Happy Christmas! Advent calendar no 8
Behind the door is…
The Eight Associates U-Value Calculator
(and a “Thermal bridging 101” jolly good read)
The Eight Associates U-value Calculator is a handy tool that generates u-values, and allows decision-making to take place in meetings to accelerate the design process.
Download now: Eight Associates U-Value Calculator
What is thermal bridging and why does thermal bridging analysis matter?
Thermal bridges are ‘weak spots’ which occur where there is a break in the continuity of, or penetration through, the insulation in a building envelope. Thermal bridging is heat loss at a junction between two thermal elements. The benefits of thermal bridging analysis are wide-ranging and applicable to building owners, developers, occupants and design teams. It’s more than avoiding ‘cold bridges’ – it is a systematic heat loss analysis and subsequent design refinement approach that can maximise available floor space, reduce build cost and reduce carbon emissions.
Commissioning thermal bridging analysis early in a project can lead to a reduction in external wall insulation thickness, adding to the overall available floor area of a scheme. Demonstration of low heat loss through linear junctions is fundamental to achieving optimal fabric energy efficiency targets, a requirement of Part L 2013, and it is a low-cost intervention in reducing a building’s operational carbon emissions. Reducing heat loss via thermal bridging decreases heating demand and improves occupant thermal comfort. In accordance with Part C, it also mitigates the risk of condensation on interior surfaces – good for occupant health and the durability of the building.
Linear thermal bridges are calculated as a ?-value (Psi-value, W/mK) and, under SAP methodology, expressed as the average Y-value (W/m2K) of overall fabric heat loss. Thermal bridging analysis calculations need to be performed to reduce the impact of thermal bridging in SAP/SBEM assessments – and the improvement in the fabric energy efficiency output figure can be substantial. For example, SAP assumes a default average Y-value of 0.15W/m2K; the figure is expressed as a penalty to the overall building U-value. It’s worth noting that when calculated, standard architectural details will often achieve an improvement over default values. For example, calculations resulting in a reduced average Y-value of 0.08W/m2K will significantly reduce this penalty.
The time and associated costs of analysis and calculation of an average Y-value depend largely on the size and complexity of the scheme in question. Cost-effectiveness can be achieved through calculating the highest-impact junctions first (such as the window reveals, roof and ground floor perimeter details) and undertaking subsequent calculations as needed until the targeted value is reached.
Thermal bridging analysis should be carried out as soon as a lower-than-default average Y-value is targeted (preferably at RIBA Stage 3, where the overall energy strategy and architectural details are being drafted), to ensure an optimised design process and prevent delays, design iterations and unnecessary costs. With a good thermal bridging package typically achieving carbon reductions of 8-10% over the Part L baseline, it is often an invaluable method of achieving high carbon and energy saving targets.
Thermal bridging analysis reduces the costs associated with the fabric specification and triple glazing, and potentially the need for additional renewables elsewhere in the design. Confirmation of reduced thermal bridging can prevent the need for last-minute addition of further thermal insulation or improved air tightness to achieve fabric energy efficiency targets, saving both time and costs in the process. As long as an expert practitioner carries out the calculations, and has access to sufficient information early enough – thermal bridging analysis can, and should, be easily incorporated into any project.