The short answer
PIR insulation is a rigid foam board used on commercial roofs, walls and floors. Its thermal conductivity is typically about 0.022 W/mK, as manufacturers declare it. It suits flat, true surfaces in a designed build-up. On an existing profiled roof, compare the joints, supports and moisture details with a continuous sprayed layer.
If boards are your starting point, keep them in the comparison. A well-designed PIR system is a good answer in the right construction. The useful question is how it fits the building you already have, especially where the roof has purlins, deep profiles and a working floor underneath.
01What PIR is
What PIR insulation is
Start with the material, then look at the assembly it belongs in. PIR stands for polyisocyanurate. It is a rigid foam insulation, commonly supplied as a foil-faced board. Other forms combine it with a lining or place it within an insulated panel. Those finished systems need their own specification.
| PIR at a glance | What it means for your building |
|---|---|
| Material | A rigid foam thermal-insulation board. |
| Typical declared conductivity | About 0.022 W/mK, as manufacturers declare it; use the chosen product's actual value in the calculation. |
| Common forms | Foil-faced boards, boards combined with a lining, and insulation within composite panels. |
| Where it fits | Designed roof, wall and floor constructions with support and details suited to the board. |
| Joints | Edges, penetrations and perimeters need completing as part of the air and vapour-control design. |
| Thickness | Set by the required U-value and complete construction, rather than the board size available. |
| Fire | Check the product and assembly evidence against the building's fire strategy. |
The conductivity tells you about the material, not the whole roof. Your sheet or deck, supports, retained layers and junctions also affect heat transfer. That is why a thickness quoted for another building is a clue, rather than your specification.
The conductivity tells you about the material, not the whole roof.
02Where PIR works well
Where PIR works well
With those basics clear, consider where boards have a practical advantage. On a new build with flat, clean surfaces, a designed board system competes well. The team can coordinate supports, membranes, joints and finishes before access becomes restricted by the building's use.
PIR also belongs in liner and built-up systems, floors and suitable wall constructions. A designed warm-roof assembly is another established use. These are complete constructions, not loose boards attached wherever there happens to be space.
That distinction matters when you compare quotations. A properly designed liner system can provide a true surface, planned fixing points and a continuous vapour-control route. Price those parts with the insulation, so the figure describes the finished work.
For other materials across the building, read types of insulation. For the wider roof-route comparison, see types of roof insulation.
03PIR vs mineral wool
PIR vs mineral wool
If you are choosing between boards and a supported wool system, compare the result each construction needs to achieve. PIR's low declared conductivity can allow a thinner thermal layer. Mineral wool has useful thermal and acoustic roles, with non-combustible products available. Check the chosen product and its installed assembly.
Both need their supports, joints and moisture details designed. The vapour route does not disappear because the material is fibrous or a board is foil-faced. Ask how the finished liner meets the perimeter and services, and what evidence supports the required fire performance.
The right choice follows the building's thermal, fire and acoustic requirements. A material name alone does not tell you which installation will work best beneath an occupied warehouse roof.
04Where PIR struggles
Where PIR struggles on existing commercial buildings
Now bring the comparison back to the roof above your working floor. An existing corrugated or ribbed sheet is a different receiving surface from a new, flat deck. Boards can still be used in a designed system, but their installation brings details that the material price cannot show.
Section · profiled roof sheet · not to scale
Fitted board systemDesigned support, joints and a vapour-control route to complete
- Same sheet, same purlinsAn existing corrugated or ribbed sheet on its purlins.
- Void at each ribA flat board cannot follow every rise and trough in a profiled sheet.
- Flat boardFoil faced and supported, cut around the steelwork and services.
- JointEdges, fixing positions and perimeters need completing as part of the assembly.
- Purlin and fixingSteel passing through the insulation route; include its effect in the U-value.
- Vapour-control routeFoil facings contribute; joints and perimeters complete it. Left open, a detail lets warm, moist air reach the cold sheet.
Sprayed layer, bonded beneathClosed-cell applied in place, following the profile
- Same sheet, same purlinsThe sheet is kept in place; the surface is prepared first.
- Closed-cell layerBonded beneath the sheet, following the profile. No void at the ribs, no joint to complete.
- ContinuousOne layer across troughs and crowns, applied in place.
- Purlin and fixingThe layer continues around the steel and the same fixing.
- Vapour-control routeThe bonded layer itself. Application boundaries and junctions are specified and checked.
