XPS Insulation

XPS Insulation

Extruded polystyrene (XPS) insulation provides reliable thermal performance, durability and structural integrity for a floors and basements.

What is XPS insulation?

Celotex XPS is a type of rigid insulation board made from extruded polystyrene. It provides low thermal conductivity (from 0.032 W/mK), high compressive strength and excellent moisture resistance. It’s also lightweight and easy to move around on-site. Its performance benefits and straightforward installation make it a popular choice for new-build and renovation projects in both residential and commercial applications. It can be used in ground floors (under or over the slab) and basements.

How is XPS insulation manufactured?

Celotex XPS is a closed cell extruded polystyrene (XPS) insulation. During production, polystyrene polymer is melted and combined with a blowing agent such as carbon dioxide (CO₂), under controlled temperature and pressure. The pressurised mixture is extruded through a die, where the CO₂ expands and the material cools to form a rigid foam with a uniform closed-cell structure.

The gas in the cells has lower thermal conductivity than air, helping give the XPS insulation lambda values as low as 0.032 W/m⋅K. The result is a foam board with a smooth outer skin and a closed-cell structure. This means that each cell is fully enclosed, rather than interconnected, giving the boards moisture resistance and high compressive strength.

The performance benefits of XPS insulation

XPS extruded polystyrene insulation brings several benefits to commercial and residential projects, helping to improve performance and supporting compliance with requirements such as the Building Regulations Part L (Conservation of fuel and power).

• Thermal performance – Celotex XPS offers a lambda value from 0.032 W/m⋅K, contributing to a building envelope's low U-values.
• Dimensional stability – XPS insulation demonstrates low shrinkage or expansion when exposed to temperature variations, helping to ensure long-term durability.
• High compressive strength insulation – The compressive strength of Celotex XPS ranges from 300 to 700 kPa, making it suitable for heavy-load applications such as commercial floors.
• Lightweight – Since extruded polystyrene foam is 98% air, XPS insulation boards are lightweight and easy to handle.
• Easy to cut – XPS boards can be cut using standard saws or hot wire cutters. This helps to ensure accurate detailing, reducing the potential for air gaps or thermal bridging.
• Moisture-resistant insulation – Thanks to its closed-cell structure, XPS offers better moisture resistance than some other insulation materials such as PIR or mineral wool. Depending on the design and site conditions, a damp-proof membrane (DPM) may not be required when using XPS insulation.
• Reduced risk of rot or decay – Because polystyrene is a non-organic material, it does not provide a food source for mould or mildew.
• Recyclable – Polystyrene is recyclable, and off-cuts can be collected for recycling. At end of life, XPS can potentially be recovered and reprocessed, depending on local facilities.

XPS insulation: Typical applications

XPS is a versatile insulation material suitable for a wide range of projects. While it can be potentially used in various applications, its moisture resistance and compressive strength make it particularly suited for the following applications:

• Ground floors: In many projects, XPS floor insulation can be placed directly on compacted ground below the concrete slab in a strip or raft foundation. Alternatively, it can be installed above a ground-bearing slab, in a suspended concrete floor or as part of a concrete beam-and-block floor.
• Basements – For basement installations, XPS is typically installed externally, outside the basement’s waterproofing layer, providing a continuous layer of insulation and resisting moisture ingress from the ground

XPS insulation products to consider

Celotex offers three types of XPS insulation in a range of compressive strengths and thicknesses, providing design flexibility and suitability for a variety of applications and performance requirements.

	Compressive strength (kPa)	Thickness range (mm)	Lambda value (W/m⋅K)
XPS 300 SL FN	300	40–280	0.032-0.033
XPS 500 SL FS	500	60–280	0.033-0.035
XPS 700 SL FS	700	60–280	0.033-0.035

 

Celotex provides a range of resources to support designers, specifiers and installers. Try our online U-value calculator to specify suitable roof insulation for your next project.

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Insulation products to consider

Product Name	Thickness Range	Sizes	Lambda
Celotex XPS 300 SL FS	40-280mm	1250 x 600mm	0.032-0.033
Celotex XPS 500 SL FS	40-280mm	1250 x 600mm	0.032-0.035
Celotex XPS 700 SL FS	60-280mm	1250 x 600mm	0.033-0.035