Can you double glaze existing aluminium windows? Retrofit, secondary glazing or replace

"Can you double glaze windows you already have?" is one of the most common questions we get about aluminium windows, and the answers online disagree. One Melbourne uPVC seller says retrofit rarely suits standard aluminium frames. Retrofit suppliers say most sound aluminium joinery can take a sealed unit. Both are partly right, because the answer sits in your frames, not in the glass.

There are three routes: retrofit, secondary glazing and full replacement. Here is how each works, where each one stops, and what to check before you pay for any of them.

The three routes

Retrofit double glazing. The single pane comes out and a sealed double-glazed unit goes into the existing frame, usually with a new profile or adaptor to hold it. You keep the frame and the look of the house.

Secondary glazing. An independent second pane is fitted on the room side of the existing window. The original window stays as it is, which is why it is often suggested for heritage homes and rentals. It is a separate product from double glazing, with its own limits.

Replacement. The whole window comes out and a new frame and glass go in. It is the most work and usually the highest cost, and it is the only route that changes the frame.

What the sources say, and where they disagree

The leading Melbourne uPVC seller says: "Retrofit double glazing rarely suits standard residential aluminium frames," and it generally recommends full replacement (Windows Republic). It makes the fair point that "Aluminium conducts heat and cold through the frame itself", so a glass-only upgrade has a ceiling.

Retrofit suppliers see it differently. One says: "For the most part, aluminium windows and door frames can be retrofitted," with exceptions for frames that are too weak or lack the space (Retroglaze via ArchiPro). Another describes its product as compatible with most existing aluminium joinery, provided the joinery is inspected first and is in sound condition (Metro Performance Glass via EBOSS).

Two notes on those sources. Both retrofit suppliers are New Zealand products, and the second says installation must follow New Zealand requirements, so treat them as evidence of what the approach can do, not as a specification for a Melbourne job. And every one of these sources sells something. The useful part is where they agree: it depends on the frame.

A checklist for your frames

Before a retrofit is worth quoting, these are the things to look at.

1. Frame condition. Bent, corroded or leaking frames are better replaced. New glass in a tired frame keeps the problem.
2. Frame age and system. Some older joinery is too light. One supplier warns that joinery from the 1960s and 1970s may be too flimsy to hold the additional weight of genuine double-glazing units.
3. Rebate depth. A sealed unit is thicker than a single pane. If the frame has no room for it, there needs to be a profile or adaptor that solves it, or the answer is no.
4. Weight. A double-glazed unit is heavier. Sliding windows and doors need rollers, tracks and sashes that can carry it, so a worn roller is better dealt with at the same time. Our sliding door guide covers what that looks like.
5. Seals and drainage. Check that the frame's seals and weep paths are working before glass goes in.
6. Sliding doors. Retrofit suppliers say they can generally be upgraded with suitable adaptors, depending on the age and design of the joinery. Ask for that to be confirmed for your door, not assumed.

If your frames pass, retrofit is a sensible way to improve the glass without replacing the window. If they fail, the extra money for a new frame is better spent than on glass that will outlast the frame it sits in.

Secondary glazing: a fair look

Secondary glazing suits a few situations well: a single noisy or cold room, a rental where the window should not be altered, and a heritage home where the original window needs to stay. It is reversible and leaves the original window alone.

It also has limits. It does not repair a draughty frame or a failed seal, and with a standard aluminium frame the frame still conducts heat. For a room you use all day, compare it with a sealed unit or a new frame before you decide.

The Victorian Government's Victorian Energy Upgrades program lists a cheaper option that adds glass or acrylic to an existing single-glazed window to create an air gap, alongside double-glazed windows. Its indicative discounts are $210 for four large double-glazed windows and $140 for the air-gap option, and the final amount is set by your accredited provider (Victorian Energy Upgrades). Private renters are eligible. If you want to use it, ask whether your provider is accredited for window upgrades; not every listed provider offers them.

When replacement is the better answer

Replace the window when the frame is corroded, bent, leaking or too light, when you want a different style, or when the main goal is comfort and energy use. A standard aluminium frame conducts heat, so if heat loss is what bothers you, ask about a thermally broken frame for the rooms you use most.

For typical Melbourne price ranges for retrofit and new windows, and what moves them, read our double glazing cost guide. If noise is the main problem, the quieter and warmer windows guide explains why glass type is only part of the answer. If you already have double glazing and it has fogged up, see fog between double-glazed panes. And if your frames are timber sash rather than aluminium, repair or replace sash windows is the better place to start.

The short version

Whether old aluminium windows can be double glazed depends on the frame: its condition, age, depth and strength. Retrofit can work on sound frames, secondary glazing suits single rooms, rentals and heritage homes, and replacement is the answer when the frame is the problem. Get the frames inspected before you compare prices, and ask each quote to say which route it is.

Our double glazing and sliding doors page covers the options we work with and how we quote.