uPVC vs aluminium vs timber - what to actually buy.
The frame matters more than most people are told. You can put a superb argon Low-E unit into a frame that throws away half its benefit, and plenty of houses in this corridor have exactly that. Here is what each material really does, what it costs in 2026, and why the answer changes street by street rather than being settled by a league table.
The most important distinction is not between materials at all.
Before uPVC versus aluminium versus timber, there is a bigger split: thermally broken, or not. A plain aluminium frame is a single continuous strip of metal joining the cold outside air to your warm living room. Aluminium conducts heat roughly a thousand times better than the still argon inside your IGU. So the frame becomes a bypass around the insulation you just paid for - you have built a beautiful wall and left a metal pipe through it.
A thermally-broken frame cuts that metal in half and rejoins it with a polyamide strut, so the inner and outer skins never touch. From the footpath the two look identical. In performance they are different products entirely: roughly 6.0 W/m2K for plain aluminium against 2.2-3.0 for thermally broken.
This is why we so often quote full frame replacement on 1980s-2000s aluminium housing rather than reglazing it. Dropping a $900 IGU into a plain aluminium frame is fitting a good door to a tent. It is also the number one thing to check on any cheap quote you have been handed - the word "aluminium" alone tells you nothing.
The three materials, honestly.
- uPVC - about 1.3-1.8 W/m2K. The thermal winner, because it is a poor conductor by nature and the profiles are hollow multi-chamber sections full of still air. Chunkier sightlines, because it needs more material to carry the same load, and very large sliders get awkward. Life 25-35 years. Insist on Australian-market profile: grey-import European uPVC is formulated for milder sun and it chalks here.
- Thermally-broken aluminium - about 2.2-3.0 W/m2K. Slimmest sightlines, strongest by far, and the only sensible answer for big stacker sliders and wide openings. Life 30-40 years, powder coat may chalk on a west wall from year 15 and can be recoated. The pragmatic default across most of this corridor.
- Timber - about 1.6-2.2 W/m2K. Better than any aluminium, close to uPVC, and the only material with no expiry date at all - repaint it and it simply continues. The cost is that repaint, every 8-12 years by elevation. Unbeatable when you already own sound sashes.
- Plain aluminium - about 6.0 W/m2K. Not a double-glazing frame. If a quote does not say "thermally broken", assume it is this.
Three houses, three different right answers.
House one: 1994 brick veneer, Cranbourne North, 16 openings, mostly standard sizes, owner staying twenty years and cares mainly about winter. uPVC. Per the ranges on our pricing guide, roughly $1,800-$2,800 per medium opening puts the job near $24,000-$32,000, less a VEU rebate around $3,600. The sightlines are a bit heavier than the originals and in a 1994 house nobody notices or cares. What they get is the best U-value per dollar available and a house that holds heat like a different building.
House two: 2008 render-look place in Berwick with a 4.2-metre rear stacker and floor-to-ceiling glass. Thermally-broken aluminium, and it is not close. uPVC at that span needs so much material the sightlines become clumsy and the panel weight gets difficult. Aluminium carries the load on a slim section, keeps the look the house was designed around, and at $2,400-$3,800 for the large openings still lands the whole house near $28,000-$38,000. They give up perhaps 0.7 W/m2K against uPVC and gain a room that still looks right. The door detail is on our double-glazed doors page.
House three: 1918 weatherboard cottage in Old Berwick, twelve original timber sashes, heritage overlay on the street. Timber, by keeping the timber they already have. We machine new beads and drop a slim IGU into the existing rebate at $880-$1,400 per opening - roughly $14,000-$18,000 for the house. No frames bought, no permit needed because the external appearance is unchanged, the 108-year-old sashes stay, and thermally it lands between the other two houses. This is the approach on our retrofit existing frames page and it is the single most under-sold option in the industry, because there is no frame in it for anyone to sell you. More on the split between old and new stock here on our Berwick page.
What the standards require, whatever you pick.
