Which double glazed glass makeup actually suits a Cranbourne house
Straight answer for nine houses out of ten here: a 4mm soft coat Low-E outer pane, a 12mm argon filled cavity on a warm edge spacer, a 4mm clear inner pane. Whole of window U-value 2.0 W/m²K or better, SHGC 0.45 to 0.55 on south and east glass and 0.30 to 0.40 on west. Over a plain clear air filled unit that is about $240 to $500 per medium window, and it is the best value line on any glazing quote you will read this year.
Most people arguing about double glazing are arguing about frames. The frame matters, and we cover it in the uPVC versus aluminium versus timber comparison, but the glass makeup inside the sealed unit is where two quotes quietly diverge by $400 a window and where the disappointments come from. Two quotes can both say "double glazed, argon filled" and be $6,000 apart across a house, because one of them is a clear air unit on an aluminium spacer with a 6mm cavity and the other is what is described below. This page is the glass half of the decision, in the order you actually have to make it.
Glass makeup comparison for Cranbourne and the Casey growth corridor
Costs below are 2026 rates for a medium opening of roughly 1.2m wide by 1.5m high, supplied and installed in Cranbourne, Cranbourne East, Clyde North, Hampton Park or Narre Warren. The first figure is the installed price in a standard aluminium frame; the figure in brackets is what that glass makeup adds over the cheapest clear air filled unit. Life expectancy assumes Climate Zone 6: damp 3 to 6 degree winter mornings, 38 to 42 degree February afternoons, and clay ground that moves under the house all year.
| Glass option | Typical 2026 cost | Expected life here | Maintenance load | Warranty | Who it suits |
|---|---|---|---|---|---|
| 4/6/4 clear float, air fill, aluminium spacer | $880 - $1,150 (base) | Seal 10 to 15 years, edge condensation from year one | Wipe the spacer line most winter mornings | 5 to 10 year seal | Garages, sheds, a room you are about to demolish |
| 4/10/4 hard coat Low-E, air fill, aluminium spacer | $1,000 - $1,300 (+$120 to $180) | Seal 15 to 20 years | Low, occasional edge condensation | 10 year seal | Rentals and sheltered south elevations on a tight budget |
| 4/12/4 soft coat Low-E, argon, warm edge spacer | $1,150 - $1,600 (+$240 to $500) | Seal 20 to 25 years, coating lasts the life of the unit | Clean the glass and the drainage slots twice a year | 10 year seal, 10 year coating | The default recommendation for almost every room |
| 4/12/4 toned solar control Low-E, argon, warm edge | $1,220 - $1,700 (+$310 to $560) | Seal 20 to 25 years | Same as clear Low-E | 10 year seal, 10 year coating | West and north west glass, unshaded estate lots, big living windows |
| 6.38 laminated outer / 12 argon / 4 clear inner (acoustic) | $1,200 - $1,850 (+$280 to $580) | Seal 20 to 25 years, interlayer longer | Low | 10 year seal, delamination cover varies by maker | Bedrooms facing the Princes Highway, the rail corridor or a main road |
| Grade A safety glass where the standard requires it | +$90 - $220 per pane | Same as the base unit | Never cut, grind or drill it after manufacture | As per the base unit | Doors, side panels, low level glazing: not optional, it is code |
| Triple glazed 4/10/4/10/4 Low-E, argon | $2,100 - $3,300 (+$900 to $1,800) | Seal 20 to 25 years, heavier hardware wears faster | Hardware adjustment every few years on large sashes | 10 year seal | Passive House builds and severe noise cases only, not a general upgrade |
Indicative 2026 supplied and installed rates for the Casey and Cardinia area. The full rate card is on the pricing page. A firm number needs a measure.
What the numbers on the glass label actually mean
Every performance claim in a glazing quote should trace back to four numbers. If a salesperson cannot give you all four for the exact unit they are quoting, you are being sold a story rather than a window.
- U-value in W/m²K, lower is better. How fast heat escapes. The single glazed aluminium sliders that went into most of Cranbourne and Hampton Park before about 2010 sit near 6.2. A clear Low-E argon unit in a thermally broken frame is about 1.9. Here is the trap: the figure on a glass brochure is usually the centre of glass number, which ignores the spacer and the frame entirely. The number that matters is the whole of window U-value, and it is always the worse of the two. Ask which one you have been quoted.
- SHGC, a number between 0 and 1. The share of the sun’s heat that gets through. High is good in winter and bad in February. In Climate Zone 6 you want it high on the north and east where winter sun is free heating, and low on the west, where the afternoon sun on an unshaded Clyde North estate lot hits the living room glass at the hottest part of a 40 degree day.
