Thermal Break Aluminium Windows: PA66 vs PVC, and How to Check

Two aluminium windows can look identical, carry the same profile series name on the paperwork, and still differ by five to ten percent in what they cost. The difference is a part of the window you will never see, cannot verify, and cannot replace once it is built.

It is called the thermal break — the strip that separates the inside face of the aluminium profile from the outside face. A good PA66 strip costs more per tonne than the aluminium wrapped around it. Which means there is a quiet, real incentive to cheapen it: of everything on a window, this is where a saving is hardest to see.

Get it wrong and you do not find out today. You find out in year eight, on a wall you cannot open up.

What is a thermal break in a window — and why this part is different

A thermal break is the insulated connection between the two halves of an aluminium profile. Its job sounds simple, but it is doing two things at once.

The first is insulation. Aluminium conducts heat extremely well, so without a break between the inside and outside faces, an aluminium window is a radiator with a frame around it.

The second is structure, and this is the half that gets forgotten. The thermal break is not a spacer. It is the structural connection between the inside and outside of the profile.

The two aluminium halves are joined through that strip. It carries the wind load. It carries the weight of the sash. On a large sliding door, it carries the weight of the glass. If the strip fails, the window does not simply leak a little more than before. The frame loses its integrity.

And you cannot see what it is made of, you cannot repair it, and you cannot replace it once the unit is assembled.

Polyamide thermal break strip: the three ways "PA66" is not PA66

First, the short version of the two materials. PA66 is nylon — in window profiles almost always PA66 GF25, polyamide with 25% glass fibre, which is where the strip gets its strength. PVC is cheaper, softer, loses stiffness as it gets hot, and goes brittle as it gets cold. In a mild climate, in a small opening, the difference takes years to show. In a large unit, in a cold climate, under snow and wind load, it does not.

Now the three ways the word on your quotation fails to describe the thing in your window.

One: the word is copied, not specified. The sales person writes "PA66" because PA66 is the good answer. It is the answer that closes the question. Whether the profile actually contains it was never checked by anyone.

Two: it is PA66 by name, not by grade. There is a large difference between a certified PA66 GF25 strip from a recognised maker and an unbranded strip that contains some polyamide and some filler. Both can be described as PA66 in a sentence. They do not behave the same way in year eight.

Three: it was never the window factory's decision at all.

That last one changes what you should ask.

Who actually decides your thermal break (it is not the window factory)

The thermal break is specified by the profile system, not by the window factory.

A window fabricator buys extruded profiles from a profile brand. That brand designs the profile — including the thermal break, its material, its geometry and its wall thickness — and publishes a specification. The fabricator cuts, welds and assembles. They usually do not choose the strip.

So the question "is my thermal break PA66" has an answer that does not depend on trusting your supplier at all. It depends on knowing which profile series you were quoted, and then reading that profile brand's own data.

It also explains something you may have noticed. When a supplier cannot answer a question about the thermal break, it is often not because they are hiding something. It is because the person you are talking to genuinely does not know. They sell windows. They did not design the profile.

So the test is not whether they say "PA66". The test is whether they can tell you the profile series, and where the strip came from.

What to ask before you pay — four questions, ten minutes

If you read nothing else on this page: get the profile series. Everything below explains why that one answer is worth more than any claim about quality.

Four questions. The answers are short when the supplier knows them, and evasive when they do not.

1. Which profile series is quoted, and who makes the profile? A real answer is a brand and a series number — and from that, the profile's own documentation can be checked independently of anything you are told.

2. What is the thermal break material, and what is the glass-fibre content? "PA66 GF25" is a complete answer. "PA66" alone is a partial one. "Good quality" is not an answer.

3. Who makes the strip? Serious strip comes from a small number of specialised makers. A name is checkable.

4. Can I see a cross-section? A cut profile shows you whether a thermal break is present, how it is formed, and how thick it is. It does not tell you the material — but it tells you whether the profile is built the way the specification says. While you are asking, ask whether there is a test report for the assembled profile, not just a material certificate.

The check that costs nothing: look at the mark

Thermal break strip usually carries a mark — the name of the company that made it. A serious strip maker puts its own brand on what it produces, because the strip is the thing its reputation rests on.

That mark is the simple version of everything above. If your strip carries no name at all, or carries the window maker's own name instead of a strip maker's, that is worth a question.

Here are the names, so you can compare them with what is printed on your own strip.

Every name is given in its Latin form and in the Chinese characters, so you can match either against what is printed on your strip. This is not a ranking and not a recommendation.

One thing we noticed when we wrote our own factory visit checklist. The thermal break is not something a factory volunteers. On the sheet we take into a workshop, the brand and grade of the strip is a starred item — meaning it has to be asked for and written down — because it does not appear in the documents that get handed to a buyer at all. It was not in ours either, until we went looking for it.

