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celotex vs rockwool

PIR vs Mineral Wool: Which Insulation for Your Extension?

PIR or mineral wool? Compare thermal performance, thickness, fire, sound and moisture, plus Celotex vs Rockwool and what suits a rear extension.

The Extension Works 11 November 2026
PIR vs Mineral Wool: Which Insulation for Your Extension?

PIR vs Mineral Wool: Which Insulation for Your Extension?

PIR board and mineral wool in an extension

PIR board gives you the most insulation per millimetre, so it suits walls, floors and roofs where depth is tight. Mineral wool is cheaper, non-combustible, vapour-open and far better at absorbing sound, so it suits cavities, studs, lofts and anywhere noise or fire matters. Neither is “best” in general. The right one depends on the building element, the depth you have and how the construction handles moisture.

This guide compares the two on the things that decide a real project, answers the Celotex vs Rockwool question, and shows how the choice plays out in a single-storey rear extension in England.

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TL;DR

  • PIR has a lower thermal conductivity (around 0.022 W/mK) than mineral wool (typically 0.032 to 0.044 W/mK), so it hits the same U-value at roughly two-thirds of the thickness.
  • Mineral wool wins on sound absorption, fire behaviour (Euroclass A1 for stone wool) and vapour openness. It is also the cheaper material.
  • Part L sets a U-value target, not a thickness. For a new extension wall the target is 0.18 W/m²K, and either material can reach it if the wall is designed around it.
  • Fire rating, moisture and acoustics depend on the whole build-up, not just the board or roll. Always work from the exact product data sheet.

Table of Contents

What actually separates PIR from mineral wool

PIR (polyisocyanurate) is a rigid foam core, usually faced with foil. Mineral wool is a fibrous material made from stone (often sold as Rockwool-type slabs) or glass, supplied as rolls, batts or semi-rigid slabs. Because one is closed-cell foam and the other an open fibre mat, they behave differently in five ways.

Property PIR board Mineral wool
Thermal conductivity Around 0.022 W/mK Typically 0.032 to 0.044 W/mK
Thickness for the same U-value Thinner Thicker
Fire (reaction to fire) Product-specific, often Euroclass F Stone wool Euroclass A1
Sound absorption Limited Very good
Vapour behaviour Highly vapour-resistant Vapour-open (mu-value about 1)
Cost Higher Lower

The thermal and acoustic figures come from a UK homeowner comparison. The sections below take each row in turn.

Thermal performance and thickness

Lower conductivity means more resistance to heat flow at the same thickness. A Celotex GA4000 data sheet listing declares 0.022 W/mK for boards from 50mm to 100mm. A stone wool slab such as Rockwool RW3 declares 0.034 W/mK. That gap is why PIR is the usual answer when you are short of depth.

In practice for an extension wall, one building guide puts a 0.18 W/m²K wall at about 150mm of full-fill mineral wool, or 75 to 100mm of PIR with a 50mm residual cavity. Those are typical build-ups rather than a design, since your actual wall, ties and finishes change the result. We cover the targets themselves in our guide to extension U-values.

Thermal conductivity and typical wall thickness comparison

Fire behaviour

This is where the two differ most on paper. Stone wool products such as RainScreen Duo Slab are rated Euroclass A1, which means non-combustible. The Celotex GA4000 listing gives Euroclass F to BS EN 13501-1 and notes that the board is designed to sit behind a lining or covering, not be left exposed.

That does not make PIR unsafe in a house. It means PIR must be installed as the manufacturer specifies, behind plasterboard or another protective board. A guide to PIR vs mineral wool makes the key point: reaction to fire belongs to the exact product, while fire resistance belongs to the complete tested wall, roof or floor.

Height and boundaries change the rules. In England, Approved Document B recommends A1 or A2-s1,d0 materials through the external walls of residential buildings above 11m. A single-storey rear extension sits far below that, but if your wall is close to a boundary, your building control officer may look harder at the material, and A1-rated wool is often the simple answer there.

Fire behaviour

Sound insulation

Mineral wool’s fibrous, open structure absorbs sound well. PIR gives very little acoustic benefit. A UK supplier guide notes that Rockwool-type products are specified for party walls, floors, partitions and fire-stopping, and another says PIR is not normally the first choice when sound is the main requirement.

For most rear extensions this matters in two places: a wall shared with a neighbour, and any new floor or partition between living spaces. If you want a worked example, see our note on 70 to 100mm stone wool and Part E acoustic insulation.

