From: Phil Slinger, Chief Executive, Council for Aluminium in Building

Since the last change to our legislative window U-values for new homes in Part L in 2013, little has changed, but, that is all set to change following the Government’s Future Homes Standard Consultation.  Our team at CAB have been working with our members to determine what these changes will mean for the industry and how practicable they are to achieve and whilst the legislation is concerned with new dwellings, it is likely that existing homes and new and existing commercial buildings will closely follow the incoming regulations for new homes.  CAB will continue to work with its membership and advise where we can in the developing legislation.

It is likely that the updated Part L (with the Approved Document to be split into Volume 1 for dwellings and Volume 2 for buildings other than dwellings) will be finalised and released at the end of 2021 to be introduced into legislation early in 2022, this will likely mean the introduction of window systems having to meet a lower U-value of 1.2 W/m2K (in England) for the ‘notional’ building and the possible introduction of triple glazing.  Following this change, it is likely that Part L will be further revised in 2025, possibly taking window U-values down to 0.8 W/m2K.  The changes are the result of the Government mission to achieving net zero carbon by 2050 and ensuring that new homes are built to be “zero carbon ready” as the National Electricity Grid decarbonises.

With the introduction of a revised air tightness requirement, down from the limiting value of 10 m³/h.m² at 50Pa to 8 m³/h.m² at 50Pa (with 5.0 m3/h.m2 at 50Pa for the notional building) using the ‘blower door’ test method or a new ‘Pulse’ test method, our building structures are likely to become even more airtight.  It is suggested that a house achieving an airtightness of 5 m³/h.m² will use 40 per cent less energy on space heating than a house built to 2013 standards of 10 m³/h.m².  With tighter airtightness this calls into question how we can retain heat in our buildings in winter whilst maintaining healthy indoor air quality for well-being with controllable natural ventilation.

Due to buildings increasingly experiencing overheating due to their designs, the overheating criteria currently within Approved Document Part L (currently Criterion 3) will be removed and a separate new Building Regulation and Approved Document for limiting overheating will be introduced.  This could begin to limit the use of glazing in buildings which contribute to summer overheating due to solar gain.  It is critical that any new regulation and guidance on overheating works with the existing energy efficiency requirements, and does not lead to any confusion, It is also important that appropriate methods of solar shading are fully considered before any reduction in the use of glazing. Two methods will be introduced for assessing the impact of overheating within new developments, a simplified method and dynamic thermal analysis.  Assessment will include the need to achieve adequate ventilation to remove excess heat.

During our recent webinars and technical meetings with our members, it is encouraging to see that our members are well prepared for the uplift in thermal requirements.  As most members have a range of systems available with various thermal capabilities, specifiers can choose systems based on capability and cost.  With future demands, older less efficient window systems will simply be phased out when sales volumes reduce.  It is important to remember that currently aluminium window system can achieve window U-values as low as 0.5 W/m²K and doors down to 0.6 W/m²K.

Will the change in thermal requirements in Part L and the revised guidance in Approved Document L Volume 1 change the material of choice for specifiers? Probably not, as both of the predominant materials chosen for new construction, aluminium and PVC, can achieve the U-values required, albeit they will be wider, more thermally efficient, may carry triple or quadruple glazing and be more expensive.

There will likely be a move to adopting differing designs of windows and doors in the future to make the most of high insulation glazing. This may mean a move to sliding systems for large glazed opening doors and the adoption of tilt turn window systems. Will this herald a move away from the UK’s love of the outward opening ‘casement’ window?  Time will tell if there is a move in this direction, but again CAB members offer various ranges already which include various types of opening window styles.

What does seem to be missing in this rapid move towards increasing thermal efficiency and airtightness in our future builds is the need for air changes, particularly in our homes. This is crucial for our well-being and continued health.  We know that sealing up a small dwelling will eventually lead to condensation appearing on the coolest elements in the room, sometimes a wall or an abutment with a ceiling, where dampness can turn quickly into an unhealthy mould growth.  This is particularly relevant in social housing where occupants may be reluctant to ventilate due to the cost incurred in reheating.  It therefore seems logical for future legislation to include mechanical ventilation with efficient heat recovery, or indeed cooling, of incoming fresh air depending on the time of year.

Another issue that we do need to attend to as an industry is the robust detailing of interfaces between windows and doors and the building structure.  Cold bridges often reduce the claimed efficiencies within a building construction that can reduce the effectiveness in use of a designed build by up to 50% of its claimed thermal efficiency.  As an industry, should we be doing more to incorporate or encourage specification of interface designs which reduce cold bridging to an absolute minimum?

The next big question is how will we bring all our existing housing stock up to a level of ‘zero carbon’ and who will pay?  We also await to see what the government are looking to legislate in our commercial buildings following the recent Future Buildings Standard consultation.

If you are currently in the window supply chain, why not join in the discussion by joining the association and being involved in the future of your Industry?

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