There is also a habit in the Chinese aluminium window market that you will not read about in a buying guide: fabricators often put their own logo on the strip, or have strip made under their own name.

Ask about it and you will be told it is another brand's custom specification, produced for them. Sometimes that is exactly what it is.

But compare the two cases. When a window maker uses strip from a recognised strip brand, the strip is usually left as it comes — the original mark stays on it. The window maker's name goes on the frame, not on the strip.

So if strip from a recognised brand is left alone, why is the mark replaced on some strips and not on others? The explanation I have been given most often is that the fabricator would rather you did not work backwards from the strip brand and series to what the window cost them. There may be other reasons, and it is not my place to guess at a particular supplier's motives.

If you look at the strip and it carries the window maker's name instead of the strip maker's, you have not learned that the strip is bad. You have learned that one route to checking it independently has been closed.

Two possibilities, and only one of them is checkable

If your windows are custom-made in China — and Foshan, where we are based, is one of the biggest concentrations of window and profile production anywhere — the strip inside them will be one of two things.

One: strip carrying the mark of the maker who produced it — a name from the list above, or a maker of comparable standing, printed on the strip as it left their factory.

Two: strip carrying the window manufacturer's own logo.

Three of the strip makers whose marks you will find on polyamide thermal break strip: Technoform, insulbar by Ensinger, and ROHIL (融海). Marks shown for identification only; no affiliation or endorsement is implied.

A strip carrying the fabricator's own mark looks different in one obvious way: the name on it belongs to the company that made your window, not to a company that makes strip. You will not find that name on any strip maker's list, because it is not a strip maker.

Those are the two normal cases. There is no long tail of other possibilities, and you are not being asked to evaluate an unknown material. You are being asked which of the two you are holding.

The burn test, and why the usual advice fails

Search for how to tell PA66 from PVC and you will find the same answer everywhere: burn a piece with a lighter.

PA66 is a polyamide, so it smells like burnt hair or burnt wool, and it tends to self-extinguish and form a bead. PVC contains chlorine, so it gives off an acrid smell, a greenish flame and black smoke.

The chemistry is correct. The advice is still close to useless, and not for the reason you might think.

Ask yourself where you are going to do it.

At the factory, in front of the people selling to you, before you place the order? Or at home, after the container has arrived, on a window you have already paid for?

And then ask who accepts the result. A burn test you performed yourself, in your own yard, is not evidence. It does not go into a file. It does not settle a claim. If you read it wrong, you are wrong with no recourse.

Burning is a real method. It belongs in one place: a third-party laboratory, run as a proper test on the assembled profile, reported by an organisation that is neither you nor the supplier. That is the only version of this test that produces a document you can hold someone to.

Which leaves the practical question: what do you do before you are at that stage?

You read the profile series. You ask who makes the strip. And you look at the mark.

If the container is already on the water

You are not helpless, but be realistic about what you can still establish.

A sliver of scrap profile, burned outdoors away from anything flammable, will tell you which family of plastic you are looking at. That is worth knowing, and it is worth little more than that: it will not tell you the glass-fibre content, the grade, or who made it, and it will not convince anyone but you.

What you can do properly is put the question in writing to your supplier, before the balance payment: which profile series, which strip maker, what glass-fibre content, and can it be confirmed in the profile documentation.

A supplier who has the documentation will send it. A supplier who does not will change the subject. Either response is information.

The end of the story

A buyer wrote to me with a question I get often, in one form or another: "What is the thermal break in my windows made of?" His quotation said PA66. His windows were already on the water.

So I asked him three things in return. Which company makes the strip? What is the glass-fibre content? What is the wall thickness of the strip itself? He took those questions back to the person who sold him the windows. That person could not answer any of the three.

He did get his answer, in the end — from the profile brand's own documentation, rather than from the people who sold him the windows. The material was what the quotation said. He had not been cheated.

But he spent two weeks unable to confirm it, on an order already on the water. That is the real cost of a supplier who repeats a word without knowing what stands behind it.

The thermal break is the one part of a window that is permanent, structural and invisible. It is worth ten minutes of asking before you pay — and it is a very expensive thing to become curious about afterwards.

We do not sell windows and we take no commission from any factory. If you want your profile specification read before you commit, that is the work we do.

Next in this series: Thermal break continuity — where the break is lost in an aluminium window frame — what a sawn section shows that a test report cannot.

Series: Spotting a Real Factory

• Part 1: The WeChat Test — remote checks, and why the sequence matters
• Part 2 (this article): PA66 vs PVC — and how to check
• Part 3: Thermal break continuity — where the break is lost in a frame