U-values they are a-changing

Window U-values have made the headlines recently in the Future Homes / Building Standards and Home Energy Model (HEM) consultations, but there are also changes impacting on the U-values of glass.

Now closing on 27th March (extended from 6th March), the consultations propose how the government intends to achieve the Future Homes Standard and Future Buildings Standard in England, including the new HEM calculation methodology.? They also set out intended changes to the Building Regulations, the associated Approved Document guidance and calculation methods.

Surprising to many, for new residential buildings, no changes have been proposed to the current limiting (‘back stop’) and notional U-values for walls, roofs, floors and windows.? This seems to be a missed opportunity to reduce energy demand and minimise running costs.?

However, the proposals include a change to the way in which U-values are calculated for windows and doors.? Currently, they are based on a ‘reference’ size – with an opening of 1.23 m wide by 1.48 m high, with a central divider and comprising one opening light and one fixed light - which is larger than most residential windows.? The proposal is for U-values of windows and doors to be calculated based on the actual size and configuration of the windows and doors being used.?

In a separate development, recent and forthcoming changes to standards affecting the calculation of centre pane U-values for glass should not be overlooked.? The changes are only small, but it will mean providers of U-value calculators will need to make adjustments to their methodologies.

EN 12898: Glass in building – Determination of the emissivity was revised in 2019, resulting in changes to the way in which emissivity is calculated and used in the determination of the (centre pane) U-value of glass in accordance with EN 673: Glass in building - Determination of thermal transmittance (U value) - Calculation method.

The main changes compared to the previous (2001) edition are the following:

— introduction of a method to determine the emissivity using Fourier Transform Infrared (FTIR) spectrophotometers where the spectral range is limited;

— provision of a new method for the calculation of corrected emissivity; and

— clarification of rounding rules for normal emissivity.

The last two changes in particular have a (small) impact on the calculation of the U-value of the glass.?

Furthermore, EN 673 is currently undergoing revision, including a modification of the calculation of the internal heat transfer coefficient for vertical glazing for consistency with EN ISO 6946: Building components and building elements - Thermal resistance and thermal transmittance - Calculation methods.? This is also anticipated to have a (small) impact on the calculation of U-value.? In EN 673, very minor changes have been proposed to some of the gas properties for consistency with ISO 52022-3: Energy performance of buildings — Thermal, solar and daylight properties of building components and elements — Part 3: Detailed calculation method of the solar and daylight characteristics for solar protection devices combined with glazing.

Fortunately, help is at hand with glass manufacturers releasing new versions of calculation software. ?Pilkington Spectrum, for example, already incorporates the 2019 changes to EN 12898 and plans are in place to release an update with the forthcoming changes to EN 673, as and when the latter is published.

We’re in changing times regarding U-values, not just in relation to the way in which windows may be considered for compliance with the Future Homes Standard and Part L, but also in determination of the contribution from the glass element.? The industry, and especially providers of U-value calculators, will need to get up-to-speed and introduce these changes.

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