Cutting around purlins and profiles
A flat board cannot follow every rise and trough in a profiled sheet. The design needs to explain where the insulation sits and how it is supported. Cutting around steelwork, services and changes of direction adds joints and fitting work, especially where access is tight.
At Derriford District Centre in Plymouth, boards could not follow the ribbed galvanised metal-deck soffit and its access. The recorded alternative was a sprayed hybrid build-up. That is a construction-specific reason to use foam, rather than a claim that boards never suit new buildings.
Laps, fixings and thermal bridges
The next question is what happens at every interruption. Board edges, fixing positions and the junction with a wall all need finishing. If the insulation stops short or a detail is left open, the drawing and the installed layer no longer describe the same construction.
Steel supports can also pass through the insulation route. Include their effect in the U-value calculation. A low board conductivity is useful, but it cannot account for an unfinished joint or an omitted thermal bridge.
Condensation behind the boards
A cold metal sheet can still be reached by warm, moist air where joints or perimeters are incomplete. That is why the designer must set a continuous vapour-control route for the actual build-up. Foil facings can contribute to that route; they do not complete it on their own.
Check the existing water problem before specifying any layer. A roof leak and condensation can occur together, and they need different work. If water is entering through a failed detail, get the roofer to repair it first.
Correctly diagnosed and specified, closed-cell spray foam stops roof condensation. The depth is typically 25 mm or 50 mm closed-cell, depending on the roof make-up, confirmed before we spray, with a condensation risk analysis where needed.
Fire performance of the assembly
With moisture understood, check the fire strategy alongside it. We assign no blanket fire class to PIR. Ask for the board's declared classification, then check there is test evidence for the whole build-up: what it sits on, the lining, the joints and the finish.
The same discipline applies to sprayed insulation. We quote fire performance only as the manufacturer's tested build-ups, and record the specified finish in your proposal.
Your designer or fire engineer sets the requirement, agreed with building control. Read fire performance and coatings with the proposal so the required finish is priced and recorded.
Access and downtime
Finally, look below the roof. Boards need transporting, supporting, fitting and finishing at the installation position. Racking, fixed machinery and vehicle routes affect how that work can be carried out. Ask what space the contractor needs and which areas must be released.
A sprayed application also needs protection, access equipment and controlled work areas. Compare both programmes against your operating hours. What you want is a written plan showing which areas are handed over, and when.
05PIR vs spray foam
PIR vs spray foam on a commercial roof
Once those details are visible, compare complete installed systems. This table gives you the questions to put alongside the drawings and quotation. It includes places where PIR has a strong case as well as the situations that favour a bonded sprayed layer.
| Question | Designed PIR system | Closed-cell spray foam |
|---|---|---|
| Flat surfaces planned into a new build | A strong fit when the supports, membranes and finish are designed together. | Useful where the shape or access calls for application in place. |
| Ribbed or irregular underside | Requires support, fitting and a plan for the voids and joints. | Follows the prepared surface as a continuous bonded layer. |
| Joints and fixing positions | Must be completed as part of the assembly. | Application boundaries and junctions are specified and checked. |
| Condensation on a cold sheet | Depends on the complete moisture and vapour-control design. | Stops correctly diagnosed condensation when the roof and depth are specified. |
| Fire requirement | Use the selected product and assembly evidence. | Use the manufacturer's tested build-up and specified finish. |
| Working building | Access and protection follow the board and liner installation programme. | Agreed work bays allow phasing where the building supports it. |
| Cost basis | PIR on a new liner, £30 to £50/m² installed. | Foam only: 25 mm £16 to £25/m² + VAT; 50 mm £22 to £35/m² + VAT. |
Typical UK industry figures, 2026. Send photographs or drawings for a free estimate.
PIR's thin thermal layer and planned board construction can be the right choice. A sprayed layer earns its place where the surface defeats board fitting or where retaining a sound roof and treating the underside suits the brief. Compare the finish, moisture design and access, then the total.
06Fibre-cement roofs
PIR on fibre-cement and asbestos-cement roofs
If the roof information identifies fibre cement or suspected asbestos cement, establish the material and work requirements before discussing fixings. Fragile sheets are not a platform to walk on, and a new insulation-support detail needs its own design. Send existing photographs taken safely; do not disturb sheets to obtain them.
