Insulation, shading and making evaporative cooling work harder

Evaporative coolers are blamed for work that belongs to the building. A ducted unit on a dark corrugated roof, over an uninsulated ceiling, feeding a west-facing glass box, will run all afternoon and still lose the rooms by four o’clock. The pads are evaporating. The house is collecting heat faster than a wet fibre can cancel it.

The cheaper move is rarely a larger Bonaire or Braemar. It is to stop treating the cooler as a correction for a shed. Bulk insulation, pelmets, external shade on west glass, a saner roof colour, and a night purge when the dry air finally arrives — these are the works that let a correctly sized machine look competent.

The cooler is an air changer, not a vault

Refrigerated air conditioning can, within limits, hold a sealed room against a hostile envelope. That is why people install it in brick boxes with the windows shut. An evaporative cooler cannot do that job. It cools a stream of outdoor air and asks the house to pass that air through and out. The rooms stay comfortable only if the heat entering through roof, walls and glass is modest compared with the cooling in the stream.

A tin roof in January is not modest. Unshaded west glass is not modest. A ceiling without bulk insulation is a radiator aimed at the beds. You can feel this without instruments: the rooms that fail first are the ones under dark roof sheets and behind the afternoon windows. The cooler outlets in those rooms are not broken. They are outgunned.

This is also why windows must stay open even after you insulate. Insulation is about heat. Openings are about the moist air leaving. Confusing the two is how a well-meant retrofit produces a clammy house.

Bulk insulation, the unfashionable layer

Ceiling bulk insulation is the first conversation in any Australian climate where evaporative cooling still earns its keep — Perth, Adelaide, inland Victoria, the NSW slopes, the Queensland west. Heat through the roof dominates the afternoon load. A bare ceiling under metal sheets will re-radiate long after the fan has been doing honest work.

If the ceiling is empty, fill it. If the batts have slumped, been moved by electricians, or never covered the edges, fix that before you talk about a new rooftop unit. The cooler sits on the same roof that is cooking the rooms. Reducing the roof load cuts the hours the machine must run, which cuts both electricity and water. That is the link to running costs.

Wall insulation helps, especially on west walls that see sun. It is harder and dearer in an existing weatherboard or brick veneer, and it is still worth doing when the wall is being opened for other reasons. Do not wait for a perfect envelope. Do the ceiling.

Reflective foil has a place under roofs as a radiant barrier. It is not a substitute for bulk. Houses that received only a thin foil in a past decade often feel as if “insulation does not work”. The foil was asked to be a batt.

Pelmets and the forgotten top of the window

A curtain that stops short of a closed pelmet, or that has no pelmet at all, leaves a slot where cooled air drops behind the fabric, picks up heat from the glass, and returns to the room as a convection loop. You have paid to cool air that then sits against a 50-degree pane.

Pelmets close that slot. They are joinery, not gadgetry. In older Australian houses they were ordinary. In renovations they disappear because they look like someone’s aunt. Put them back on the west and north windows that see summer sun, and use a close-woven curtain or a proper blind during the worst hours.

External shade is better than any curtain. A verandah, an awning, a deciduous vine, a shutter — anything that stops the sun hitting the glass. Once the heat is in the pane, the room already has the problem. Internal blinds are a rear-guard action. They are still better than nothing.

West glass is a special case

The western window is where Australian houses lose the afternoon. A living room that is pleasant at eleven can be uninhabitable at four if that wall is glass and the sun has a run-up across a pale driveway. An evaporative outlet in that room will feel weak because the load is concentrated on one surface.

Shade first. Tint or replace glass only after you understand the numbers; do not treat film as a personality. If the room is a late addition with a huge west face, accept that the cooler will not make it into a cave. Close it off on the worst days, run the rest of the house, and stop asking one rooftop unit to redeem a solarium.

East glass is a morning problem and usually more forgiving. South glass in southern Australia is the easy one. Designers who “bring the outside in” on the west were not thinking about wet-bulb temperature.

Roof colour and the unit on top

Dark corrugated iron is a solar collector that happens to keep the rain out. Light colours reject more of that load. A reroof is the moment to choose a lighter colour if the street and the planning rules allow it. Painting an existing roof can help and can also be a warranty argument with the sheeting; treat that as a building decision, not a cooler accessory.

The rooftop unit itself sits in that climate. A cabinet on a dark roof, in full sun, with tired pads, is starting the afternoon already warm. You cannot shade every unit, and you should not wrap a machine that needs airflow. You can keep the lid intact, the pads wet and even, and the ceiling below insulated so the ducts are not lying in an attic oven.

Winter is the other half of the roof story. An uncovered cooler is a hole. Cover the unit and close the ceiling vents so July does not pay for January’s appliance. That is care, and it is also heating cost.

Night purge, the free shift

Dry-climate towns often get a night that is worth using. When the outdoor air is cooler than the house, and dry enough, open the house and run the fan without water, or simply open opposite windows. You are dumping the day’s stored heat. The pads do not need to work as hard the next afternoon if the building did not stay charged overnight.

This is not a humid-coast trick. Sydney and Brisbane nights can arrive sticky. Perth, Adelaide and the inland still get many nights when a purge is the most honest cooling available. Do it before you sleep if the security and the insects allow, or run the fan on a timer.

A night purge does not replace the cooler on a 42-degree day. It means the day starts from a lower base. Combined with insulation, that is hours off the machine.

Why the cooler cannot outrun a tin roof

Physics here is blunt. Evaporation can take a fixed amount of heat out of a given airflow, set by the wet-bulb depression — how dry and how hot the outdoor air is. The roof and west glass can add heat without asking the wet-bulb’s permission. When the additions exceed what the airstream can absorb, rooms rise. Turning the fan up moves more air and also pulls more heat through any leaky ceiling. It feels like doing something.

A larger unit on the same roof, into the same glass, repeats the disappointment at a higher water rate. That is how people decide evaporative cooling “does not work here” in a climate where it has worked for decades.

Fix the envelope until the machine is sized to the house you actually have. Then keep the pads honest, the windows open, and the hours matched to dry air. Compare that system with reverse cycle only after the building is no longer a collector; the evaporative versus reverse cycle argument is otherwise a comparison between a tool and a complaint.

A cooler that is allowed to do its job — wet pads, open house, a roof that is not an oven — will take the sting out of a dry summer. A cooler asked to outrun corrugated iron in the western sun will run, and run, and lose. The iron was always going to win.