Frequently asked questions about XPS insulation
Find answers to common questions about XPS insulation boards, thermal performance, moisture resistance, load-bearing applications and inverted roof systems.
What is XPS insulation?
Extruded polystyrene, commonly known as XPS, is a rigid thermal insulation material manufactured using a continuous extrusion process.
This process creates a closed-cell structure with a smooth outer skin. The resulting boards combine thermal insulation with resistance to moisture and high compressive loads.
What are the main benefits of XPS insulation?
XPS insulation combines low water absorption, high compressive strength and good thermal performance. The boards are also lightweight, durable and relatively easy to cut, handle and install.
SOPRAXPS is also available with a ship-lapped edge profile, helping to reduce thermal bypass at board joints. These properties make XPS particularly suitable for demanding applications in which insulation may be exposed to moisture or sustained loads.
Where can XPS insulation be used?
Depending on the product, XPS insulation can be considered in applications including:
- Inverted flat roofs
- Green, blue and ballasted roofs
- Floors and concrete slabs
- Foundation walls and below-ground construction
- Basements
- Cold-storage and industrial environments
Always consult the relevant product datasheet and certification to confirm suitability for the intended construction.
Why is XPS insulation commonly used on inverted flat roofs?
In an inverted roof, the insulation is installed above the waterproofing layer and may therefore be exposed to rainwater, temperature changes and imposed loads.
The closed-cell structure, low water absorption, compressive strength and durability of XPS make it well suited to this position. The boards are normally loose laid and held in place by a suitable finish or ballast, such as paving slabs, gravel, a podium deck or a green roof build-up.
What thermal performance does XPS insulation provide?
The declared thermal conductivity of XPS varies according to the specific product, board thickness and compressive strength. SOPREMA XPS products can have declared lambda values ranging approximately from 0.032 to 0.035 W/mK.
For inverted roofs, a design lambda may be used in the U-value calculation to account for the conditions in which the insulation will operate. The required thickness should therefore be established for the complete roof build-up.
Is XPS insulation waterproof?
XPS has very low water absorption and is highly resistant to moisture ingress because of its closed-cell structure. This makes it suitable for inverted roofs, foundation walls and other applications where insulation may encounter moisture.
However, XPS insulation is not a substitute for a designed waterproofing system. The complete construction must still include the appropriate waterproofing, drainage and protection layers.
How strong is XPS insulation?
XPS boards are available in different compressive-strength grades for applications ranging from standard floors and roofs to heavily loaded decks, industrial floors and parking areas.
The appropriate grade should be selected according to the expected permanent and imposed loads. Product-specific compressive strength and compressive creep performance must be checked during specification.
What is the difference between XPS and PIR insulation?
PIR generally offers lower thermal conductivity, meaning that a thinner layer may achieve a given thermal resistance. XPS, however, offers particularly low water absorption and high compressive strength.
PIR is widely used in walls, floors and conventional warm roof constructions. XPS is often selected for inverted roofs, below-ground applications and heavily loaded areas. The correct choice depends on the complete construction, exposure conditions, loads and target U-value.
How durable is XPS insulation?
When correctly specified, installed and protected, XPS can retain its thermal and mechanical performance throughout the design life of the construction.
Its closed-cell structure helps it resist moisture and repeated loading in demanding environments. Refer to the applicable product documentation, maintenance requirements and independent certification for project-specific durability information.
Can XPS insulation be recycled?
XPS boards can be recycled after removal where suitable collection and recycling routes are available. Clean installation offcuts may also be recoverable through appropriate waste-management schemes.
Keeping boards dry, clean and free from bonded materials can improve the potential for recovery and recycling at the end of their service life.
What should be considered when specifying XPS insulation?
Product selection should take account of the application, required thermal performance, compressive strength, moisture exposure, board thickness, edge profile and compatibility with the surrounding system.
Fire performance must also be assessed as part of the complete construction rather than from the insulation board alone. Review the product datasheet, Declaration of Performance and relevant third-party certification before specification or installation.