120mm Celotex XR4120 PIR Insulation Board - 2400mm x 1200mm

Polyisocyanurate (PIR) insulation boards are rigid foam insulation boards backed with a foil layer, renowned for exceptional thermal performance within a small footprint. It is a strong and light type of foam that keeps structures warm by trapping air inside, which makes it ideal for cavity wall insulation, and internal wall insulation.

PIR insulation is lightweight, durable, offering excellent fire resistance and thermal insulating properties, which makes it a super DIY-friendly solution that can easily be cut to size to fit rafters, joists and stud walls, and used as Cavity Wall Insulation. We also sell a Celotex PIR Insulated Plasterboard which is ideal to save time in internal wall insulation and loft conversion projects.

Standard Celotex insulation boards measure a standard size of - 2400mm x 1200mm, and are available in a variety of thicknesses to suit every size of floor, cavity or project such as:

25mm, 30mm, 40mm, 50mm, 60mm, 70mm, 75mm, 80mm, 90mm, 100mm, 110mm, 120mm, 130mm, 150mm and 200mm. 

The most popular sizes are 100mm Celotex, 50mm Celotex and 25mm Celotex. 

Celotex Cavity Wall Insulation Boards (CW4000) and Celotex Thermaclass Cavity Wall Boards have a narrower width of 450mm to make them easier to install in the construction of cavity walls.

Due to it’s versatile nature, there are many different uses for Celotex Insulation and therefore many different ways to correctly install Celotex insulation. However, generally Celotex should be purchased to fit the depth of the cavity or space it is to fill other than when used as partial-fill insulation in cavity walls. It can be cut to size to fit the height and width of cavities, joists and rafters using a sharp utility knife or handsaw for thicker boards. It is typically designed to be friction-fit to a space, meaning the boards should fit tightly and stay easily in place with no additional fixtures and fittings.

For precise installation instructions for your specific use case, it is recommended to consult your builder, architect or building inspector to ensure installation complies with building regulations. 

Celotex boards are rated Euroclass F Reaction to Fire classification, according to BS EN 13501-1.

The R-Value of Celotex PIR Boards increases with the thickness of the board. The higher the R-Value, the greater the resistance of the board and therefore the more effective the thermal insulation. 

Celotex PIR boards have a thermal conductivity of 0.022 W/m.K.

R-Value (m².K/W)

• 50mm - 2.25
• 60mm - 2.70
• 70mm - 3.15
• 75mm - 3.40
• 80mm - 3.60
• 90mm - 4.05
• 100mm - 4.50

To calculate the U Value of your wall, you must consider the entire structure of the wall, including air gaps. A general formula for calculating the U-value:

U = 1 over R

Where:

U = U Value

R = The total thermal resistance of the building component, including all layers of materials and air gaps.

Use the Celotex U-Value Calculator to help calculate the correct u-value of your project to ensure it complies to all of the required building regulations.

Celotex PIR boards have a thermal conductivity of 0.022 W/m.K.

Celotex boards are made of a closed-cell structure which means they do not specifically retain moisture, although they are not entirely waterproof.

There are two types of Celotex insulation most commonly used in Cavity Wall insulation: Celotex Thermaclass insulation and Celotex CW4000. Both are a rigid PIR Insulation boards with a foil facing, which are sized at a 450mm width to make it easy to install in cavity walls. The main difference between them is that Celotex Cavity Wall CW4000 is designed as a partial-fill insulation, meaning it is designed to sit in the cavity with an air-gap, and it has smooth, squared-off edges. Celotex Thermaclass is a full-fill insulation board designed to fill the entire cavity (with a small 10mm gap to allow for ease of fitting and mortar spread when constructing the wall) with a tongue-and-groove shaped edge for a seamless fit.

As well as walls, ceilings and roofs, Celotex can be used as floor insulation in a variety of different use cases. As well as fitting between timber joists, Celotex is also suitable for use in concrete flooring above the concrete slab and below the final screed layer, as long as appropriate damp proofing is applied.

Due to it's lightweight properties, Celotex PIR Insulation is a great option for Roof insulation in loft conversions. It is typically installed between the rafters with a "friction fit" which doesn't require any additional fixings or adhesives. It is particularly suitable for this purpose as it is easy to cut to size with minimal mess.

As it is such a versatile product, Celotex is popular for a lot of internal wall insulation projects, too. It's narrow depth relative to it's effectiveness makes it a popular choice where the depth of the wall would otherwise not allow sufficient room to reach target u-values.

Due to it's effective thermal performance relative to it's narrow form factor, Celotex is a great option for insulating sheds and garden buildings to allow maximum floorspace.