Celotex Insulation Boards

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Celotex Insulation Boards are a versatile PIR insulation board popular suitable for a variety of uses including wall, roof and floor insulation. They are popular due to their DIY-friendly nature - being easily cut to size, with an ability to friction-fit between joists and studs, and are lightweight with a small form-factor, without sacrificing effective thermal insulation. 

What is PIR Insulation?

Polyisocyanurate (PIR) insulation is a type of rigid foam insulation backed with a foil layer, that is renowned for its exceptional thermal performance. It is a strong and light type of foam that keeps structures warm by trapping air inside. PIR insulation is lightweight, durable, and offers excellent fire resistance and thermal insulating properties, making 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 stud wall applications. 

What Sizes Do Celotex Insulation Boards Come in?

Our Celotex insulation boards all 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 Installation - How to Install

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, 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 PIR Insulation Fire Rating

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

Celotex PIR Insulation R-value

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


U = U Value

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

Celotex PIR Insulation Thermal Conductivity 

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

Celotex PIR Insulation Moisture Resistance

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

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