thermal mass (i.e. not covered with insulative materials such as carpet) in combination with other passive design elements, ensure that temperatures remain comfortable all night and, if well-designed, on successive sunless days. This is due to a property known as thermal lag — the amount of time taken for a material to absorb and then re-release heat, or for heat to be conducted through the material.
Thermal lag times are influenced by
- temperature differentials (ΔT) between each face
- conductivity and density
- texture, colour and surface coatings
- exposure to air movement and air speed.
- Rates of heat flow through materials are proportional to the temperature differential between each face.
- External walls have significantly greater temperature differential than internal walls and thermal mass must be insulated externally.
- The more extreme the climate, the greater the temperature differential and the more insulation required.
- Even 300mm-thick adobe and rammed earth walls require external insulation in cool and cold climates. Avoid using high mass in hot climates.
- The useful thickness of thermal mass is the depth of material that can absorb and re-release heat during a day−night cycle.
- For most common building materials this is 50−150mm depending on their conductivity. Longer lag times are useful for lengthy cloudy periods but must be matched by solar input (see ‘Glass to mass ratios’ below).
- This water filled balustrades provide high thermal mass suitable for current climate conditions but have the potential for low-cost mass reduction (if drained) as climate change progresses.
- In warmer temperate climates, external wall materials with a minimum time lag of 10 to 12 hours can effectively even out internal−external diurnal temperature variations. In these climates, external walls with sufficient thermal mass moderate internal−external temperature variations to create comfort and eliminate the need for supplementary heating and cooling.
- The use of high mass solutions is becoming questionable because climate change will increase summer temperatures and cause longer and more extreme heatwaves. In general, moderate mass or well-insulated lightweight construction is generally a more appropriate solution for the life span of housing built today.
- Extremely high thermal mass levels (e.g., earth covered housing) can even out seasonal temperature variations. Summer temperatures warm the building in winter and winter temperatures cool it in summer. In these applications, lag times of 30 days are required in combination with the stabilizing effect of the earth’s core temperature.
- Low mass solutions with high insulation levels work well in milder climates with low diurnal ranges. Low mass construction gives faster response times to auxiliary heating and will prove more flexible in warming climates. Additional flexible thermal mass options are available (see Thermal mass).
By following these principles, passive solar heating design can significantly reduce energy costs and increase the comfort and livability of buildings.