Passive and low-energy cooling for Australian workplaces: reduce the heat before you buy the cooling

Australian summers are getting harder on buildings and the people in them. Warehouses under uninsulated metal roofs reach temperatures that make work unsafe. Offices with large west-facing windows overheat every afternoon. Factories add heat from compressors, ovens and motors to the summer load. The usual response is to buy more air conditioning, which adds capital cost, peak electricity demand and running costs, and in large, open industrial buildings often cannot achieve comfortable conditions at any reasonable price.

There is a better order of operations. First reduce the heat that enters or is generated in the building. Then remove what remains with the least energy-intensive methods available, such as ventilation, air movement and, where the climate suits, evaporative cooling. Only then size refrigerated cooling for the load that is left. This approach, often described as passive and low-energy cooling, can make buildings more comfortable and resilient while reducing both capital and running costs.

This article explains, in general terms, how heat enters buildings, the main passive and low-energy measures, how they suit different Australian climates, how comfort is affected by air movement and adaptation, and how to prioritise improvements in offices, warehouses and factories. It is general information. Building work must comply with the National Construction Code and relevant approvals, and significant design decisions should involve qualified building services and building physics practitioners.

Where the heat comes from

Cooling loads come from several sources:

• Solar radiation through windows and absorbed by roofs and walls.
• Conduction through the roof, walls and glazing when outside surfaces are hotter than inside.
• Infiltration and ventilation, bringing in hot outdoor air.
• Internal gains from people, lighting, computers, motors, compressors, ovens and other processes.

In single-storey industrial and commercial buildings, the roof is often the largest single source of heat gain in summer, because it receives intense sun for most of the day. In offices, windows, particularly those facing west, and internal equipment loads often dominate. Understanding which source dominates in a particular building directs effort to where it pays most.

Reduce heat gains first

Shade windows from the outside

Shading outside the glass stops solar radiation before it enters. Internal blinds help glare and some heat, but much of the solar energy is already inside the building once it passes through the glass. Useful approaches include:

• horizontal shading such as eaves and awnings on north-facing windows, which block high summer sun while admitting lower winter sun;
• vertical or adjustable shading on east and west windows, where the sun is low in the morning and afternoon;
• external blinds, screens and vegetation; and
• high-performance glazing with a low solar heat gain coefficient where shading is impractical.

Insulate and lighten the roof

For buildings with large roof areas, roof insulation and reflective, light-coloured or “cool” roof surfaces can dramatically reduce heat gain. Cool roof coatings reflect more sunlight and emit absorbed heat more effectively, reducing roof surface temperatures. Insulation reduces conduction into the space. Together they often transform conditions in warehouses and factories.

Seal the building

Uncontrolled air leakage brings hot air in and conditioned air out. Sealing gaps, managing large doors with seals, fast-acting doors or air curtains and controlling outdoor air quantities reduces cooling load.

Reduce and relocate internal heat

Efficient lighting, turning equipment off when not needed, insulating hot pipes and ovens, extracting heat at its source and relocating heat-producing equipment such as air compressors outside the occupied space can remove large loads. Heat recovery from compressors and processes can turn waste heat into useful hot water.

Remove heat with low energy

Natural and mixed-mode ventilation

When outdoor air is cooler than indoor air, ventilation is the cheapest cooling available. Cross ventilation uses openings on opposite sides of a space; the stack effect uses warm air rising and leaving through high-level openings, drawing cooler air in at low level. Ridge vents and wall louvres in industrial buildings use both effects. Mixed-mode buildings use natural ventilation when conditions allow and mechanical cooling when they do not.

Thermal mass and night purging

Heavy materials such as concrete and masonry absorb heat during the day and release it later. Combined with night purge ventilation, which flushes the building with cool night air, thermal mass can reduce daytime peak temperatures. The approach works best in climates with a large difference between day and night temperatures, common in inland and many southern parts of Australia, and less well in humid tropical climates where nights stay warm.

Air movement

Moving air increases the rate at which the body loses heat, so people feel cooler at the same temperature. Ceiling fans in offices and high-volume, low-speed fans in warehouses and factories can make spaces comfortable at higher temperatures than still air would allow, allowing higher cooling setpoints or avoiding refrigerated cooling altogether in some areas. Fans use far less energy than refrigerated cooling.

Evaporative cooling

Direct evaporative cooling passes outdoor air through wetted pads, cooling it as water evaporates. It is very effective in hot, dry climates and uses much less electricity than refrigerated air conditioning, although it uses water and adds humidity. Indirect evaporative cooling cools air without adding moisture to the supply, extending usefulness into more humid conditions. Evaporative systems need regular maintenance and water treatment to manage hygiene risks.

Other options

Ground-coupled systems, radiant cooling through chilled ceilings or slabs and spot cooling of workstations each have roles. Radiant systems in particular need careful control of humidity so that surfaces stay above the dew point.

Matching strategies to climate

Australia’s climates vary widely, and the National Construction Code uses climate zones to reflect this. In broad terms:

Comfort is not only temperature

Comfort depends on air temperature, radiant temperature, humidity, air movement, clothing and activity. Hot roofs and windows raise radiant temperature, making spaces feel warmer than the thermostat suggests, which is another reason to reduce heat gains at the source.

People in naturally ventilated buildings, who can open windows and adjust their environment, tend to accept a wider range of temperatures than those in sealed, fully air-conditioned buildings. Adaptive comfort models in international standards reflect this. Designing for adaptive comfort, with operable windows, fans and flexible dress codes, can reduce energy use while maintaining satisfaction.

In workplaces, heat is also a health and safety issue. Employers have duties under work health and safety legislation to manage risks from heat, and regulators publish guidance on working in heat. Passive and low-energy measures are often the most effective long-term controls.