Moisture and vapour

Foil-faced PIR is effectively vapour-proof for practical purposes. Other facings typically give a mu-value of 30 to 50, while mineral wool sits at about 1, according to the same supplier guide. The Celotex GA4000 data sheet states a water vapour resistance above 150 MNs/g.

Two practical consequences follow:

  • Seal PIR joints carefully and put the vapour control layer on the warm side so moisture does not condense inside the build-up.
  • Mineral wool lets moisture pass through, which suits breathable constructions, but it must be kept dry and protected from driving rain by a sound outer leaf or a cladding system with a ventilated cavity.

If you already have damp patches or salt staining on an existing wall you plan to extend from, find the cause before you insulate. This piece on exterior wall moisture problems explains why diagnosing moisture comes first.

Celotex vs Rockwool: the brand question

People searching for Celotex vs Rockwool are usually comparing a PIR board against a stone wool slab. Celotex is a PIR brand, and Rockwool is a stone wool brand, so the comparison is the same as PIR vs mineral wool, with product-level numbers.

Product Type Declared conductivity Reaction to fire
Celotex GA4000 PIR, foil-faced both sides 0.022 W/mK (50 to 100mm) Euroclass F
Rockwool RW3 Stone wool slab 0.034 W/mK Euroclass A1
Rockwool RainScreen Duo Slab Stone wool slab 0.034 W/mK (under 90mm), 0.035 W/mK (90mm and over) Euroclass A1

Sources: the GA4000 product datasheet describes the foil-faced PIR core, with conductivity and fire class from a stockist listing; the RW3 page and the RainScreen Duo Slab data sheet cover the stone wool products.

One warning from the PIR vs mineral wool guide: do not treat “mineral wool” as one specification. Products vary in density, stiffness and purpose, so use the exact data sheet and Declaration of Performance for what is going into your wall. The same goes for PIR, where facings and formulation change the numbers.

On installation, rigid PIR boards cut cleanly, fix quickly and give a flat surface, which is why they are common between studs, under floors and on flat roofs. Slabs and batts of mineral wool friction-fit between studs or sit in a cavity. Mineral wool is easy to fit but needs a bit more care to avoid gaps, because gaps are where the performance is lost.

Which one suits which part of the house

A renovation guide sums up the usual UK split: mineral wool is the go-to for many domestic jobs because of its low cost, ease of installation, fire resistance and acoustic performance, while PIR suits limited-depth cases that need high thermal performance per millimetre. For a rear extension, here is how that looks element by element.

Cavity walls

Masonry extensions with a cavity can take either full-fill mineral wool or partial-fill PIR with a retained cavity. Full-fill wool is simpler and cheaper. Partial-fill PIR keeps the wall slimmer, which can matter when you are close to a boundary or trying to protect internal floor area. Our comparison of masonry vs timber frame for a rear extension covers how the wall type affects what goes inside it.

Timber frame walls

Between the studs, mineral wool is a natural fit because it fills the cavity snugly. Many builds add a layer of PIR across the inside or outside to cut thermal bridging through the timber and to bring the wall to its U-value without making it very thick.

Flat roofs

Flat roofs need vapour-tight build-ups, so PIR is the standard choice, laid in a warm roof arrangement with a suitable membrane. One comparison summarises the logic by depth and moisture profile: flat roofs need vapour-tight systems, solid-wall interiors need breathable ones.

Floors

Under a concrete slab or screed, rigid PIR is typical because it resists compression and keeps floor build-up shallow. Where a floor separates rooms and you want less noise, mineral wool in the void does more for sound.

Lofts and pitched roofs

For an existing loft, mineral wool rolls are the cheapest option, and Part L calls for 270mm of mineral wool or equivalent thermal performance for new loft insulation. On a typical 4m by 6m loft area, installed costs are quoted at £600 to £900 for mineral wool against £1,400 to £1,800 for PIR. Those are 2026 guide figures and vary by contractor and access.

Quick rule of thumb by depth

The same OMEGA guide suggests choosing by available depth: 100 to 200mm points to PIR or a hybrid, and 200mm or more points to mineral wool. Treat that as a starting filter, then check moisture, fire and acoustic needs.

What Part L asks of an extension

The building regulations do not name a material. Approved Document L sets a U-value, 0.18 W/m²K for a new wall in an extension, and the thickness follows from the product you choose. That is why the PIR vs mineral wool decision is really a decision about depth, cost, acoustics and moisture, with the U-value as the pass mark either way.

