Evaporative cooling: where it suits NSW homes and where it does not

Evaporative cooling is one of the oldest ways of cooling a house, and it still suits a good number of NSW homes. The principle is simple. Air is drawn across wet pads, water evaporates into it, and the air that comes out is cooler than the air that went in. The catch is in the first word of that description, because the amount of cooling depends entirely on how much moisture the outside air can still take up.

That single dependency is why evaporative cooling is a good recommendation in one part of the state and a poor one in another. It is also why a builder who recommends a cooler without checking the climate, the outlets and the building is likely to hear about it from the client within a season. This article covers where evaporative cooling suits, where it does not, and what has to be planned around it before it goes in.

The article is about climate and airflow suitability, not a comparison with reverse-cycle air conditioning. A client who has already settled on refrigerated cooling will find nothing here about it. The question is narrower: in this house, in this climate, does an evaporative cooler make sense, and if it does, what has to be done to the building to make it work?

What evaporative cooling actually does

The cooler is a fan, a pump and a set of wet pads in a box. The pump keeps the pads wet, the fan draws outside air through them, and the air loses heat as water evaporates from the pad surface. The air leaving the unit is cooler, and it is also more humid. That second effect matters more than most clients expect, because it is what the house has to get rid of.

The cooling is a temperature drop, not a change in the air itself. The drier the outside air, the bigger the drop the pads can produce, and the more the house feels cooler. In air that is already carrying a lot of moisture, the pads have little room to work, and the air coming through them is barely cooler and noticeably damper. The cooler is still running and still using water, but the room is not getting cooler.

This means the cooler does not have a fixed performance. It depends on the weather on the day, the moisture in the outside air, the airflow the house allows and the condition of the pads. Two identical units in two different NSW towns will not perform the same way, and a specification that promises a set result for every site is promising something the physics will not deliver.

Why the climate decides the answer

Climate is the first filter, and it should be applied before anything else is designed. The useful question is how the outside air behaves during the warm months, not how hot the summer gets. A place can be very hot and still suit evaporative cooling, provided the air is dry. A place can be only moderately warm and be a poor site for it, because the air is humid all the time.

Dry air and hot afternoons

Inland NSW, across the western plains and the inland slopes, is where evaporative cooling does its best work. The afternoons are hot and the air is dry, and the cooler produces a clear drop in temperature that a family can feel. Evenings often cool off as well, which means the house can be flushed with cooler outside air once the afternoon heat has passed. The combination of dry air and a large day-to-night change is the one the technology was made for.

Humid air and the coast

Coastal NSW is a different story. The coast is often humid through much of the warm season, and humid air has little room left to take up moisture from wet pads. The cooler then adds moisture to the house without dropping the temperature much. The result is a room that feels sticky, that can develop condensation on cold surfaces and that may encourage mould in the wrong places. The cooler is switched off and the client concludes the builder got it wrong, which is a fair conclusion if the climate was not checked.

The middle ground is harder. Some places sit between the two, with humid stretches that come and go. Evaporative cooling can still be useful there, but only as one part of a wider approach, and the expectation has to be set honestly with the client about which weeks the cooler will help and which weeks it will not.

Where it suits NSW homes

Evaporative cooling suits a house that has a clear, dry climate in the warm months, a practical exhaust path, and a building that has been designed to stay cool for most of the day. Those three conditions usually come together in inland homes on larger lots, where the outlets can be placed on the far side of the house and the water supply is straightforward. Single-storey homes with good cross-flow are better candidates than sealed two-storey homes with small windows.

It also suits clients who are prepared to use it actively. A cooler works best when the house is opened in the evening, closed in the morning and managed through the day with the windows and blinds. A client who wants to set it and forget it, with the house closed up all day and the windows never opened, is not going to get the result they expect, and the builder should say so at the start.

Where it does not suit

Evaporative cooling does not suit a house that has to be closed up tightly, a house without a clear outlet, or a house on a humid site. It is also a poor choice for a client who expects the house to feel like it has refrigerated air conditioning, because the air is cooler but more humid and the result is different. Setting that expectation early avoids a difficult handover.

Think also about the house itself. A building with a large amount of glazing facing the afternoon sun, little shading and a heavy roof load will gain heat faster than a cooler can remove it. The cooler is not a substitute for controlling the heat that enters the house in the first place. Shade, insulation and well-chosen glazing do most of the work, and the cooler adds to them.

The airflow path is part of the system

The cooler is only half of the system. The other half is the path the air takes through the house and out again, and that path has to be planned from the start. The cooler pushes air in, so the house needs an outlet that lets it leave. Without a clear outlet, the pressure inside rises, the air stops moving and the humid air sits in the rooms.

A good layout puts the cooler on one side of the house and the outlets on the other, so that air crosses each room in turn before it leaves. Each room should have a path to an outlet, and doors between rooms should be left open or fitted with a grille, so that the air can keep moving. A room with a closed door and no outlet becomes a pocket of humid air, however well the rest of the house is working.

Think about the outlets before the cooler position is fixed. Ceiling vents, open windows and purpose-built outlets all have a place, but they must be sized and placed so that the air moves through the house rather than around its edges. The outlets are where the design decisions get made, and they are the part most often left to chance.

