Everything you need to know about PIR insulation boards

Insulation boards are one of the most widely used insulating products. There are different varieties, all with specific properties, but PIR insulation boards seem to offer the best balance between price and performance. 

PIR insulation products have the potential to drastically reduce your energy bills and your home’s carbon footprint. They can line roofs, ceilings, cavity walls and floors to bring warmth and comfort in the colder months.  You can see I have used a thinner 25mm PIR board to insulate my garden office log cabin. It performs really well in both winter and summer.

garden room insulation

What Is PIR Insulation?  

Polyisocyanurate or PIR insulation is a polyurethane thermoset plastic produced as foam. The plastic foam board is bonded to an aluminium facing to make up an insulation board. 

PIR insulation boards consist of rigid foam sandwiched between aluminium facing. They are available in different thicknesses, from 20 to 150mm, and are ideally used to sheath cavity walls, external walls, slanted and flat roofs and floors. 

The aluminium facing acts as a vapour barrier and helps further block out airflow and retain heat. The high thermal efficiency of PIR boards is their main selling point, but they are also great for multiple different applications. Thinner pieces are great for tight spaces whilst the thicker pieces are great for larger cavities such as under floors. In addition, installing and cutting PIR insulation boards is relatively simple and can be done with a sharp knife or saw. 

The boards can also be combined with other materials to enhance structural integrity or increase thermal properties. For example, PIR insulation boards bonded to plasterboard are seen in masonry wall applications and in roof and loft insulation. The thickness of the plasterboard is often limited to 12.5mm, whereas overall thickness ranges between 32.5mm to 72.5mm. Boards bonded to plywood and OSB make quality flooring insulation due to superior impact and moisture resistance.  

How PIR insulation is made 

PIR or polyisocyanurate insulation owes its high thermal efficiency to the manufacturing process and the use of a combination of various organic and inorganic compounds. Polyol, MDI (methylene diphenyl di-isocyanate), a fire retardant (that adds optimal flame and heat resistance) and a blowing agent to expand the mixture into the foam under high heat and pressure. The process hardens and dries the foam resulting in a thermoset rigid foam structure sandwiched between aluminium facing and backing. 

The end product is impermeable and inert which makes for an excellent heat and vapour barrier.

How It Compares 

PIR insulation exhibits favourable properties when compared to other insulation boards: 

Higher Thermal Efficiency  

R-values, or the ability of insulation to resist heat flow, are higher in PIR boards when compared to other foam insulation boards like Extruded (EPS) and Expanded Polystyrene (XPS). It is down to the foil facing, the closed cell structure and the blowing agents used in PIR.

Polyiso has around 12 per cent higher thermal efficiency than its sibling, PUR board, 40 per cent more than unfaced XPS and 75 per cent better performance than EPS for the standard thickness of 25mm. This means that PIR boards can be much thinner than other boards to achieve the same heat resistance levels. 

The only rigid board with better R-values is the Phenolic Foam (PF) board, which retains roughly 10 per cent more heat but is much more expensive. For reference, EPS has an R-value of 4, XPS 5, PUR 6.25, PIR 7 and a Phenolic board of 7.5.  

Compared to other insulation options and building materials, PIR is more than twice more effective as glass and mineral wool and almost ten times better than plywood. It also retains five times more heat than double-insulated glass.  

cladding interior corner

Durable, Lightweight and Long-Lasting 

PIR insulation board is durable, lending structural stability to walls, floors and roofing. It is also rated to last the lifetime of the building and doesn’t rot or shrink. In addition, it is very lightweight, weighing only 3.2 kilos per square metre when optioned in a standard thickness of 25mm.  

Fire, Humidity and Pressure-Resistant 

Fire ratings for PIR prove that this is the go-to insulation when there’s concern about high heat levels and flames. Polyiso boards perform much better in this respect when compared to other insulation boards, which qualify PIR boards for higher fire safety standards (of at least 30 minutes or more). In case of a fire, it neither melts, drips, nor creates flashover, So, it won’t spread flames.  

As for humidity resistance, the aluminium foil backing and facing help as does the closed cell structure. They create a vapour barrier, making PIR boards useful in potentially damp settings, such as roofs and floors. Only XPS boards perform better in this respect.  

Impact and pressure resistance make PIR boards suitable for flooring applications. Here they can sustain high loads on a frequent basis without deforming. XPS boards display slightly higher compressive strength. 

Specifying PIR Insulation Boards 

PIR is available under different names (even from the same manufacturer) when specified for a purpose. The boards used in floors and the boards used in roofing will be called differently, but ultimately have the same chemical composition or just a slight difference in exterior foils in terms of layering and thickness. Retailers will (or should) mention this when buying a PIR insulation board.  

foil backked bubble wrap insulation for shed

Boards come in different thicknesses, with thicker boards having better thermal properties. Since space is not always a given in all projects and areas, the thinner boards start at 20mm and go all the way to 150mm, with multiple thicknesses (50, 75 and 100mm being some of the more popular sizes). 

Thicker boards (100 to 150mm) are commonly used for flooring, whereas standard thicknesses (50 and 75mm) in wall cavities and roofs. Besides this, most boards are cut to standard construction sizes of 1200 by 2400mm or 4 by 8 feet.  

Installing PIR Boards 

To save on energy all insulation materials need to be properly installed. As mentioned, PIR is a versatile and extremely lightweight material and cutting PIR insulation boards is done with a simple trimming knife or smaller insulation saw. Since particles can dislodge from the foam, it’s necessary to use safety gear such as goggles and masks. 

Individual pieces and boards snap together, though, for the best fit, it’s good to use adhesive tape on the joints. This prevents air pockets, drafts and moisture buildup that can lead to mildew or mould. Ensure that there is no gaping between boards. When used with other facer and building materials, boards can be positioned in place with sealant and some types accept fasteners for a more secure fit.  

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