All you need to know about water flow reducing layers (WFRL), ETAG 031, declared lambda versus design lambda and moisture conversion factor calucations...
The inverted roof principle is easy to understand. Placed on top of the waterproofing membrane, the insulation is straightforward to install and provides simpler access for repairs to the concrete deck. Inverted flat roofs are a very common feature in modern buildings and offer architects some design advantages given their ease of use for green roofs and blue roofs.
Extruded polystyrene (XPS) insulation boards are the best solution for inverted roofs due to their low water absorption and high compressive strength. Thermal performance is exceptional: Ravatherm XPS X 300 SL from Ravago Building Solutions has a declared lambda of 0.031 W/mk, requiring just 200mm of insulation to achieve a U-value of 0.15 W/m2K (200mm concrete deck, 7.5mm waterproofing membrane at a project in London).
How does the WFRL impact U-value calculations?
It is important to understand the difference between declared lambda and design lambda and their intended purposes. Declared lambda is a measurement of thermal capability and is included in the product’s Declaration of Performance. This takes account of the ageing of the material, so that the thermal performance is known over the building’s lifetime. This set of testing and rules is contained in EN ISO 10456 and is part of the CE Marking of the XPS under EN 13164.
However, once the insulation is used within an inverted roof system, there are corrections that need to be considered as part of the calculation. ETAG 031 (European Technical Approval Guideline for inverted roof kits) requires that a design lambda must be used for U-value calculations, which includes the moisture conversion factor where the WFRL has a pivotal role.
The moisture conversion factor needs to be accounted for because, in an inverted roof, the insulation will be partially exposed to the elements. Some rainwater will have contact with both the insulation and the concrete deck, affecting the thermal performance of the whole system.
How is the moisture conversion factor calculated?
The WFRL plays an important role in an inverted roof system and is not to be confused with a waterproofing membrane. It is loose-laid on top of the insulation material and reduces the volume of cold rainwater that reaches the roof’s waterproofing layer, which temporarily affects the rate of heat loss from the roof. ETAG 031 Annex C outlines a test for WFRL performance that measures the proportion of water that passes through a build-up.
The fraction of water measured is the drainage factor (f) and there is a general acceptance that 2.5 per cent should be used as a minimum factor, even when the test result is 0, as this is not a waterproof layer. When combined with the standard factor for increased heat loss (x) of 0.040 Wday/m²Kmm this results in a correction factor of fx=0.001 to be applied to the declared lambda to calculate the design lambda. Effectively, 1mW/mK is added to the declared value. A thorough explanation of this calculation process can be found in BBA Bulletin 4 Issue 6.
ETAG 031 also details a process to use generic fx values without the need to carry out the WFRL testing, these values being much higher to allow for an extra safety factor, depending on the roof build-up and the edge profile of the insulation used.
The correct use and installation of the WFRL in an inverted roof system is therefore crucial for desktop U-value calculations to be realised. Further guidance on the correct procedures was recently published in Guidance Note 14 from the Liquid Roofing and Waterproofing Association.
Proper installation of the insulation is also essential to ensure that an inverted roof will perform as expected. Third-party assurance schemes such as Agrement certificates are a vital source of information and should not be perceived as mere tick box exercises when specifying materials for a project. These approved bodies make sure that the current standards are followed and properly explain best practise for a build-up. The often overlooked WFRL has a key role to play: it may look like a piece of paper, but it has a lot to say.
References
ETAG 031:2010 Guideline for European Technical Approval of Inverted Roof Insulation Kits – Part 1
EN 13164:2012 Thermal insulation products for buildings – Factory made extruded polystyrene foam (XPS) products – Specification
EN ISO 10456:2007 Building materials and products - Hygrothermal properties - Tabulated design values and procedures for determining declared and design thermal values
BBA Bulletin 4 Issue 6 - 2015
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