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If you’ve spent any time around builders, architects, or DIY enthusiasts, you’ve probably heard someone refer to insulation boards as “Celotex,” even when the boards in question aren’t technically Celotex at all. In many builders’ merchants and on job sites, the phrase “grab a sheet of Celotex” is used the same way someone might say “pass me a Hoover” when they simply mean a vacuum cleaner.
So why does this happen? Why is PIR insulation — a type of rigid foam board — so commonly called Celotex, even when it’s made by Kingspan, Recticel, Quinn, Xtratherm, or other manufacturers?
Let’s unpack the reasons behind this widely used but technically inaccurate shorthand.
Before diving into branding habits, it’s worth understanding what PIR actually is. PIR stands for polyisocyanurate, a high-performance rigid foam insulation material. PIR boards are known for:
Because PIR balances cost, ease of installation, and strong insulating properties, it has become one of the most widely used insulation types in modern construction.
Celotex is not a type of insulation — it’s a brand. The company, originally founded in the early 20th century, became particularly well-known in the UK for manufacturing PIR insulation boards. Over several decades, Celotex achieved massive market penetration. Its characteristic orange branding was hard to miss, and its products appeared almost everywhere from housing developments to loft conversions.
This widespread presence created brand dominance: Celotex became the most recognisable name in the PIR insulation category. When builders grabbed a sheet of PIR, chances were good that the branding said Celotex. Over time, the brand name started to stick in people’s vocabulary.
This phenomenon isn’t unique. It’s the same reason we say:
In linguistic terms, this is called genericisation — when a brand name becomes synonymous with a whole product category.
Several factors contributed to Celotex becoming the term people used instead of PIR:
For years, Celotex had prominence in builders’ merchants and construction supply chains. Their packaging and marketing were instantly recognisable. The more often builders saw the name, the more natural it became to use it as shorthand.
While most PIR boards are similar, Celotex offered a highly consistent range — GA, TB, and XR series — which made specifying and sourcing their products straightforward. Builders like simple, identifiable terms, and Celotex provided that.
It may sound trivial, but “Celotex” is simply easier and faster to say on site than “polyisocyanurate board” or even “PIR board.”
Tradespeople often learn terminology from whoever trained them. Once a term is adopted across generations of builders, it becomes the norm — even when the market changes.
Over time, competitors like Kingspan, Recticel (now Eurosolv), Xtratherm, and Quinn became equally dominant. Some even matched or exceeded Celotex in market share. Yet the habit of calling all PIR insulation “Celotex” remained.
Why? Because once a generic name catches on, it rarely fades. Even if another brand takes the lead, the original becomes part of the construction vocabulary.
Celotex also continues to be available, so the name never disappeared from the marketplace. And while Kingspan is also a major player, its brand name hasn’t become generic in the same way. People tend to refer to Kingspan as “Kingspan,” but PIR insulation as a category is still often called “Celotex.”
Technically, yes — but practically, not always.
When it’s fine:
Builders usually understand what someone means when they say “Celotex.”
When it’s a problem:
In these contexts, precision is important. Saying “Celotex” when you actually mean “PIR” could cause confusion, cost discrepancies, or incorrect product installation.
To avoid misunderstandings, a good rule of thumb is:
Use “PIR” when talking about the type of insulation.
Use “Celotex” only when referring to the actual brand.
If you’re ordering, specifying, or installing insulation, it’s worth checking the exact product. Many projects are flexible with brands, but not all.
The reason PIR insulation is often called Celotex is simple: Celotex became so widespread and recognisable that its name turned into the default reference for the entire category. It’s a classic case of brand genericisation, shaped by industry habits, market dominance, and everyday convenience.
While the term “Celotex” is deeply ingrained in the construction world, being aware of the distinction helps ensure clarity — especially in formal or technical settings. Whether you’re a seasoned tradesperson or a homeowner planning a renovation, understanding the difference between PIR and Celotex can help you make better, more informed decisions.
For those interested in exploring alternatives to Celotex-brand PIR (or PIR in general), see our Alternatives to Celotex page for other high-performance insulation options.
