Why Melbourne Homes Overheat Through Glass and What You Can Do About It

Melbourne homes are designed to make the most of natural light. Large windows, sliding doors, atriums and increasingly expansive areas of glazing can make a home feel brighter and more connected to the outdoors.

The downside often becomes apparent when the weather warms up.

A room that feels comfortable in the morning can become noticeably hotter once direct sun reaches the glass. West facing living areas can be particularly difficult in the afternoon, while skylights and large areas of unshaded glazing can introduce heat for much of the day.

Replacing the windows is one possible response, but it is rarely the only one. Understanding why the room is overheating makes it easier to choose an appropriate solution.

Why can a room get so hot through its windows?

Sunlight reaching a window contains energy across ultraviolet, visible and infrared wavelengths. Some is reflected by the glass, some is absorbed and some passes through into the building.

Once solar energy enters a room, it is absorbed by surfaces such as floors, furniture and walls and contributes to the temperature inside.

The amount of solar heat entering through a window depends on several factors, including the type of glass, its orientation, the size of the glazed area, shading and the time of day.

This explains why two otherwise similar rooms can behave very differently.

A large west facing window exposed to afternoon sun can create a substantial heat load, while the same glazing on a shaded elevation may cause relatively little discomfort.

Before changing the windows, it is therefore worth identifying whether the problem is the glass itself, direct solar exposure, inadequate shading or a combination of factors.

Does double glazing stop solar heat and UV?

Double glazing can substantially improve the thermal performance of a window, particularly by reducing heat transfer between the inside and outside of a building.

But double glazing, solar control and UV protection are three different things.

A standard double glazed unit can still admit a significant amount of solar energy if sunlight is falling directly on it. Its performance will depend on the particular glass configuration, including whether coatings such as Low E glass have been incorporated.

Double glazing also does not automatically provide high UV protection. Standard clear glass blocks some ultraviolet radiation, particularly UVB, but a substantial proportion of UVA can still pass through. UVA is particularly relevant to the fading and deterioration of flooring, furniture, fabrics and artwork.

By comparison, many quality architectural window films are designed to reject more than 99 per cent of ultraviolet radiation.

This can surprise homeowners who have invested in double glazing but still experience overheating or remain concerned about fading in rooms receiving substantial direct sunlight.

The distinction is important. Improving insulation addresses one aspect of window performance. Managing solar heat gain and UV exposure addresses others.

Start with external shading where practical

Stopping direct sunlight before it reaches the glass can be highly effective.

Eaves, awnings, shutters, external blinds, pergolas and appropriately positioned vegetation can all reduce the amount of solar energy reaching a window.

Good shading design also considers the changing angle of the sun. An overhang may block high summer sun while still allowing lower winter sun to enter.

However, external shading is not practical for every window.

Apartments may have owners corporation or facade restrictions. Upper storey glazing can be difficult to access. Skylights present their own challenges. Some homeowners also want to preserve an existing view or architectural appearance.

That is where other retrofit options become useful.

Window film can provide solar control without replacing the glass

Modern architectural window films can alter the solar performance of existing glazing while leaving the windows themselves in place.

For homeowners considering window tinting in Melbourne, there is now a much wider range of options than the dark reflective tint traditionally associated with cars or older commercial buildings.

Clear and lightly tinted films can reduce solar heat while retaining high levels of natural light. Darker solar films can provide greater glare reduction, while reflective products can be useful where daytime privacy is also a priority.

The appropriate film depends on the glass and the problem being addressed.

Some products are installed internally. Others are designed for exterior installation, which can be important with certain double glazed or laminated glass configurations.

This is why the highest performing film on a specification sheet is not automatically the best choice for every window.

What should you look for when comparing solar window films?

One of the difficulties for homeowners is that window film performance is described using several different measurements.

Total Solar Energy Rejected, usually abbreviated to TSER, describes the proportion of total solar energy rejected by a glazing and film system.

Visible Light Transmission describes how much visible light passes through.

Glare reduction indicates how much the film reduces visible light compared with the untreated glazing.

Infrared rejection is another commonly advertised measurement, but it should not be confused with total heat rejection. Infrared represents only part of the solar spectrum.

A good solar window film should therefore be assessed using the complete performance data rather than a single headline percentage.

This is particularly important when comparing very clear films. A product can reject a high percentage of infrared energy within a specified wavelength range without rejecting the same percentage of total solar energy.

Natural light should be part of the decision

Reducing heat is usually not the homeowner's only objective.

If a room was designed around a garden view or abundant natural light, dramatically darkening the windows may solve one problem while creating another.

The best solution is often a balance between solar control and visible light.

In living areas where maintaining brightness is important, a high light transmission film may make more sense than a dark tint. In a television room or west facing home office where glare is the dominant problem, a darker product may be preferable.

There is no single film specification that is ideal for every room in a house.

UV protection matters even when heat is manageable

Solar exposure can also contribute to fading and deterioration of interior finishes.

Ultraviolet radiation is one factor, but it is not the only cause. Visible light, heat, material composition and environmental conditions also contribute to fading.

Quality architectural window films can reject more than 99 per cent of ultraviolet radiation, making them useful where flooring, furniture, artwork or other finishes receive prolonged exposure through glazing.

They cannot completely prevent fading, but reducing UV exposure can remove one significant contributor.

This can be relevant even in homes with double glazing. Double glazing should not be assumed to provide the same level of UV protection as a dedicated UV filtering window film.

For renovated homes with new timber floors, furnishings or artwork, this can be an important consideration even where overheating is not severe.

What about curtains and blinds?

Internal window furnishings remain useful because they give occupants direct control over light and privacy.

Blockout blinds can dramatically darken a room. Sheer curtains can soften daylight and provide a degree of daytime privacy. Curtains can also contribute to insulation when closed.

However, an internal blind does not stop solar energy from reaching the glass in the first place. Some of that energy has already entered the building before it reaches the blind.

This does not mean homeowners need to choose between film and window furnishings.

The two can perform different jobs. Film can provide continuous solar and UV control while blinds or curtains provide adjustable privacy, light control and interior design.

When does replacing the windows make sense?

Window film cannot repair failed glazing.

If insulated glass units have lost their seals, frames are deteriorating, water is entering around the windows or the glazing needs significant structural work, replacement may be the appropriate solution.

Replacement also creates an opportunity to redesign the entire glazing system using high performance glass, improved frames and purpose designed shading.

But if the existing windows are otherwise in good condition, replacement can be an expensive way to solve a problem that is primarily related to solar exposure.

Retrofit options are therefore worth investigating before assuming that the glass needs to be removed.

The best solution may involve more than one approach

Melbourne's climate makes window performance unusually complicated.

A homeowner may want less heat during a hot February afternoon but still value winter sun in July. They may want daylight without glare, daytime privacy without permanently closing the curtains, and protection for their floors without changing the appearance of the house.

No single window treatment maximises all of these outcomes simultaneously.

External shading, window film, curtains, blinds, ventilation, air conditioning and high performance glazing each address different parts of the problem.

The useful starting point is therefore not asking which product is best. It is identifying what is happening at the particular windows causing the problem.

Orientation, glass type, room use, existing shading and the homeowner's priorities can then determine whether the answer is better shading, a retrofit treatment, new glazing or a combination of approaches.

For many homes, improving comfort does not necessarily require replacing perfectly serviceable windows. Sometimes the better approach is simply making the existing glazing work harder.