Upgrading to Double Glazing in an Existing Melbourne House

Can my existing windows be double glazed?

Almost always no, and the reason is physical rather than commercial.

A single-glazed sash is built to hold a pane 4 to 6mm thick. A double glazed unit in our residential systems is 18 to 24mm — three to five times thicker. The frame rebate has to physically accommodate the glass, with enough depth and enough edge cover to hold it, and a single-glazing rebate simply doesn't have the room. In our own awning systems the single-glazed and double-glazed sashes aren't even the same extrusion: they're different profiles with different glazing beads, because a bead only holds the thickness range it was made for.

So upgrading to double glazing means new sashes and frames, not new glass in the old ones. On your existing windows — whoever made them — the honest answer is that we'd need to see them, but expect the same physics to apply.

Is it worth replacing my single glazed windows?

On the numbers, this is the biggest single improvement available in an aluminium window.

Take our Comet awning as the like-for-like. Single glazed with 4mm clear it's certified at a whole-window U-value of 6.4, rating 3.8 heating stars and 1.1 cooling stars. The same window with a standard argon-filled double glazed unit comes in at 4.0 — 5.8 heating and 2.4 cooling stars. Add a good Low-E coating to that unit and it reaches about 3.0. So plain double glazing cuts heat loss through that window by roughly 37 per cent and more than doubles the cooling star rating.

One thing worth knowing before you commit to the whole house: energy targets are usually set per item, so it's frequently one or two windows that need the step up rather than all of them. We'd rather work through the schedule window by window.

Do you do secondary glazing or retrofit inserts?

No. We make windows, and everything we supply is a manufactured unit — we don't hold any retrofit, secondary or insert glazing product.

Worth understanding what secondary glazing actually is, though, because it's usually sold on noise. It's a second, separate window frame installed inside the existing one. The industry guidance is clear that ordinary double glazing with a standard cavity of around 12mm does not substantially improve acoustic performance — the gap is too small. To get a real acoustic benefit from two panes you need to separate them by at least 100mm, which is secondary glazing territory, not sealed-unit territory.

So if noise is the actual problem, don't assume a double glazed unit solves it — see the noise question below.

Why can't my sliding window take the same unit as my awning?

Because the sash is a different shape with a different pocket, and the unit thickness follows the pocket.

Our awning and casement sashes take double glazed units of 18, 20, 22 or 24mm. Our sliding window and sliding door sashes take an 18mm unit and stop there — a slider's sash has to run in a track and pass another panel, which constrains how deep the glazing pocket can be.

That difference shows up in the energy numbers. A thicker unit generally means a wider cavity and better insulation, which is part of why our double-glazed awnings and fixed lites reach lower U-values than our sliders — around 3.0 for a Comet awning against 3.2 for a Comet sliding window.

If a slider isn't reaching the target on your energy report, changing the window type is often more effective than chasing a better unit.

Will it stop the condensation on my windows?

It helps considerably, and it's the right direction, but nobody should promise you it stops.

Condensation forms when warm moist indoor air meets a surface cold enough to reach dew point. Raising the temperature of the inside face of the glass is what reduces it, and that's exactly what double glazing does — taking a typical Comet awning from a U-value of 6.4 to around 4.0, or to 3.0 with a Low-E unit. The remaining cold spot is the edge of the glass where the spacer sits, which is where a warm-edge spacer earns its place.

But the industry position, which matches ours, is that condensation is usually driven by humidity and poor ventilation in the room rather than by a fault in the window. Extraction fans and drying clothes outside still do their share. We'd be misleading you if we said otherwise.

Will new double glazed windows make the house quieter?

Less reliably than you'd expect, and our own test data is genuinely mixed.

Our Titan apartment door tested at Rw 29 double glazed against Rw 27 single glazed — a modest gain. But our Ultramax sliding window tested Rw 24 double glazed against Rw 26 single glazed, and our commercial 10044 awning tested Rw 29 double glazed against Rw 31 single glazed. In both of those the double glazed unit measured slightly worse.

The reason is that acoustic performance depends on the two panes having different thicknesses, and on the interlayer — not simply on there being two of them. A standard 12mm cavity is too narrow to do much acoustically.

If noise is your main reason for replacing windows, an acoustic laminate is the specification that moves the number. Tell us what you're trying to block out and we'll spec to that rather than to a general assumption.

Do I have to do the whole house at once?

No — and doing it selectively is often the smarter spend. Energy targets are usually set per item, so it's frequently one or two windows that miss rather than all of them. The rooms that reward the upgrade most are the ones you heat and cool hardest, and the elevations that face the weather. If you're chasing the lowest possible U-values on a few key windows, that's also where a thermally broken frame earns its place — selectively, on the openings that miss their target, not across the whole house by default.