5 February 2016
By Guest post by Thomas Lefevre of Etude

Fabric Energy Efficiency – Going beyond U-values

There is now a consensus that improving the energy efficiency of building fabric is the first fundamental step towards low carbon new buildings. Gone are the days when local authorities, developers and their engineers thought that mitigating climate change had only to do with renewable energy: we all know that efforts need to be made on all fronts, starting with the building fabric.


Over the last ten years, the building industry’s understanding of how to improve fabric energy efficiency has increased considerably. Led by the pioneering and exemplary work on Passivhaus projects and by organisations such as the Zero Carbon Hub, fabric energy efficiency is no longer only about target U-values.


There are a number of key tips we communicate to design teams to help them understand, and therefore improve, fabric energy efficiency.


First and foremost the form of the building is really important. The form factor assesses the ratio between heat loss surfaces (e.g. external walls, roof, exposed floor) and the internal floor area. The higher the ratio is, the more heat the building will lose for a given internal floor area. You may think that there is not a lot that can be done to improve a form factor but you would be surprised to see how simple decisions can have a significant impact.


Complexity of architectural design is also very important. Buildings should be energy efficient but that should not restrict the ability of architects to design the right building in its context. So far from promoting an efficient square box designed by an engineer, energy efficiency is about giving the architects the tools to understand how to balance design decisions to deliver architectural interest without negatively impacting on fabric efficiency. One such example is to reduce vertical inconsistencies but introduce horizontal variations.


U-values should no longer be a simple calculation based on the most effective section. Design values need to be chosen based on a realistic and thorough assessment of predicted thermal performance, not best case values. The effect of timber/steel/concrete bridging any insulation layers, ties and brackets and specific insulation products should all be considered in order to set targets that are actually deliverable.


The same goes for the thermal performance of junctions. Architects and structural engineers need to work more closely with energy engineers to understand, reduce or even design out inefficient junctions and details, both in terms of thermal bridges and airtightness. A direct example of this is the effect of steel support system for medium and high rise brick cladding, often wrongly ignored in U-value calculations.


How to measure fabric energy efficiency though? It is actually easier than most people think, and can be simply integrated very early in the design. Simple calculation tools such as PHPP (used on Passivhaus projects) can provide you with a detailed understanding of fabric energy efficiency, but we are also having success monitoring performance using the Fabric Energy Efficiency Standard (FEES). It is an indicator that should be used much more widely than it is at the moment. It complements the more usual Part L indicators and helps the project team understand more directly the impact of their decisions on fabric energy efficiency.


Getting the design right, is of course only half the battle; delivery on site is equally important. The performance gap is a well-discussed issue and a number of very useful publications encourage good quality construction by identifying what could go wrong on site and how to prevent those issues. It is difficult to deliver any ambition without someone taking responsibility for checking quality on site and taking pride in the delivery of the energy efficient design. So, finally we need to put on our boots and hard hat, work with site teams and instill a passion for delivering better buildings across the whole value chain.


Thomas Lefevre is Director of Etude, a sustainability engineering consultancy and a trusted collaborator with KLH Sustainability on projects, including East Wick & Sweetwater and Bristol Arena.


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