- VLT, visible light transmittance. How much daylight comes through. Plain glass is around 80 per cent, clear Low-E 70 to 75, toned solar control 40 to 55. This is the number that decides whether a room feels dim, and nobody volunteers it.
- Rw and Rw+Ctr in decibels, higher is better. The acoustic rating. Rw is the laboratory number; Rw+Ctr is corrected for low frequency traffic noise and is the honest figure for a house near a highway or a freight line. A standard 4/12/4 unit is around 29 to 31 Rw. An asymmetric laminated build reaches the middle 30s.
The independent scheme that publishes whole of window U-value and SHGC for a specific frame and glass combination is the Window Energy Rating Scheme. Ask for the rated figure for the system being quoted, not a generic glass datasheet. Windows and external glazed doors also have to meet AS 2047 for structural, water and air performance; the sealed units themselves are manufactured and tested to AS/NZS 4666; and where the glass sits in the wall decides what Grade A safety glass is required under AS 1288. A compliant window carries a label saying so, and you are entitled to see it before it goes in the opening.
The Cranbourne conditions that decide your glass
This is not a generic Melbourne answer. Four local things push the specification around, and they push it in different directions depending on which street you are on.
- Winter mornings sit in the 3 to 6 degree band with high humidity. Cranbourne, Clyde and the flats out towards Devon Meadows fog up harder than the higher ground around Berwick and Endeavour Hills. Condensation forms on the coldest inside surface in the room, and in a badly specified double glazed window that surface is the aluminium spacer along the bottom edge. That is why the spacer, not the glass, decides whether you are still wiping windows down in July.
- New estate homes carry far more west facing glass than the old stock. A 1985 Hampton Park brick veneer has modest aluminium sliders. A 2015 Cranbourne East or Clyde North house has a 3.6m stacker plus two 2.4m windows on the living side, often under a 450mm to 900mm eave that gives no shade at all after 3pm. Those houses need the SHGC pulled down on the west elevation and left up on the north, so a builder standard clear unit on every opening is the wrong answer twice over.
- Traffic and rail noise is highly directional. The Princes Highway, the Western Port and South Gippsland highways and the Cranbourne rail corridor all generate low frequency noise that plain double glazing barely touches. You do not need acoustic glass through the whole house. You need it in the two or three rooms facing the source, which is a $600 to $1,700 decision rather than a $9,000 one. There is more on our acoustic glazing page.
- The ground moves. Most of this corridor is reactive clay and sites here commonly classify as moderately or highly reactive under the residential slab and footing standard. Houses swell through a wet winter and shrink in a dry February, and that racks window openings by a few millimetres a year. The glass does not care, but the sealed unit does: an IGU packed hard against a moving frame with no setting blocks and no room to expand is an IGU that fails its seal early. The install matters as much as the glass, which is the honest argument against a do it yourself retrofit.
Want the actual glass numbers for your own windows?
We measure every opening, note the orientation, and put the U-value, SHGC and price against each one. Free, and there is no sales presentation attached to it.
The cheap specification that fails early here, and why
The spec we get called out to fix is always the same one: a 4/6/4 clear float unit, air filled, on a plain aluminium spacer, in a frame with no thermal break. It is roughly $240 to $500 a window cheaper than the spec we recommend, so across fourteen openings it saves about $3,400 up front. Here is what it does with that money.
The 6mm cavity is too narrow. Below about 10mm the air in the gap starts convecting between the panes instead of sitting still, so a good share of the insulating benefit you paid for never arrives. Twelve millimetres is the sweet spot for argon; sixteen is fine with air but the returns fall away past that. Then the aluminium spacer conducts cold from the outer pane straight around to the inner one and creates a cold strip along the perimeter. On a foggy Cranbourne morning that strip is below the dew point when the middle of the glass is not, so you get a neat band of water along the bottom of a window you were told would stop condensation. Water sits in the reveal, the paint lifts, and within two or three winters there is black mould in the corner of a brand new window.
The seal is the last part of it. A sealed unit has a desiccant inside the spacer bar to soak up the moisture that inevitably migrates in over time. Once that desiccant is saturated the unit fogs internally, permanently, and it cannot be cleaned: the pane has to be replaced. A single seal unit on a cheap spacer, in a frame that moves with the clay, will typically reach that point in eight to twelve years. A dual seal unit on a warm edge spacer, properly blocked and drained, is still clear at twenty five. That is the entire argument, and the repair job at the end of it is on our fogged unit replacement page.
What "commercial grade" and the other sales words are worth
There is no Australian standard that defines the phrase commercial grade for a residential window, and no residential grading system a salesperson can point to when they use it. In a genuine commercial context it means the frame comes from a heavier extrusion series designed for higher wind pressures and taller spans. On a suburban house in Cranbourne it usually means the frame walls are somewhat thicker, which is a real if modest benefit, and it is being used to justify a price rather than describe performance. A plain translation of the rest of the vocabulary:
- "Energy saving glass" tells you nothing at all. Ask for the U-value and the SHGC of the finished window.
