The Complete Guide to PIR Insulation


Written by Maddie

The proper use of insulation within a building, no matter if it’s a house, office, or shop, will significantly reduce energy bills and keep rooms and occupants warmer during the colder months. While there are dozens of insulation products, rigid insulation PIR foam boards exhibit several benefits compared to other existing options. 


The boards are some of the best at conserving heat, can be used in different types of floors, roofs, lofts and walls, are durable, and shield against excessive noise and moisture.

What Exactly Are PIR Boards?

Also called Polyisocyanurate insulation boards, they are a type of closed-cell rigid foam board. Liquid polyols and liquid Methylene diphenyl diisocyanate (MDI) are combined to form foam in the presence of a blowing agent and additives. It allows for a heavily cross-linked cell structure, with very low thermal conductivity achieved by the high volume of trapped gas between the cells. 

Not only do these insulation boards keep more warmth in, but they also have many other advantages when compared to other rigid boards.

Thicknesses and Sizes

Thickness and R-Values go hand in hand in determining the overall thermal efficiency of insulation products. R-values refer to a product’s ability to resist the transfer of heat. The higher the R-value, the better a product can keep in the heat.

But this is only half the story. While PIR insulation boards have some of the highest R-values of any insulation materials currently sold, thicker options provide higher thermal efficiency. 

These boards are offered in different thicknesses, ranging from 25 to 150mm PIR insulation boards. You can use thinner 25, 40, 50, 75 and, to an extent, 90 mm boards where space is limited. Where this isn’t an issue, buyers should go for standard sizes – 100, 110, 120, and 150mm.  

As for sizes, all insulation boards are offered in the standard building dimensions of 1200 by 2400mm for most uses and 1200mm by 1200mm or 1200mm by 600mm in specific areas like flat roofs. 


While solid boards without added facer and backer materials are a rarity, most uses require a composite aluminium foil material to enhance performance against flames and moisture. These also help keep thermal bridging in internal wall cavities and external walls at bay. 

In addition, PIR is the main insulation material for internal walls when bonded to plasterboard. The combination is considerably versatile as it also saves space and installation time.

Where Are These Boards Used?

Polyisocyanurate boards can fit a range of different insulation projects. Both thinner and thicker boards prove their worth in roofing applications, in pitched and slanted roofs. 

They can also be used in cold roofs (installed under the roof rafters) and warm roofs with boards above the rafters. The first type of insulation is often common in existing homes and buildings, whereas warm roofs are a feature of new builds. In addition, flat roofs can have thicker PIR or insulated plasterboard with a PIR core.  

Floors are another area where these boards find extensive use. This can be done when insulating suspended timber floors, with boards cut to size and placed either between or above the joists. In addition, solid walls in either concrete or stone are insulated above or below the masonry. As with roofing projects, foil-faced or PIR-bonded plasterboard is utilised.  

Walls account for a large portion of lost heat, and the boards can insulate different types of walls. PIR insulation boards are seen in external walls, placed behind polymer renders or dry cladding; in drylining, solid internal walls (often with bonded plasterboard) and in insulating cavity walls.  

The Benefits of PIR Insulation 

These insulation products have been on the market for quite some time, meaning prices are more affordable than ever. It makes them a viable option for getting the best possible results when insulating your home or office. Boards made of polyiso have substantial benefits over other insulating products, while the prices aren’t much higher.

The exceptionally high R-values of thermal efficiency distinguish PIR from older rigid insulation foam products. Even compared to its sibling, PUR insulation, it comes out as 10 to 15 per cent more effective. It is more pronounced when comparing these boards to polystyrene-based EPS and XPS options that need to be much thicker to achieve the same results. 

For instance, a 50mm board has the same thermal effectiveness as a 100mm EPS board. Where there’s more space, a 150mm PIR insulation board has no comparable rivals and offers the highest possible R-values.  

Besides this, the specific structure and manufacturing process lends this type of insulation board exceptional strength and durability. Boards retain their structural stability over the lifetime of the building, and their rigidity helps strengthen and support other elements in the building. 

It also means that they won’t shrink, deform or rot over time, regardless of weather conditions or outside temperatures. And in areas with high foot movement, such as floors and flat roofs, the resistance to pressure and impact means boards can maintain form even under high loads.  

What distinguishes PIR from related PUR boards, and even more to lesser options is the fire resistance. The closed-cell structure and aluminium foils on either side mean boards won’t catch fire, emit toxic substances or spread flames to other parts of the building. If your home or business needs to meet strict British fire standards, then this is the board to get.

Boards are also environmentally-friendly, as they don’t contain Ozone-depleting CFCs during production. The material can also be recycled. This type of insulation board is irritant and allergen-free, and much easier to install with simple tools than glass or mineral wool.  

Lastly, it has exceptional resistance to moisture when sourced with the appropriate metallic foils. In addition, it doesn’t attract bacteria, mould or vermin.  

All these properties imply that once installed, it does a great job at keeping the heat in and reinforces surrounding structural elements with its rigidity and durability. Most producers warrant that their boards sustain their thermal properties for 25 years or more.

Brands Sold in the UK

Quinn Therm, Ecotherm, Kingspan, Mannok, Celotex and Knauf are some of the better-known names. All have extensive lines of insulation products and PIR insulation boards for specific applications and in the appropriate thicknesses. And similar thermal performance across the board.


Since they can be optioned in thinner variants (due to the very high R-value), these boards will also reduce transportation times, especially in larger projects. Like all rigid foam boards, they are lightweight, easy to handle, and even easier to cut to size. Boards are sold in packages consisting of 2,4,6,10, 12, or 15, depending on thickness.  

Installation requires a rough estimate of the area that is to be insulated. Boards are then cut to size and can be installed horizontally or vertically. The edges of separate boards easily snap together (some are available in tongue-and-groove varieties), and joints are sealed with adhesive tape to prevent air pockets and the buildup of moisture. Sealants can be used in difficult areas such as corners and edges. Plasterboard-bonded PIR boards are installed in a similar fashion, along with the use of fasteners.  

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