According to Peter Wilcox, technical manager at insulation specialist Recticel, the construction industry has a problem.
It’s a problem that affects us all – whether designers, contractors or occupants – new buildings aren’t meeting their designed energy efficiency targets. The shortfall is known as the performance gap and various studies into why ‘as-built’ doesn’t live up to ‘as-designed’ have resulted in some common observations applicable to all building types.
The assessment model – whether SAP or SBEM – predicts the energy consumption of a building and demonstrates regulatory compliance. Both models are a ‘one size fits all’ solution, so naturally include assumptions and generalisations. This highlights the importance of assessors using the calculation software being competent and the accuracy of the input data.
The biggest fault of assessment models is that they don’t account for ‘unregulated energy’, such as plug-in devices, domestic appliances or commercial IT/communication equipment. Identical buildings with identical results could be used in different ways, resulting in very different energy use and running costs.
Another fault of the assessment is that it is not used early enough in the design process, so the calculations only highlight issues when detailed design is complete and changes are harder to incorporate, leading to possible compromises on-site.
Design should be simple enough to be delivered on-site without improvisation. Complex, hard-to-insulate features will leak warm air, but balancing simplicity and aesthetics can be difficult. Planning authorities might expect a project to be in keeping with a local vernacular. If that style precludes better energy efficiency, is it an acceptable compromise?
Speed of construction is often emphasised over the quality of construction, and training of site operatives is not focused on energy performance. When sub-contractors change the work of the main contractor – particularly where it affects insulation and airtightness membranes – whose responsibility is the making good?
The scope for building control and approved inspectors to monitor work is limited, while architects have no contractual means of forcing through changes on-site.
CE marking of building products has levelled the playing field for laboratory testing and performance declaration, but results may not correspond with in situ performance. Climate is a factor that can influence the performance of buildings on a site-specific basis. For services, the perfect example is a tangle of flexible ducts that restricts air movement and causes them to achieve nowhere near the intended level of performance.
Poorly communicated product substitutions can also cause problems. Stockists may not carry the full range, so the contractor accepts an alternative, which doesn’t perform at the same level.
The RIBA Plan of Work 2013 has a stage called In Use, which requires as-built information to be updated in response to client feedback. A period of monitoring should highlight whether waste is occurring due to flaws in the building design, construction or commissioning of services. Understanding the finished product creates the opportunity to improve assumptions and predictions. Without that opportunity, designers will continue producing flawed solutions or, conversely, fail to recognise positives that can be incorporated into future projects.
The need for better communication Long before such monitoring can be considered, addressing the performance gap begins with the design brief. Clients must communicate their expectations of quality and energy performance to the architect, while the architect needs to demonstrate the efficiency benefits of their design to the client and planning authorities.
Communication to the contractor is also vital for set targets to be met and to ensure that any specification changes are appropriate and accounted for in the assessment model.
Part L 2013 (England) and 2014 (Wales) are assisting designers with a positive first step toward change. As in Scotland, notional dwelling specifications emphasise low U-values and good airtightness, but give designers flexibility to achieve the best solution for a project.
The idea is to achieve an efficient building fabric that will last for the life of the building, rather than offsetting poor U-values with costly renewable technology such as heat pumps or arrays of photovoltaic panels.
In England, along with the familiar Target Emission Rates (TER), new houses must also meet a Target Fabric Energy Efficiency rate (TFEE) performance standard.
As U-values reduce, thermal bridging becomes responsible for a greater proportion of overall heat loss. Linear thermal bridges – window reveals, junctions of building elements etc. – contribute around 30 per cent of heat loss from an otherwise well insulated property.
Simple design minimises the number of junctions and therefore, potential heat loss. Buildability is important too: difficult-to-construct junctions are more likely to suffer discontinuity of insulation and compromise the finished building’s performance.
The fabric first approach is about ensuring that insulation is reasonably continuous throughout the building envelope and that air permeability meets reasonable limits.
The Passivhaus standard operates on this basis, offering savings of up to 90 per cent on space heating costs. That standard is not realistic for every project but continuous insulation is proven to work, if done correctly.
The performance gap affects us all and we can all contribute to the solution. Taking a responsible, thoughtful and collective approach to basic construction principles will make a difference and provide future generations with quality, efficient and healthy buildings.