WHAT TESTING IS NEEDED TO FULLY DETERMINE INVERTED ROOF PERFORMANCE?

Inverted roof construction places unique demands on the components within the build-up, and the insulation layer specifically. For specifiers and installers of inverted roofs, it is essential to have full confidence that the chosen insulation has been fully assessed against those demands. Otherwise, there is a risk of under-performance: not just thermally, but in terms of durability too.

Traditionally, only a few insulation solutions have been deemed suitable for inverted roofs; European technical approval guideline (ETAG) 031 names extruded polystyrene (XPS) and expanded polystyrene (EPS).

But recent entrants to the marketplace have increased the number of available solutions. With vacuum insulated panels (VIPs) and cellular glass options to consider as well, the time seemed right to look afresh at testing for inverted roofs: what tests should be carried out on insulation, and why is each test important?

Testing inverted roof kits in accordance with ETAG 031

As we have already mentioned it, let’s start with ETAG 031. Its guidelines cover the inverted roof ‘kit’, which is the name given to the combination of thermal insulation and water flow reducing layer (WFRL). The kit is installed, loose laid, on the waterproofed roof deck.

Arguably, ETAG 031’s most significant aspect is its description of how to test and assess inverted roof kits in terms of their WFRL restricting rainwater’s passage through the insulation layer to the waterproofed deck. Any insulation manufacturer seeking an Agrément certificate for their kit must have the WFRL tested in accordance with the ETAG 031 method.

The test results directly impact every U-value calculation undertaken for an inverted roof using that kit.

Our in-depth blog post explains more about ETAG 031 and looks at what exactly you might need to know about its guidance.

Establishing design lambda values for inverted roof U-value calculations

ETAG 031 testing often generates a result that suggests water does not penetrate the inverted roof build-up. In accordance with BS 6229:2018 and, by extension, the BBA Information No. 4 guidance document, the WFRL should not be assumed as waterproof.

BBA Information No.4 sets out industry-standard guidance on correctly applying rainwater cooling correction factors to inverted roof U-value calculations. It describes the values that should be used to establish the correction factor, including rainfall volume and drainage factor.

As well as applying a rainwater cooling correction to the U-value, the thermal conductivity (lambda value) of the insulation material must also be adjusted to account for any moisture absorption.

‘Declared thermal conductivity’ refers to the lambda value of the insulation when it leaves the factory. In inverted roofing, ‘design thermal conductivity’ is the lambda value adjusted for potential moisture absorption. The design lambda is simply the declared value with a moisture correction factor applied.

The moisture correction depends on the properties of the insulation material, which we’ll discuss shortly. It is only appropriate to use design lambdas, but we have started to see some inverted roof insulation solutions state that a “worst case” lambda value is being used. Such terminology is not consistent with standard practice and creates unnecessary confusion.

A more detailed blog post explains further the difference between declared and design lambda values.

Insulation performance: freeze thaw testing

Applying a moisture conversion factor to an insulation’s declared thermal conductivity is described in ETAG 031. The document defines two levels for the effect of moisture on the lambda value.

For warmer Mediterranean countries, freeze thaw is not a significant design issue. Design lambda can therefore be calculated taking into account only the effects of water absorption by diffusion.

For northern European countries, including the UK, freeze thaw is an issue. The water absorption caused by diffusion and subsequent freeze thaw must be taken into account. When assessing products for certification, the BBA’s calculation of the moisture conversion factor reflects that.

Inverted roof ‘kits’ offered for use in the UK should therefore feature insulation with results declared for both freeze thaw testing and water absorption testing.

Insulation performance: water absorption

A material with very low water absorption will see its thermal conductivity affected less because, in situ, the material is closer to its factory production specification.

Design lambda uses test results for water absorption by diffusion; this is an additional test for products used in inverted roof applications, alongside testing water absorption by immersion.

The more common testing by immersion assesses water absorption when the material is sitting/submerged in water. Testing by diffusion assesses water absorption when the material is exposed to high humidities.

We have a blog post dedicated to explaining more about the different tests for water absorption by immersion and diffusion. A further post looks at the specific water absorption performance of XPS insulation, offering complete transparency about performance in both cases.

For the purposes of this post, it is enough to say that an insulation manufacturer offering inverted roof insulation products should be able to state water absorption by diffusion test results, in addition to freeze thaw testing results.

For accurate inverted roof U-value calculations based on up-to-date guidance, including using design lambdas and appropriate correction factors, contact Polyfoam XPS for technical support.