What is the importance of a WFRL?

If you haven’t been involved with inverted roofing before, the acronym ‘WFRL’ may be a new one for your collection – there are lots of them in construction! If you have been involved in inverted roofing the Water Flow Reducing Layer (WFRL) is the ‘bit of paper’ that you unroll over the inverted roof insulation before the roof finishes are installed.

The WFRL isn’t actually paper, it is a spun bonded polyethylene geotextile based upon Tyvek® by DuPont, a variant of the DuPont Tyvek® breather membranes used in pitched roof and external wall constructions.  

In an inverted roof application, the WFRL performs similar functions to its wider family of applications, providing a waterproof and vapour permeable layer that prevents water flowing through it whilst allowing moisture vapour to escape. These are key functions in an inverted roof system to improve the thermal performance of the installation, enabling a target U-value to be met with a thinner insulation thickness.

This isn’t to say that no falling rainwater will get beneath the WFRL, in all U-value calculations there is an acceptance that a small degree of water will get beneath the insulation and reach the waterproofing layer. This is accounted for by the BBA through providing both a Declared and a Design Lambda value (see QI@QI May 2022) and is commonly referred to as the rainwater cooling effect.

When it comes to ensuring the minimum amount of rainwater gets beneath the WFRL, it all comes down to installation on site and the correct training of the installers so they know the importance of the WFRL. Whilst roof waterproofing remains the most important element of keeping the building dry, the correct installation of the insulation and WFRL is equally important in keeping the building warm in winter and cool in summer.

The WFRL should always be installed so that the overlaps in each run are in the same direction as the falls on the roof, just as you would with the waterproofing overlaps (if it has them).

The overlaps need to be a minimum of 300mm as marked on the surface of many WFRLs, this prevents capillary action. In practice this isn’t difficult to achieve but is does require attention to the installation, especially if it is windy, and may require some temporary ballasting to assist.  

Similarly, when detailing items such as internal or external corners, rooflights and penetration overlaps need to be maintained and a similar level of skill to waterproofing details should be applied to the installation.

Rainwater outlets provide the biggest challenge now we have such increased insulation thicknesses. This is typically the area where rainwater will get beneath the WFRL and tracks back beneath the insulation, however, as we have already said, the thermal design accounts for this and with heat from beneath the building and falls, this water will evaporate and/or drain. One way to limit this would be to form a funnel into the outlet from the WFRL or seal the WFRL to an upside down pre-formed soil pipe flashing, sealing the WFRL to the flashing with a suitable tape.

Next month: Insulating external balconies