How to get U-values right from the start

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Advice on correctly calculating U-values for inverted roof constructions are provided here from Karen Everitt, Product Manager of Polyfoam®XPS at Knauf Insulation…

In the construction industry, energy efficiency is a key consideration during both the design of the building and throughout its ongoing use. Therefore, specifying the correct U-values for the building fabric is of utmost importance because it forms the basis of any energy or carbon reduction performance assessment. In practice, nearly every external building element has to comply with thermal standards that are expressed as a limiting U-value and is a requirement of Building Regulations.

For that reason, it is crucial when specifying thermal insulation, that installers are confident that the product manufacturer is providing the correct U-value calculation for applications, including inverted roofs. Ignoring them can lead to a significant underestimate of the amount of insulation required to achieve a target U-value.

The measure of heat loss associated with a building element can be calculated quickly using U-value calculation software. However, when it comes to inverted roofs, there are a number of vital factors that must be taken into account.

Rainwater Cooling

In an inverted flat roof construction, the insulation is placed above the waterproofing layer. While this brings advantages for the application, it can introduce an additional mechanism for heat loss; if permitted, rainwater will flow beneath the insulation boards and remove heat directly from the building fabric before finally running off the roof and down the rainwater drainage outlet. This is commonly known as the Rainwater Cooling effect and must be taken into account when calculating the U-value.

The Rainwater Cooling effect can be reduced by adding a water-reducing layer over the insulation and is an essential addition to the inverted roof insulation system. For this reason, Slimline Zero Membrane is used with Polyfoam®ECO Roofboard Extra as an integral part of the BBA approved Knauf Insulation Roof System.

Inverted roof U-value calculations are produced in accordance with BS EN ISO 6946:2007 adding a correction, known as Delta U. It takes into account the average rainfall in the building’s location and the percentage of the rainfall reaching the waterproofing layer.

If a water-reducing layer is not used and rainwater cooling is not minimised, there will be a dramatic increase in the insulation thickness needed to achieve the required U-value. For example, to achieve a U-value of 0.20W/m2K using a water-reducing layer, such as Slimline Zero membrane with Polyfoam®ECO Roofboard Extra, 160mm of material is required. By omitting the water-reducing layer, the thickness of insulation increases to 400mm.

Design and declared lambda values

In addition to this, we must also assess the individual materials that make up the construction element and know their respective thermal resistance (which is derived from the thermal conductivity – also known as lambda value – and thickness). For any building material used including insulation layers, manufacturers are required to provide accurate information for insulation products used in the U-value calculation.

The declared lambda value (thermal conductivity) refers to the value that is obtained for 90% of production, with a 90% confidence level, thus increasing assurance in the product’s thermal performance. In essence, it is the thermal conductivity of the product when it leaves the factory.

In practice though, the insulation in an inverted roof is subjected to environmental conditions (rainfall as well as freeze-thaw and moisture absorption by diffusion), which can have an effect on the efficiency of the thermal insulation.

Best practice and guidance on the design of inverted roofs can be found in the European Technical Approval Guidance (ETAG 031-1), which confirms that the declared lambda value should be corrected to take into account the roof construction and local climate conditions.

ETAG 031-1 states that the possible moisture absorption over time affects the efficiency of thermal insulation. Therefore, thermal values need to be corrected by assessing the total water absorption potential. By applying moisture correction factors, (in accordance with BS EN ISO 10456) the insulation is assigned a design lambda value, and it is these values which ETAG 031-1 state should be used in any U-value calculations for inverted roof systems.

In practice, due to the nature of the exposed conditions, only extruded and expanded polystyrene are suitable for inverted roof applications. In general, the moisture corrected design lambda values favour extruded polystyrene (XPS) products over expanded polystyrene (EPS) as the correction factor from declared to design lambda values can be far less. In simple terms, by using the lower XPS design lambda insulation boards, you will require less insulation to meet the U-value requirement.

To achieve a U-value of 0.18W/m2K, typical thicknesses are required:

XPS/EPS

XPS

EPS

Declared lambda value of – 0.033

Design lambda value of – 0.034

Design lambda value of  – 0.038

175

180

200

For example, an extruded polystyrene insulant such as our Polyfoam®ECO Roofboard is the ideal thermal insulation for an inverted roof, as it benefits from closed cell technology. It is highly moisture resistant and has high compressive strength, thereby delivering a robust solution.

Designers should, therefore, liaise with a reputable insulation manufacturer to obtain the correct design lambda value and rainwater cooling correction factor for inverted roof applications, to ensure the target U-value is achieved – and the roof fully complies with the Building Regulations.

Knauf Insulation is a fully accredited member of the prestigious and industry leading TIMSA/British Board of Agrément (BBA) U-value and Condensation Calculation Competency Scheme. This provides specifiers and contractors with utmost confidence that the inverted roof U-value and condensation calculations supplied by the UK’s leading insulation manufacturer are in line with all the relevant industry standards.

For more information, please visit www.knaufinsulation.co.uk/roofboard-extra

Karen Everitt

Product Manager of Polyfoam®XPS

Knauf Insulation

01744 76 66 66

technical.uk@knaufinsulation.com

www.knaufinsulation.co.uk

@KnaufUK

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