Preparing the building first

The cooler works best in a house that is already doing the right things. Shade the windows on the western and northern sides, and use blinds or external shading where the sun hits the glass in the afternoon. Insulate the roof and the walls to the level the design specifies, and use glazing with a performance that suits the orientation. These are the measures that stop the house gaining heat, and a cooler cannot make up for a house that gains heat all day.

Air sealing needs a careful balance. A house that is sealed too tightly traps the moisture the cooler adds and makes the outlet planning harder, while a house with large gaps wastes the airflow the cooler is trying to create. The sensible approach is to seal the house against uncontrolled leaks while leaving deliberate outlets open. The envelope and the ventilation have to be designed together, and it is worth reading how air sealing fits into the building envelope before you settle the detail.

Night flushing is the other part of preparation. A house that can be opened at night and closed in the morning can carry a cool store of air through the warm day. Houses that cannot be opened at night gain the heat during the day and hold it, and the cooler then has to fight a stored load it cannot beat. Plan the openings so that the night air can get in and out without security or noise problems.

The wider design of the roof and the outside surfaces matters too. Light-coloured and reflective roof materials reduce the heat the roof carries into the house. The article on cool roofs and heat-resilient homes covers this in more depth, and it is worth reading alongside any cooling decision.

Worked example: a cooler in a humid house

Consider a builder in a regional town on the NSW coast, where the summers are warm and humid. A client wanted a cooler on the strength of a neighbour’s experience, and the builder had been asked to install it in a new single-storey house. The house was well shaded and reasonably insulated, which was the good news. The bad news was that the layout had only small windows on the one side, the bedrooms were closed off with solid doors, and there was no planned outlet.

The builder explained the climate problem before installing anything, and the client agreed to a trial. The cooler went in on the roof with a duct to the living area, and the bedrooms were left without a path. Within the first humid week the house felt damp, the bedroom walls showed condensation on the cold corners, and the client switched the unit off. The cooler had been working correctly. The house had not been prepared for it.

The fix was to cut two ceiling vents to the bedroom passages, fit grilles under the doors, and add a dedicated outlet at the far end of the living area. The builder also suggested the client use the cooler only in the drier spells and rely on shading and night ventilation during the humid weeks. The client accepted that the cooler would not be the answer in every week, and the house became comfortable for most of the year. The lesson was that the outlet and the climate had to be checked before the unit went on the roof, not after the first complaint.

Running and maintaining the system

The cooler needs a routine, and the routine has to be part of the handover. The pads need replacing on the schedule the manufacturer gives, because dry, mineral-crusted pads cool poorly and can smell. The water tray and the supply need checking, and any standing water should be drained and cleaned out rather than left to stagnate. Keep the drainage clear so the unit does not overflow into the roof or the ceiling.

Water quality matters. Hard water leaves scale on the pads and the tray, which reduces the cooling and shortens the life of the unit. Where the water is poor, a filter or a treatment may be needed, and the manufacturer should be asked what the unit tolerates. Keep the water side of the system clean and do not let a tank or a tray stand full for long periods.

Write the routine down and give it to the client with the unit’s documentation. Show them how to check the pads, clear the tray and set the fan and pump settings. A client who knows the routine is far more likely to keep the cooler working than one who has been left to work it out, and a cooler that is not maintained becomes the complaint that comes back to the builder.

Explaining the choice to the client

The conversation with the client is where most evaporative cooling problems are either avoided or created. Explain the climate first, using the plain facts about humid and dry air, and then explain the airflow path. Be clear about what the cooler will and will not do in the weather they actually get. Clients accept a cooler that works in the months it suits far more readily than one they were promised would work in all of them.

Put the expectations in writing. The climate assessment, the outlet plan, the preparation works and the maintenance routine should all be on file, signed off by the client. That protects the builder if the client later expects the cooler to do something it was never designed for, and it gives the client a clear picture of what they have bought.

Where to check the requirements

The energy provisions of the National Construction Code set the framework for how a building is assessed, and the certifier or the energy assessor will confirm how a cooler fits into that for the job. Do not assume that adding a cooler changes the assessment, and do not describe a cooler as a compliance measure to the client without checking. The window rating scheme, WERS, gives a way of comparing the performance of glazing, which matters where the windows are the main source of heat gain.

The guidance on passive cooling published by Your Home is a useful starting point for the design side. It sets out the measures that reduce the need for mechanical cooling, and it is the right place to begin before any cooler is specified. Where the detail matters to the approval, the current code and the certifier’s requirements are the final word.

Keep the sources for every cooling decision on the job file. If a client later asks why the cooler was recommended or not, the record should show the climate assessment, the design checks and the advice given, in the order it was made.

The courses on PointCPD are for the continuing professional development you need for your own licence, and they sit alongside your day-to-day technical work rather than replacing it. If you are weighing up which courses to take for your renewal, the guide to how NSW builder CPD works explains the rules.

The PointCPD home page lists the current range.

Questions builders ask

• Cooling
• Energy efficiency
• Ventilation

Published 6 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.