HOLVEM does not test for asbestos or replace roof sheets, and never removes asbestos sheets. If the roof is safe to keep, we encapsulate it; if not, removal is a job for asbestos removal professionals, licensed where HSE requires it.
Read insulating a fibre-cement roof from below for our retained-roof method, and the fibre-cement roof decision for the wider choices. The asbestos survey or register and roof condition guide which work can proceed.
07The wrong answer
When spray foam is the wrong answer
A fair comparison must leave room for another recommendation. If the roof is leaking, have it repaired before insulation. HOLVEM is not a roofing contractor, and spray foam is not waterproofing. If the sheets or supports cannot be retained, price the required roofing work first.
A surface that cannot receive the proposed bonded layer needs preparation or a different construction. Where your fire strategy requires a different assembly, use the construction with applicable evidence. A moisture design that needs drying routes must also be resolved before choosing the insulation.
We explain those findings before agreeing the installation. A good specification answers the building's problem; it does not force every roof into the same product.
08Costs per m²
Costs: PIR boards and spray foam per m²
To turn the comparison into a budget, keep the complete work beside each band. These are typical installed costs for each method, not the price of the boards on their own.
| Method | Typical installed cost per m² |
|---|---|
| PIR boards on a new liner | £30 to £50 installed. |
| Mineral wool on a liner | £20 to £30 installed. |
| Closed-cell foam, 25 mm | £16 to £25 + VAT, foam only. |
| Closed-cell foam, 50 mm | £22 to £35 + VAT, foam only. |
| Overcladding | £60 to £90. |
| Insulated panels or replacement | £100 to £160. |
Typical UK industry figures, 2026. Send photographs or drawings for a free estimate.
Ask whether each figure includes VAT. Ask each contractor to confirm it and include supports, membranes, access, protection and finish. Additional coatings, external foam systems and calculated thermal depths need their own project price. Our cost guide explains the foam-only bands.
09Recorded projects
Projects where the surface decided the method
The examples below show why the drawing and installation position matter. They describe recorded work and results, not what every building will achieve or what those clients paid.
At Derriford District Centre, the ribbed deck and access ruled out boards. We applied 25 mm closed-cell under 160 mm open-cell without fixings into the deck. The build-up met its design U-value of 0.18 W/m²K.
At The Box, Plymouth, irregular profiles, tight access and service penetrations also ruled out boards. We applied 120 mm closed-cell beneath 787 m² of concrete soffit. The build-up was calculated to the specified U-value of 0.18 W/m²K; we probed the depth and tested adhesion across the soffit zones, and the work passed site inspection.
At TJW Grab Hire, the task was condensation beneath about 505 m² of sound box-profile steel. We applied 50 mm closed-cell in two working days. The customer reported condensation eliminated and a warmer, drier interior.
At the Midlands distribution warehouse, 75 mm closed-cell went on in a continuous layer, bay by bay while the rest of the warehouse carried on. The client reported condensation stopped, a substantial fall in energy use and cost, steadier temperatures and less rain noise.
10Getting a price
Getting a price for your building
To start your own comparison, send photographs or drawings for a free estimate. We include a virtual survey using aerial imagery to measure and check the roof remotely. Clear photographs often let us return the estimate the same day, without a site visit.
We tell you the next step after reviewing that information. Straightforward jobs can be confirmed on the day we install, before work starts. Larger or complex jobs receive a free survey, then a firm written quote. No obligation either way.
A binding quotation is issued in writing before work starts, from a survey or from the photographs and drawings you send. Include your proposed build-up or thermal target if you have one; you do not need to choose the material before enquiring.
11Why HOLVEM
Why HOLVEM
If your building is a new build with flat, true surfaces, a designed PIR system may well be the right answer, and we will tell you so. If it is an existing roof with purlins, laps and a cold sheet, that is where we come in.
We specify certified spray foam systems, which we apply as an accredited installer for the manufacturer, with the roof calculations and a QA evidence pack that show your specifier, surveyor or insurer what is proposed and completed.
Correctly diagnosed and specified, closed-cell spray foam stops roof condensation. Guaranteed to stop condensation when we confirm it is condensation at survey, set the depth, and the roof has no existing leaks.
The price sits beside the board alternative: our foam starts at £16/m² + VAT, against £30 to £50/m² installed for PIR on a new liner. Typical UK industry figures, 2026. Send photographs or drawings for a free estimate.