- "German engineered" usually refers to the hardware or the profile design, not where anything was made. Ask where the frame was extruded and where the sealed unit was assembled.
- "A grade glass" is not a thing. Grade A safety glass is a real term from AS 1288 and it means toughened or laminated glass in the positions where a person could fall against it. If a quote does not name it for doors and low level glazing, that is a genuine problem, not a detail.
- "Triple sealed" normally means three rubber gaskets in the frame, which is good, and has nothing whatever to do with the sealed glass unit. Two different things, one word.
- "Lifetime warranty" almost always means the lifetime of the product as the manufacturer defines it, not yours. Read what it actually covers, which we unpack on the warranty page.
Two documents are worth more than all of those phrases combined: the AS 2047 compliance label on the window itself, and a WERS rated U-value and SHGC for the exact frame and glass combination being supplied. Ask for both in writing before you pay a deposit.
Our recommendation for a typical Cranbourne house
For a four bedroom brick veneer on a standard estate lot, with a north or west facing living area and fourteen to sixteen openings, this is what we would put in our own house.
- South and east openings: 4/12/4 clear soft coat Low-E, argon, warm edge spacer, SHGC around 0.50 so you keep the winter light. Roughly $1,150 to $1,600 per medium opening installed.
- North openings under a real eave: the same clear Low-E unit. Do not tone north glass that is already shaded in summer, because you will be paying to lose free winter heat.
- West and north west openings: toned solar control Low-E, argon, warm edge, SHGC 0.30 to 0.40. Add about $70 to $110 per window over the clear unit. This is the single change that stops a living room reaching 32 degrees inside on a 40 degree day.
- The two bedrooms closest to the road or the rail line: asymmetric acoustic build, 6.38 laminated outer against a 4mm inner. Add $280 to $580 each, and only on those rooms.
- Doors, side panels and any glass below about 500mm from the floor: Grade A safety glass. Not a discussion, it is a requirement.
- Everywhere: a thermally broken frame or uPVC, a warm edge spacer, and a properly foamed and flashed perimeter. A perfect sealed unit in a frame that leaks air around the reveal is money burnt.
That specification across fourteen openings lands in the $19,000 to $27,000 range in 2026 for full replacement, or roughly $12,000 to $19,000 if the existing timber frames are sound and we can retrofit units into them, which is covered on the retrofit page. The staging options are set out on our budget page, and if you want to work out how much glass area you actually have before you ring anyone, start with the glass area estimator.
When we tell people not to upgrade the glass
We would rather quote honestly than sell a specification nobody needs, so here are the cases where the answer is no.
- Rotten or racked frames. If the timber is soft at the sill or the aluminium sashes no longer close square, glass is the wrong conversation. Fix or replace the frame first, otherwise you are fitting a 25 year sealed unit into a 3 year opening. The symptoms are listed on our signs your windows need replacing page.
- Rooms you do not heat. A laundry, a garage conversion used twice a week, a south facing hallway with one small window. Plain double glazing or nothing at all is the right call there. Put the money in the bedrooms.
- A house you are selling inside twelve months. Double glazing does not reliably return its cost at sale in this corridor. Sell first and glaze the next one.
- Toned glass on shaded south facing windows. Some quotes tone the whole house because it is simpler to order. It makes south rooms dim for no thermal gain at all. Push back on it.
- Triple glazing as a default. We will supply it, but in Climate Zone 6 that money almost always does more work somewhere else in the same house.
What we do not do at all: we are not a security screen or plantation shutter business, we do not manufacture heritage timber sashes in house, and we do not sell finance. If your job needs one of those we will say so and point you at the local trades list.
Six questions to ask whoever quotes you
- Is the U-value you have quoted centre of glass or whole of window, and can I see the WERS figure for this exact frame and glass?
- What is the SHGC on each elevation, and did you set it differently for the west facing glass?
- What spacer is in the unit: plain aluminium, stainless, or a polymer warm edge?
- Is it a single seal or a dual seal unit, what is the cavity width, and is the gas argon or plain air?
- Which openings are getting Grade A safety glass, and does the quote list them individually?
- Who manufactures the sealed units, and what happens to my warranty if that supplier stops trading?
Any glazier worth hiring answers those six in about four minutes. If the answers come back as adjectives instead of numbers, keep looking. When you are ready, the Low-E glass service page covers what we stock and the Cranbourne page covers how we work in your area.
Get a written glass specification and a fixed price per opening
Send this through and we will measure every window, note its orientation, and put the U-value, SHGC and price against each one, back to you within 7 days.