Designing new buildings

For a new building or a major refurbishment, passive cooling decisions cost least when made early. Orientation, building form, window placement and roof design are almost free choices at concept stage and very expensive to change later. Useful early decisions include:

• Orienting the long axis east-west, so most glazing faces north and south, where shading is easier.
• Limiting west-facing glazing, or shading it with vertical or adjustable devices.
• Specifying roof insulation and reflective finishes well beyond minimums where large roofs and hot climates make heat gain the dominant load.
• Designing for ventilation, with openings positioned for cross flow and high-level outlets.
• Grouping heat-producing processes in zones that can be ventilated or extracted separately.
• Allowing for fans, with structure and power provided from the start.

The National Construction Code sets minimum energy efficiency requirements for building fabric and services. Going beyond the minimum is often justified by lower running costs and better conditions over the building’s life.

Controls and monitoring

Passive and low-energy measures only work if they are operated well. Night purge ventilation needs openings or fans that operate at the right times and close when outdoor air becomes warmer than indoor air. Mixed-mode buildings need clear rules, automated or manual, for when to switch between natural ventilation and mechanical cooling. Evaporative coolers need controls that consider humidity, not just temperature.

Simple temperature and humidity loggers, inside and outside, show whether measures are working and help fine-tune operation. Occupants also need to understand how the building is meant to work. A building designed for night purging performs poorly if windows are shut every evening for security without an alternative such as secure louvres.

Prioritising improvements

A practical sequence for existing buildings is:

1. Measure: log temperatures and humidity in problem areas, and identify the main heat sources.
2. Fix the envelope: shading, roof insulation and coatings, sealing.
3. Cut internal gains: efficient equipment, extraction at source, relocation.
4. Add air movement: fans, often the cheapest comfort improvement.
5. Use ventilation and evaporative cooling where the climate suits.
6. Right-size refrigerated cooling for the remaining load, with good controls.

The HVAC for commercial and industrial buildings article covers system choices and controls for the mechanical cooling that remains.

A worked example

This is an illustrative example. The building, figures and U-values are hypothetical assumptions chosen to show the reasoning, not design data.

A small manufacturer occupies a 2,000 m² factory in western Sydney with an uninsulated, dark metal roof. On hot afternoons, temperatures under the roof make work uncomfortable, and quotes for refrigerated air conditioning of the whole space are very expensive.

An engineer estimates roof heat gain to show the scale of the problem. Assuming, for illustration, an effective heat transfer coefficient of about 6 W/m²K for the uninsulated roof and an effective temperature difference of about 25 K, accounting for the sun-heated roof surface, the roof gain is about 6 × 2,000 × 25 = 300,000 W, or 300 kW. With insulation and a reflective coating, assuming an effective coefficient of about 0.5 W/m²K and the same effective temperature difference for simplicity, the gain falls to about 0.5 × 2,000 × 25 = 25,000 W, or 25 kW. In reality a cool roof also lowers the effective temperature difference, so the improvement would be larger still.

The business insulates and coats the roof, installs ridge vents, adds high-volume, low-speed fans over the production floor and relocates two air compressors outside with ducted intake air. It installs refrigerated air conditioning only in the offices and a small quality room. Afternoon temperatures on the floor fall noticeably, workers report better comfort with the fans, and the cooling equipment purchased is a fraction of the original quotes.

How this applies to a small Australian business

• Find the dominant heat source before buying cooling.
• Shade outside the glass, especially on west-facing windows.
• Insulate and lighten roofs on warehouses and factories.
• Use fans generously. Air movement is cheap comfort.
• Match evaporative cooling to the climate.
• Relocate or extract heat-producing equipment.
• Treat heat as a work health and safety issue, not only a comfort one.
• Log conditions before and after changes, so you know which measures worked.

Common mistakes

• Buying air conditioning before reducing heat gains.
• Relying on internal blinds instead of external shading.
• Using evaporative cooling in humid conditions without considering performance and humidity.
• Ventilating with hot daytime air, which adds heat instead of removing it.
• Ignoring compressors, ovens and other internal heat sources.
• Oversizing refrigerated systems for loads that envelope improvements could remove, then running them inefficiently at part load.

Frequently asked questions

Are cool roof coatings worthwhile? On buildings with large, sun-exposed roofs, they often reduce roof temperatures and heat gain substantially, particularly where the roof is poorly insulated. Their benefit depends on climate, roof construction and maintenance of the coating’s reflectivity.

Can fans replace air conditioning? In many warehouses and factories, fans combined with envelope improvements provide acceptable conditions for much of the year. In very hot or humid periods, or for sensitive processes, additional cooling may still be needed.

Is thermal mass always helpful? Only when it can be cooled, for example by night ventilation. Heavy construction that never cools down can hold heat and keep spaces warm overnight and into the next day.

Questions to ask

• What are the main sources of heat in our building, and how large are they?
• Are our windows shaded externally, and is our roof insulated and reflective?
• Where could air movement improve comfort cheaply?
• Does our climate suit evaporative cooling or night purging?
• Can heat-producing equipment be relocated or its heat extracted at source?
• What cooling load would remain after these measures?

Bringing it together

Low-energy cooling starts with reducing heat: external shading, insulated and reflective roofs, a sealed envelope and lower internal gains. Then it uses the cheapest ways to remove the remaining heat: ventilation, night purging where the climate suits, air movement and evaporative cooling in dry conditions. Refrigerated cooling, sized for what is left and well controlled, completes the system. Following this order improves comfort and safety, lowers peak demand and running costs and makes buildings more resilient to increasingly hot Australian summers. The cutting energy use in business premises article places cooling within a wider energy review.

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Source: KEVOS editorial notes. Examples and figures in this article are illustrations. Building work must comply with the National Construction Code and relevant approvals.