A builder rang me last month halfway through a kitchen‑table argument with his client. She wanted uPVC. He’d quoted aluminium. The architect had floated alu‑clad timber. “Just tell me which one’s best,” he said. I get that call a lot — and the honest answer annoys everyone, because “best” isn’t a material.
If you’re weighing up uPVC vs aluminium windows — or throwing timber and composite into the mix — the brochure will hand you a tidy ranking. Aluminium is strong and slim. uPVC insulates and is cheap to run. Timber is the warm, natural one. Composite, or alu‑clad timber, is timber on the inside with aluminium taking the weather. All of that is true. Almost none of it is the number a certifier signs off.
Start with what the frame actually does to heat. Aluminium is a brilliant conductor — roughly a thousand times more conductive than timber — so a bare aluminium frame behaves like a cold bridge: it hands heat straight out and drops the temperature of the inside face. A thermal break — a polyamide strip run through the frame — cuts that down a lot, which is why thermally broken aluminium exists; but every break is different, and not every break insulates as much as the name suggests — the thermal break that isn’t covers what the break often misses. uPVC, hollow and multi‑chambered, is a genuine insulator. Timber insulates naturally. Alu‑clad timber keeps the timber doing the thermal work while the aluminium takes the sun and rain — so it behaves much like timber, not like the aluminium you can see.
Strong, slim, durable — and the coldest inside face of the four. Without a thermal break it’s the frame most likely to carry condensation.
A polyamide break cuts the heat path sharply. Much warmer than bare aluminium, still not uPVC or timber — and the size of the gain depends on the system.
Multi‑chambered and genuinely insulating, with a warm inside face. Strong on condensation and running cost; performance still rides on the glass and size.
Timber does the insulating; on a composite the aluminium skin just takes the weather. About the warmest of the lot — but it’s a property, not a certificate.
So on raw material the ladder runs roughly: uPVC and timber warm, composite close behind, aluminium coolest unless it’s broken. But here’s the part the league table hides. A warm frame is a material property. Thermal performance is a result — and the result depends on the window that actually gets installed. Uw is area‑weighted, so the frame’s share of the number shifts with the real project size, and the glass and spacer you pair with that frame can move the figure as much as the frame choice itself. Two makers quoting “the same” uPVC window can land in different places. That’s the whole reason the window you specify isn’t the window that gets installed.
That gap used to be academic. It isn’t now. NCC 2025 was adopted in Victoria on 1 May 2026 (New South Wales and Queensland have deferred to 2027), and it tightened the two things frame choice touches most: condensation management has widened, and for commercial work the Section J energy budget has been roughly halved. Embodied carbon is moving from a talking point into Green Star / NABERS territory — where timber and uPVC each have a story, but only once it’s measured.
Where the frame choice really bites is window condensation compliance. A cold inside frame face is where mould starts, and under F8 / H4 that’s now a compliance question, not a winter callback — how windows grow mould under NCC covers the compliance mechanics. This is exactly where uPVC and timber pull ahead of bare aluminium — the warmer inside surface. But “warmer” isn’t evidence; the surface temperature is. You can’t read it off the frame material, and you can’t read it off a rating that tested a different size.
A warm frame is a material. Thermal performance is a result — so the only honest answer to “uPVC or aluminium?” is the window you actually install.
So when that builder asks me which frame is best, I don’t answer from the brochure. We model the actual window — uPVC, aluminium, thermally broken, timber or alu‑clad — to ISO 15099, at the real size, with the project’s glass and spacer. One model returns the Uw, the SHGC and an fRsi surface‑temperature screen for condensation, so you can see where each frame lands on the job in front of you — not a generic catalogue size pulled from a ratings database. More than a rating: the full picture of your window. It’s the same ISO 15099 method that sits behind WERS, run on the window you’re actually comparing.
Nine times out of ten the “best frame” argument dissolves once there’s a number on the table, because the real answer is “it depends on the window, the glass and the opening” — and now you can see it instead of arguing it. If you want the long version on any one material, we’ve written up how we work with uPVC, timber and aluminium companies.
Pick a frame. Send us one window.
Choose the material you’re leaning toward — uPVC, aluminium, timber or composite — and send us one window. We’ll model the actual system free and send the numbers back, so you can compare it against the one you were about to rule out.
Send us one window → Try BTW yourself →About AFRC Lab. AFRC Lab (Advanced Fenestration Research & Certification) is an independent, product‑neutral fenestration consultancy. It is not the AFRC and is not affiliated with WERS or the Australian Fenestration Rating Council. We provide verification data and modelling; where testing or certification is required it is delivered through our accredited specialist partners. Project‑specific thermal simulation to ISO 15099 / NFRC 100, with condensation assessed via the surface‑temperature ratio (fRsi) as a screening estimate; modelled results are indicative to the systems and sizes provided, and final NCC compliance is confirmed by your assessor or certifier. Comparisons between frame materials depend on the specific systems, glass and sizes modelled. State adoption of NCC 2025 varies — check the edition that applies to your project. BTW™ and TIRS™ are trademarks of AFRC Lab Pty Ltd (patent pending).