Frame material does not change compliance. Every new window unit must be AS 2047 tested and wind-rated for the site, which across suburban South East Melbourne is typically N2, or N3 for elevated ground around Old Berwick, Harkaway and hilltop Endeavour Hills under AS 4055. Nor does the frame material change the permit position: swapping like-for-like into an existing opening needs no building permit anywhere in the City of Casey or the Cardinia Shire, but widening an opening is structural and does, and a heritage overlay - the Old Berwick streets among them - brings a City of Casey planning permit into play regardless of what the frame is made of. Every IGU should be AS 4666 compliant, which is what makes the 10-year seal warranty mean something. And AS 1288 mandates Grade A safety glass - toughened or laminated - in any door, sidelight, window with a sill under 500mm, or wet area, regardless of frame. The full reference list is on our local trades and standards page.
One last thing worth knowing. If noise rather than warmth is your driver - and near the Monash, the Princes Highway or the flight path it often is - frame material matters far less than glass build and air sealing. An asymmetric laminated IGU in a well-sealed aluminium frame beats a symmetrical unit in uPVC every time. That is a different conversation and it is on our acoustic glazing page. And whichever frame you choose, what determines whether it is still clear in 25 years is drainage, not material - maintenance covers that in twenty minutes of reading.
Frame material questions.
Which window frame is best for double glazing - uPVC, aluminium or timber?
On thermal performance alone, uPVC wins, timber is close behind, thermally-broken aluminium is third and plain aluminium is not in the conversation. But best on paper is not best for your house. uPVC is the right answer for a cold-priority whole-house retrofit where you do not mind the chunkier sightlines. Thermally-broken aluminium is right for big openings and slim modern sightlines. Timber is right when you already have sound timber sashes worth keeping. The frame follows the house, not a league table.
Is uPVC actually better than aluminium for Melbourne conditions?
Thermally, clearly. A uPVC frame runs around 1.3-1.8 W/m2K against roughly 2.2-3.0 for thermally-broken aluminium and about 6.0 for plain aluminium, because uPVC is a poor conductor by nature and its profiles are hollow multi-chamber sections. The trade-offs are real though. uPVC needs more material to carry the same load, so sightlines are chunkier and very large sliders get difficult. Buy Australian-market profile that is UV-stabilised for our sun - grey-import European profile is formulated for a milder climate and it chalks.
Why is plain aluminium so bad if it looks the same as the thermally-broken version?
Because a plain aluminium frame is one continuous strip of metal running from the cold outside to the warm inside, and aluminium conducts heat about a thousand times better than the still air in your IGU cavity. The frame becomes a highway around the insulation you just paid for. A thermally-broken frame cuts the metal and bridges it with a polyamide strut, so the outside and inside halves are no longer touching. From two metres away they look identical. In performance they are not remotely the same product.
Is timber worth it, or is it just for heritage homes?
Timber is genuinely excellent thermally - around 1.6-2.2 W/m2K, better than any aluminium - and it is the only frame material with no expiry date, because you can keep repairing and repainting it indefinitely. The cost is maintenance: a repaint every 8-12 years depending on elevation. Where timber makes overwhelming sense is when you already have it. If your Old Berwick cottage has sound original sashes, dropping a slim IGU into the existing rebate keeps the character, avoids heritage permit issues and costs $400-$700 less per window than replacement.
What does each frame material cost in South East Melbourne in 2026?
Per medium opening, installed: thermally-broken double-glazed aluminium $1,500-$2,400, uPVC double-glazed $1,800-$2,800, timber double-glazed $1,800-$3,800 depending on species and profile complexity. Retrofitting an IGU into an existing sound timber sash is the cheapest path at $880-$1,400 because you are not buying a frame at all. Across a 16-opening house that spread is roughly $5,000-$8,000 between the cheapest and dearest route, which is real money but rarely the deciding factor once you have seen the U-values.
Where we measure.
Let the house choose the frame.
We measure first and recommend after - including when the honest answer is to keep the frames you already own. Free in-home measure across South East Melbourne.