For the full set of figures, including roof and floor targets, read our guide to extension U-values. If you are weighing running costs too, why an energy efficient rear extension keeps bills down is worth a few minutes.

Insulation behind timber cladding: cedar vs larch

Many rear extensions finish with timber cladding over insulated walls, so the insulation question often arrives with a cladding question. A few points tie them together.

The Timber Cladding Handbook from the TDCA describes cladding as sitting in the wet zone of the wall, fixed over a drained and ventilated cavity with vents at top and bottom, separated from the dry zone by a breather layer. The same handbook says insulation behind the cladding should be either rigid and fixed to the backing wall to carry the battens, or carried within a framework to prevent thermal bridging, and that thickness should be no more than 100mm unless the support is designed for lateral loads. Rigid boards and stone wool slabs both fit that picture, which is why the ventilated-cavity systems mentioned earlier often use A1-rated stone wool.

On the species themselves:

In short, cedar is the usual pick for a consistent, warm tone with less preservative worry, while larch is a sturdy, often more affordable option that rewards regular treatment. Either can sit over PIR or mineral wool, provided the cavity behind is ventilated and the fixing detail is right.

Perspective

On the extensions we design and build, we do not choose insulation by habit. We choose it by element. Our single-storey rear extensions are designed in-house, so the same team that draws the wall also specifies what goes in it and checks it against the U-value targets in the regulations.

That matters because the PIR vs mineral wool debate is rarely about which material is better. It is about where each one earns its place. Stone wool in a wall that shares a boundary or needs good acoustics, rigid board where we want a slim build-up or a vapour-tight flat roof. Our live 3D model lets you see the effect of a thicker wall on your floor area before you commit, and our fixed VAT-inclusive price means the specification is settled up front, not renegotiated on site.

Our advice to any homeowner: ask for the build-up to be written down, with each layer, thickness and declared conductivity, and make sure someone has checked it against the regulations. Our approach is built to do exactly that, from first sketch to final sign-off.

Esskay

Getting the insulation right in your own extension

If you are planning a single-storey rear extension in Sutton, Morden or the wider South East, the insulation decision is one small part of a design that also has to cover planning, structure and building control. We handle all of it in-house: design, planning routes including Permitted Development and prior approval, architectural plans, structural calculations, project management and construction, with a fixed VAT-inclusive price and a 12-month workmanship warranty.

The Extension Works

You can price and visualise your extension online with our instant quote tool, then secure your slot with the £499 booking fee described on the how it works page. Our home page explains the full service, and our guide to structural calculations for extensions shows what sits behind the design.

Checklist before you decide

  1. Confirm the target. For a new extension wall, the figure to meet is 0.18 W/m²K.
  2. Measure the depth you can spare. Tight on space points to PIR, generous depth points to mineral wool.
  3. Decide where sound matters. Party walls and floors between rooms favour mineral wool.
  4. Check proximity to boundaries. Closer to a boundary, A1-rated wool is often the simpler route.
  5. Match the moisture profile. Flat roofs need vapour-tight layers, breathable walls need vapour-open ones.
  6. Use the exact data sheets. Conductivity, fire class and facings are product-specific.
  7. Get the build-up in writing. Each layer, thickness and who checks it against Part L.

FAQ

Is PIR better than mineral wool?

PIR is better for thermal performance per millimetre, so it suits places with limited depth. Mineral wool is better for sound absorption, fire behaviour and cost. Which is better for you depends on the part of the building and the depth available.

Are mineral wool and fibreglass the same?

No, though both are mineral wools. Glass wool is made from glass and stone wool from rock. A mineral wool explainer is not needed to see the practical difference: stone wool tolerates much higher temperatures than glass wool, which is why it is used in fire-protection roles. Check the product data sheet for the exact material and fire class.

Can you walk on PIR insulation?

PIR boards are rigid, but they are not designed as a walking surface on their own. Under floors they are covered by a screed or deck, and on roofs they sit under a protective layer. Follow the manufacturer’s loading guidance for the specific board and use boards or a deck to spread weight during installation.

What is the R-value of 100mm PIR?

In the UK we talk about thermal resistance in m²K/W rather than R-value. Dividing thickness in metres by conductivity gives an estimate: 0.1m divided by 0.022 W/mK is about 4.5 m²K/W for a board with that declared conductivity. Use the declared value on your board’s data sheet for design.

Sources