Keeping a sound roof avoids a strip-out programme. We agree the working bays and protection around your use, as at the Midlands warehouse where the rest of the building carried on.
Over 500 installs include the ribbed deck at Derriford and the irregular concrete soffit at The Box. Those jobs show the practical reasons boards were ruled out.
Backed by a 25 year product and workmanship warranty.
12Boards or a bonded layer
Three questions point to the first step.
Answer the three below. The result names the first step.
PIR insulation
Your roof, in three answers.
If the roof is leaking, have it repaired before insulation. HOLVEM is not a roofing contractor, and spray foam is not waterproofing.
Roof leak or condensation?HOLVEM never removes asbestos sheets. If the roof is safe to keep, we encapsulate it; if not, removal is a job for asbestos removal professionals, licensed where HSE requires it.
Fibre-cement encapsulationSend existing photographs taken safely. We tell you the next step after reviewing them.
Send photographsOn a new build with flat, clean surfaces, a designed board system competes well. Price the supports, membranes, joints and finish with the insulation, so the figure describes the finished work.
Types of roof insulationAt The Box, Plymouth, irregular profiles, tight access and service penetrations ruled out boards. We applied 120 mm closed-cell beneath 787 m² of concrete soffit.
Concrete soffitsCorrectly diagnosed and specified, closed-cell spray foam stops roof condensation. The depth is typically 25 mm or 50 mm closed-cell, depending on the roof make-up, confirmed before we spray.
Metal roof undersideSteel supports can pass through the insulation route; include their effect in the calculation. A low board conductivity cannot account for an unfinished joint or an omitted thermal bridge.
U-values and Part LAgreed work bays allow phasing where the building supports it. What you want is a written plan showing which areas are handed over, and when.
Start a project brief13Questions, answered
Questions about PIR insulation
What is PIR insulation used for on commercial buildings?
PIR is used within designed roof, wall and floor constructions, including liner and built-up systems. Price the supports, joints, vapour-control route and finish with the board so the proposal describes a complete assembly.
What are the disadvantages of PIR insulation?
On an existing profiled roof, boards need fitting and support, with joints, fixing positions and perimeters completed. The moisture and fire design also matter. These are installation considerations to price and resolve, rather than reasons to reject PIR in every building.
Is PIR better than mineral wool?
PIR can provide a thinner thermal layer; mineral wool has thermal and acoustic uses, with non-combustible products available. Compare the chosen products as complete assemblies for the building's heat, moisture and fire requirements.
Can you fit PIR boards under a corrugated metal roof?
Yes, with a designed support or liner system and continuous vapour-control details. The board does not follow every trough by itself. The proposal needs to show what happens in the space behind it and around each joint.
Why does condensation form behind PIR boards?
Warm, moist air reaching a cold sheet through incomplete details can condense behind the boards. Check the complete moisture design and the cause of existing water. Closed-cell is an alternative for a suitable roof with confirmed condensation; leaks need a roofer first.
Is PIR insulation fire safe?
Check the manufacturer's declared classification and the proposed assembly against the fire strategy. We assign no universal class to PIR. For our sprayed systems we quote only the manufacturer's tested build-ups and specified finishes.
Is spray foam better than PIR?
It is a strong option on suitable irregular or profiled undersides. PIR competes well on flat, true surfaces designed for boards. The roof condition, moisture route, fire requirements and complete installation decide which one fits.
How much does PIR insulation cost per m² installed?
The typical band for PIR on a new liner is £30 to £50/m² installed. Our foam-only bands are £16 to £25/m² + VAT at 25 mm and £22 to £35/m² + VAT at 50 mm. Typical UK industry figures, 2026. Send photographs or drawings for a free estimate.
14In five lines
PIR insulation, in five lines.
- PIR is a rigid foam board with a declared conductivity of about 0.022 W/mK. It suits flat, true surfaces in a designed build-up.
- On an existing profiled roof, boards need support, fitting and completed joints; a sprayed layer follows the prepared surface as a continuous bonded layer.
- Compare complete installed systems: supports, joints, the vapour-control route, the manufacturer's tested fire evidence for the build-up and access to the working floor.
- Correctly diagnosed and specified, closed-cell spray foam stops roof condensation. Leaks go to a roofer first.
- PIR on a new liner is typically £30 to £50/m² installed; our foam-only bands are £16 to £25/m² + VAT at 25 mm and £22 to £35/m² + VAT at 50 mm.
