Polyisocyanurate (PIR) and extruded polystyrene (XPS) are both lightweight, rigid foam boards used for thermal insulation in buildings.

The required thermal transmission – aka U-value – for a construction element (floor, wall or roof) determines the thickness of thermal insulation that has to be incorporated within that element’s build-up.

The required thermal transmission – aka U-value – for a construction element (floor, wall or roof) determines the thickness of thermal insulation that has to be incorporated within that element’s build-up.

For architects and specifiers looking to include an inverted flat roof as part of a construction project, ETAG 031 is a document that is very likely to be referenced in technical literature for inverted roof solutions

At the end of March 2021, consultation closed on a draft Code for Construction Product Information (CCPI).

BS EN 13164 is one of a number of harmonised European Standards relating to the manufacture of thermal insulation products for construction. It covers extruded polystyrene (XPS) specifically.

BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings. Code of practice describes best practice for the design, construction and maintenance of flat roofs. A flat roof is defined as having a pitch of no greater than 10 degrees to the horizontal, but the recommendations in BS 6229 can be applied to roofs with a steeper pitch.

BRE Report (BR) 443 Conventions for U-value Calculations is a document produced by the Building Research Establishment in the UK. As its title suggests, the report details best practice that should be followed by anybody carrying out U-value calculations for building elements.

‘Declared thermal conductivity’ is the lambda value of an insulation product as it leaves the factory. In the context of inverted roofing, ‘design thermal conductivity’ is the lambda value of the insulation once it has been adjusted for potential moisture absorption.

When calculating the U-value of an inverted flat roof, the thermal conductivity of the insulation layer should be the product’s design lambda value and not its declared value. Because of the insulation’s position above the roof waterproofing layer, it is expected to be exposed to some rainfall and therefore a moisture correction factor should be applied accordingly.

A water flow reducing layer (WFRL) is a loose laid membrane used as part of inverted flat roof constructions. It is installed over the roof’s thermal insulation layer, and below the roof covering. A WFRL is a barrier to rainwater, reducing the volume of water that can reach the waterproofed roof deck. It also stops fines from entering the roof system.