Building stock accounts for nearly half of all emissions in the UK. Paul Forrester, technical specialist at Recticel Insulation, explains why architects and specifiers need to think beyond simple U-value targets to get the best out of their designs and ease the route to compliance.
For the construction industry, the General Election campaign has meant an indeterminable wait to see whether the Government’s long-promised 2016 introduction of ‘Zero Carbon’ into the Building Regulations goes ahead.
But amid the speculation about future regulations, it’s easy to forget that 2015 will mark the first anniversary of the current Part L regulations in England and in Wales.
Transitional arrangements – whereby sites that had seen work commence before the introduction of the new Approved Documents could still meet the previous version – mean that architects and specifiers could easily have spent much of the last twelve months not having to think about Parts L1A and L2A 2013/14. Given that the U-value targets in Part L1B and L2B were unchanged, the need to consider the Fabric Energy Efficiency Standard (FEES) (England) or tighter U-value backstops (Wales) may still be relatively new.
For Scotland, October 2015 will see a revised Section 6 (Energy). Conversely, its contents have been available since October 2014 in order to give designers time to get acquainted.
Up and down the country, construction professionals are getting to grips with tighter and more complex energy efficiency requirements.
Basic principles
Beneath the surface of these different approaches, however, the same fundamental principles apply – wherever you are in the country, the specification of a new building must be entered into SAP (Standard Assessment Procedure) or SBEM (Simplified Building Energy Model) and compared to a ‘notional specification’ of the same design.
The effect of FEES and tighter limiting U-values is to encourage greater focus on the complete fabric of the building, rather than compensating for poor U-values with expensive ‘eco-bling’ technology that may not meet its efficiency claims and/or has a limited service life.
Of course, building fabric can fail to live up to its intended performance too, but when built correctly it will last for the life of the property. It’s the perfect illustration of ‘fit and forget’, meaning occupants can enjoy the benefits without having to think about changing or replacing it. To achieve this, designers need to turn away from thinking purely in terms of using U-values as a way of meeting targets.
Thermal bridging
When considering the performance of the building fabric, thermal bridging is an important factor. Designers are used to considering repeating thermal bridges, such as timber rafters or studs at specific centres, but perhaps less obvious are linear thermal bridges.
Wherever a thermal element changes direction or forms a junction with another element, the geometry of the element is altered and increases heat loss – particularly if the design fails to allow for the continuity of insulation at the junction. Wall/floor details are an obvious example, as everybody is familiar with perimeter upstand insulation. Insulated cavity closers around door and window openings illustrate another common linear bridging solution.
The heat loss attributed to a linear thermal bridge is called a psi-value. The simplicity of a design to minimise the number of junctions, and to make sure they are ‘buildable’, needs careful consideration to keep psi-values to a minimum. The importance of thermal bridging has increased as U-values have lowered for the simple reason that they now account for a greater proportion of heat loss (up to 30 per cent in an otherwise well insulated building).
But how much thought do you give to the issue when working on a new-build property? Do you:
• Arrange for the calculation of bespoke psi-values for each junction on every new project?
• Adopt pre-calculated psi-values, like Accredited and Enhanced Construction Details, in the design and ensure they are built accordingly on site?
• Take no account of thermal bridging and adopt a conservative value to calculate total heat loss due to thermal bridging, as offered by the regulations?
If the last option is your answer, are you aware of the likely effect on your compliance calculations? Take a SAP calculation for Part L1A 2013 in England: adopting the conservative approach to thermal bridging and then specifying everything else to match the compliant recipe offered in the Approved Document would mean failure in terms of both Target Emission Rate (TER) and Target Fabric Energy Efficiency (TFEE).
The result? It would take even lower U-values – and a much greater level of air tightness – than prescribed in the recipe to make sure the design met the necessary targets. Every design is different of course, but one example is needing to achieve a U-value of 0.10 W/m²K for the floor, walls and roof – compared to their respective recipe values of 0.13, 0.18 and 0.13W/m²K.
That’s a lot of extra insulation to accommodate! And even though the regulations for Wales and Scotland don’t feature a TFEE, the same stringent recipe approach means it is still necessary to think about thermal bridging details – and that won’t change in future editions of the regulations.
No need to wait
Adopting a fabric first approach by concentrating on the design and construction of thermal bridging details, pays off with instant performance benefits that will ultimately help to limit the impact of climate change.
Whichever part of the UK you are working in, good building fabric improves thermal comfort and reduces heating demand – and that can be achieved under current regulations, without waiting for politicians to make their minds up.