One Year On: Thermal Regs in 2015

The merry-go-round of a General Election campaign ensures that meaningful political decisions are put on hold. For the construction industry, that means an interminable wait to see whether the Government’s long-promised 2016 introduction of ‘Zero Carbon’ into the Building Regulations actually happens – and if not in 2016, then when?

But amid all the speculation about future regulations, it’s easy to forget that 2015 will mark just the first anniversary of the current Part L regulations in England and in Wales.

Transitional arrangements 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 Fabric Energy Efficiency (England) or tighter U-value backstops (Wales) may still be relatively new.

For Scotland, meanwhile, 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, but they are based on the same theories. Up and down the country, then, 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 UK, the specification of a new building must be entered into SAP or SBEM software 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. Where the mindset of the designer needs to change is in getting away from thinking purely in terms of U-values to achieve the necessary performance.

Thermal Bridging

One aspect of building fabric common to the English, Welsh and Scottish regulations is accounting for thermal bridging. 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 a prime example, where perimeter upstand insulation needs to be carefully detailed (especially on low energy projects like a Passivhaus). 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 maximise their ‘buildability’, needs careful consideration to keep psi values, and therefore thermal bridging heat loss, low.

Thermal bridging’s importance has increased as U-values have lowered, for the simple reason that they account for a greater proportion of heat loss (up to 30% in an otherwise well insulated building). On some projects, however, psi values are not even being accounted for, instead adopting a conservative value offered by the regulations.

What effect does that have on 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 meets 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 0.13 W/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

The great thing about adopting a fabric first approach, including concentrating on the design and construction of thermal bridging details, is that it pays off with instant performance benefits.

Whichever part of the United Kingdom 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 get elected and get their act together.