Why XPS insulation is typically used on a flat roof


XPS, also known as Extruded Polystyrene Insulation, is manufactured using a process of extrusion. The continuous process results in a closed cell structure with a smooth skin on the top and bottom of the board. The closed cell structure prevents water penetrating the structure of the insulation board and provides long term strength and durability.

Where can XPS be used?

XPS insulation can be used in interior walls, foundation walls, flat roofs, pitched roofs, floors and parking, cavity walls and sidings. SOPREMA manufacture and supply an extensive range of XPS insulation solutions for all of these applications, called EFYOS XPS. See the table below for an overview of which EFYOS XPS products can be used where.

Why is XPS insulation used on inverted flat roofs?

XPS insulation is the most common type of insulation used on an inverted roof. Here are some of the key reasons why:

• Exceptional compressive strength and therefore often used under slabs, concrete floors, foundations and underground works, etc.

• Excellent moisture-resistance and particularly suitable for inverted roofs due to its closed cell structure

• Low thermal conductivity which guarantees a high thermal insulation performance

• Lightweight, easy to cut and install

• 100% recyclable

XPS insulation is able to perform in some of the most challenging environments and is often used in areas where other types of insulation cannot. This is largely due to its exceptional compressive strength and consistent lambda values year after year.

Lambda values explained

Our range of EFYOS XPS insulation is given a lambda (λ) value when it leaves our factory; this refers to the thermal conductivity of the XPS. When insulation is then laid over an inverted roof, its efficiency can be affected by water run off, should it enter the insulation system.

Once installed, the initial lambda value of the insulation will be adjusted and given a ‘design lambda’ based on the value of the thermal conductivity of that material under specific condition. This is then used in the U-value calculation for the inverted roof.

Should you choose to work with SOPREMA on your waterproofing project, we have a technical team able to support you on your design, specification and technical advice including U-value calculations.

Don’t forget the water dispersion layer

At SOPREMA, we offer a water dispersion layer (also known as a water control layer) called ROOF 115. This sits on top of the insulation and can help contribute to the thermal performance of the roof by retaining the water so it does not penetrate to the waterproofing layer. The water dispersion layer, ROOF 115, has a high resistance to bacteria and is resistant to UV, rot and bitumen to ensure its longevity.

Options for covering the XPS insulation

XPS insulation and the water dispersion layer are loose laid on the roof so a ballast is needed to stop them blowing away or moving on the inverted roof. Some options include slabs, concrete floors, loose laid gravel, plaza podium decks and green roofs.

You can find more details about our EFYOS XPS insulation product range on our website.  Please contact us for more information and to find out how we can work with you to provide a total waterproofing solution.