Double Glazing in Melbourne — What It Actually Improves

How much does double glazing actually improve a window?

Taking a like-for-like comparison in our Comet awning system, single glazed with 4mm clear gives a U-value of 6.4 and an SHGC of 0.64, rating 3.8 heating stars and 1.1 cooling stars. The same window with a standard 4mm–12mm argon–4mm clear double glazed unit gives a U-value of 4.0 and an SHGC of 0.56, rating 5.8 heating stars and 2.4 cooling stars.

So plain double glazing cuts heat loss through that window by roughly 37 per cent and doubles the cooling star rating. Adding a Low-E coating to the double-glazed unit takes it further again, down to about 3.0. That's the single biggest performance step available in a standard aluminium window.

Can I double glaze my existing windows?

Almost always no — a single-glazed sash holds 4 to 6mm of glass and a double glazed unit is 18 to 24mm, so an upgrade means new sashes and frames rather than new glass in the old ones. The full retrofit picture — what's physically possible, what it involves, and the secondary-glazing question — lives on our Upgrading to Double Glazing in an Existing House page.

Is Low-E glass worth it if I can't afford double glazing?

In our experience, yes — it's the most cost-effective single upgrade available. In our Comet awning system, going from 4mm clear single glazing to 6mm Low-E single glazing takes the U-value from 6.4 to 4.8 and lifts the rating from 3.8 heating stars and 1.1 cooling stars to 4.9 and 2.3.

That's most of the way to plain double glazing, which sits at 4.0 for the same window, in a single pane. A 6.38mm Low-E laminate performs the same at 4.8 and adds acoustic and safety benefits. It won't match a good Low-E double-glazed unit at 3.0, but if the budget is fixed, Low-E single glazing is where we'd put the money first.

What is Low-E glass?

Low-E stands for low emissivity. It's a near-invisible metallic coating applied to one surface of the glass that reflects radiant heat back where it came from — keeping warmth inside during winter and reflecting solar heat out during summer — while still letting light through.

It's the single biggest driver of a window's energy rating. In our Comet awning system, switching from 4mm clear to 6mm Low-E single glazing improves the U-value from 6.4 to 4.8. Low-E comes in different grades with genuinely different performance, and it also comes as a laminate, which combines the coating with acoustic and safety benefits in one pane.

What is argon gas in double glazing and does it help?

Argon is an inert gas used to fill the cavity between the two panes instead of air. It conducts heat less readily than air does, so it improves the insulating performance of the unit without changing how it looks.

It's effectively standard on the double-glazed units we supply — the large majority of the double-glazed options in our energy database are argon filled. We wouldn't specify a double-glazed unit without it for a Melbourne project. It's worth understanding that argon is a contributor rather than the main event: the Low-E coating and the cavity width do more of the work, with argon adding to both.

What's a spacer, and does a warm-edge spacer matter?

The spacer is the bar that separates the two panes of a double-glazed unit and sets the cavity width — commonly 10, 12 or 14mm. A standard spacer is aluminium, which conducts heat and creates a small cold bridge right at the edge of the glass. A warm-edge spacer replaces that with a less conductive material, which reduces the cold edge and, in our experience, helps most with condensation around the perimeter of the glass on cold mornings.

We'd recommend it where condensation is a concern. We're not going to put a number on the improvement, though — warm-edge units are only thinly represented in the certified energy data we hold, and we won't quote a figure we can't stand behind.

Will double glazing stop condensation?

It helps considerably but it isn't a guarantee. Condensation forms when warm moist indoor air meets a cold surface, so 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. If you’re weighing this up for an existing house, our upgrade guide covers the retrofit side.

The remaining cold spot is usually the edge of the glass, where the spacer sits, which is where a warm-edge spacer helps. But ventilation and indoor humidity matter just as much as the window. We'd be misleading you if we promised double glazing alone would eliminate condensation in a poorly ventilated room.

Does double glazing help with noise as well?

It can, but not as reliably as people expect, and the results in our own test data are 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 cases 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. If noise is your main concern, an acoustic laminate is usually the more effective specification, and we'd want to know what you're trying to block out.

Can double glazed units be re-cut or adjusted on site?

No. A double glazed unit is a sealed assembly — two panes bonded to a spacer with a sealed edge and usually an argon fill — and cutting it destroys the seal. Once it's made, the size is fixed.

This is worth knowing at design stage rather than on install day. With single glazing there's some scope to adjust on site if an opening isn't quite as documented; with double glazing there isn't, and a mistake means remanufacturing the unit. We measure carefully before ordering glass for exactly this reason, and we'd rather delay an order by a day to confirm a dimension than replace a sealed unit later.

Can I get obscure glass in a double glazed unit?

Yes. Obscure and privacy options are available within double-glazed units, including translucent and privacy laminates, and they're certified for energy performance the same way clear units are.

One thing worth planning for: a sealed double-glazed unit cannot be re-cut once it's made. With single glazing there's some flexibility on site; with a double-glazed unit the size and specification have to be right before it's manufactured. We also match obscure patterns exactly across a job rather than mixing them, since reeded, fluted, satin and translucent finishes look quite different side by side.