What is a thermal bridge?
A thermal bridge (traditionally known as ‘cold bridge’) is defined as a part of the building envelope where the otherwise uniform heat flow path is locally altered. The most severe cases occur where the insulation is ‘short circuited’ by materials with a great thermal conductivity.
The temperature at internal surfaces is usually colder near a thermal bridge (e.g. window lintel or wall-to-ceiling junction). This can potentially lead to surface condensation or mould growth.
Thermal bridges typically allow an extra over heat flow through them, which is additional to U-values. This could be invisible in a DEAP assessment but definitely not in heating bills!
Why assess a thermal bridge?
Thermal bridging tends to be especially significant in well insulated buildings. Different studies carried out on the Continent (consolidated in an EPBD Information Paper) have measured that thermal bridge heat loss generally represents around 20% of the whole primary energy consumption of a low-energy building (such as any new-build compliant with Part L 2011 of the Irish Building Regs). This is so significant that it can easily overcome the solar hot water input! Minimising thermal bridging is a quick, smart and cost-effective way to limit that underperformance, without requiring additional energy saving measures elsewhere.
Thermal bridges are particularly concerning in internal insulation retrofits, due to the existing wall becoming colder. The risk of surface condensation at thermal bridges can potentially be higher after the retrofit than it was before.
Accurately assessing the thermal performance of each junction detail will become essential for progressive architects and developers of increasingly energy-focused clients.
A thermal bridge calculation:
· can ensure that a detail is free of mould and surface condensation
· can quantify the additional heat loss at a junction and identify ‘weak points’
· can be used, alongside ACDs or other details, to manually calculate a y-value to be entered in DEAP or SAP, improving the energy rating of the house and helping Part L compliance
· can allow rating a detail as ‘thermal bridge free’ so that it is suitable for Passive Houses
Our range of services
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Linear (2D) thermal bridge calculation
Linear thermal bridges are the most common type of thermal bridge: these happen at all junctions between plane elements (walls, floors, roofs and opes). A two-dimensional calculation (conforming to EN 10211 and BR 497) using Psi-Therm 2D software allows us determine:
· The additional heat loss at the linear thermal bridge, measured by the linear thermal transmittance (Ψ-value).
· The risk of surface condensation, measured by the temperature factor (fRsi).
Point (3D) thermal bridge calculation
Point thermal bridges happen where there is a point penetration of the insulation layer, such as a fixing or a steel post or beam. Unlike linear thermal bridges, these need to be calculated three-dimensionally. A three-dimensional calculation (conforming to EN 10211 and BR 497) using Psi-Therm 3D software allows us determine:
· The additional heat loss at the point thermal bridge, measured by the point thermal transmittance (χ-value).
· The risk of surface condensation, measured by the temperature factor (fRsi).
Calculation of whole-dwelling thermal bridge factor (y-value)
The y-value is an input in DEAP & SAP that accounts for the total thermal bridge heat loss of a dwelling. Default values are allowed but these are inaccurate and often penalise well-built dwellings. A calculated y-value (as opposed to default) can improve the DEAP or SAP rating, thus easing compliance and offering greater design flexibility.
· In order to calculate the y-value of a dwelling, Ψ-values for all its junctions need to be known
· The same dwelling can mix ACDs and custom (improved) details.
· There are Tables listing Ψ-values for all ACDs (Acceptable/Accredited Construction Details)
· For custom details, we can calculate Ψ-values (see linear thermal bridge calculation above)
Support in improving thermal performance at junctions
In addition to the thermal bridge calculation, we can offer guidance on how to improve the performance of a detail. While doing that we can model iterations of the same detail in order to quantify the value of the changes we propose.
U-value calculation
We can calculate U-values as per EN 6946 (the regular layered method) or EN 10211 (using thermal modelling software). The latter allows for a more accurate assessment of:
· Repeating thermal bridges happening at regular intervals (e.g. steel studs or fixings)
· Construction products of non-rectangular shape (e.g. insulating concrete blocks)
Case studies
Case1: 5907 DCC: supporting a public body with thermal assessment & design
Case 2: 5112 E2 Architecture: y-value calculation for architectural practice
Case 3: 5123 Kieran Carew: support for NSAI Agrément holder
Case 4: 5211 Thermal Bridge Mitigation: assisting in creation of new construction product
Case 1
Building Life Consultancy carried out a study for Dublin City Council on the retrofit of the Glover Court flats near St. Stephen’s Green. While the retrofit works were funded by the Department of Environment as a test case, working with us to analyse relative value was purely a DCC Architects Department initiative. DCC had a range of alternative propositions and needed guidance as to why, and by how much, one was better than the other. We had several exploratory meetings and carried out approximately 150 thermal bridge calculations across the four different options assessing Ψ-values and temperature factors for junctions in a limited number of indicative flats. With our understanding of architecture and building physics, we could argue the relative merits of various strategies at different levels.