Lee Davies, technical director at building envelope specialist, CA Group Limited, explains the role of robust detailing in delivering thermally efficient buildings.
The issue of improved airtightness has become a key focus for architects both in striving to deliver increasingly efficient buildings for their clients and in meeting the ever-demanding enhancements to building regulations.
Tests have proven that one of the primary causes of air leakage, or heat loss from a building, is poor detailing. Poor detailing leads to thermal bridging: the transference of heat from the inside of a building to the outside, often via poorly designed details, flashings at junctions and interfaces such as drip flashings, gutters and parapets.
To put this into context, some details such as drip flashings, which run around the entire perimeter of the building at the base of the walls, can, if they are not designed and installed with care, act as huge heat conductors, drawing heat from the warm interior and allowing it to be wasted, by passing to the exterior of the building envelope. Thermal bridging also increases the risk of condensation inside the building.
Pre-empting the issue at the building’s conception through the specification of thermally robust details, which mitigate this ‘heat-sink’ effect, can deliver a thermal performance increase of up to 10 per cent. This translates into a direct saving on the building’s energy consumption, reducing Psi values and greatly enhancing compliance with Approved Document L2A.
The application of robust detailing can significantly increase levels of airtightness, reducing a building’s associated heat loss by as much as 30 per cent. This realisation has led to some building envelope specialists assigning technical teams to develop a ‘gold standard’ which highlights precise detail specifications, along with their individual U and Psi values, to facilitate to input of calculations into SBEM.
In some cases, these details are delivered as standard, at no extra cost. However, where a choice of ‘standard’ and ‘enhanced’ details are on offer, the architect should carefully examine the options to determine which will provide the greatest protection against air leakage, prior to agreeing the specification.
The extent of the thermal bridging effect can vary from one material to another. Interface details comprised of different materials, such as concrete and brick, are common thermal bridges. 3D thermal modelling can be used to accurately assess the areas most likely to cause thermal bridging, enabling those designing the building to make the necessary adjustments and reduce, and sometimes even eliminate, heat loss from specific areas of the roof and walls at the design phase, greatly reducing costs.
Two examples of major heat loss due to poor design are drip details and parapets:
• A typical drip detail, with a drip flashing pinned to the base cladding rail (sketch 1), will produce a psi-value of approximately 1.60W/mK, whereas with properly designed details (sketch 2), this could be as low as 0.00W/mK
• A building with an inboard boundary wall gutter, complete with 1.1m high parapet (sketch 3), will produce a psi-value similar to the drip detail of 1.60W/mK, whereas if the detail was designed with an outboard boundary wall gutter, removing the parapet, the heat loss could be reduced to 0.02WmK.
The thermal performance of the two details can be easily equated back to a plane element U-value. The table9in gallery) indicates a variety of building sizes, adopting the same two details mentioned previously:
The provision of training for designers in robust detailing is vital and will facilitate the understanding of the associated benefits of improved airtightness from an architectural point of view. Contractors and installers also need to be trained to ensure that the theory is understood and carried through into practice. Regular site checks will ensure the necessary steps have been taken and that the work meets the required standards.
Where robust detailing has been specified, value engineering, at main contractor level, must also be closely scrutinised. It is at this stage, in an attempt to increase profit margins, that the finer attention to detail is often ignored in favour of a cheaper, less effective, alternative. Architects can ensure that main contractors do not deviate from this requirement by specifying precise details – which are available for download from reputable building envelope specialists – along with a stipulation that these should not be changed.