Thermal Innovation
Frequently asked questions
What is a thermal bridge?
Structural Thermal Breaks adderss thermal bridges in a building’s envelope. Thermal bridges are weak points/areas in a building’s insulated envelope where heat bypasses other, well insulated, elements. This is really easy to visualise in the infrared image here which highlights cold areas around the framing, floor connections and windows where heat is escaping an otherwise well insulated building.
What is a Structural Thermal Break?
A Structural Thermal Break is an insulating element that interrupts the flow of heat across a thermal bridge where there is also structural load. Examples where a Structural Thermal Break could be utilised include: • Column base, e.g. roof top plant room • Floor/wall connection • Lintels over doors or windows • Balcony connections • Slab-edges • External balustrate or staircase connections Our primary products are Structural Thermal Break plates which are rigid, structural elements that are placed in a structural connection, either between two steel plates or between a concrete and steel connection. The Thermal Conductivity of our Structural Thermal Breaks are about (product dependent) 100 to 1000 times lower than that of steel, meaning they significantly reduce the amount of heat transfer through the connection. There are many other factors (fasteners, detail geometry etc.) that also require consideration.
What are the benefits of using Structural Thermal Breaks?
1. They improve the thermal efficiency of a building. Minimised heat losses > reduced HVAC loading > minimised OPEX and minimsed operational carbon footprint. “Thermal bridges caused by uninsulated concrete slab edges and balconies can reduce the effective R-value of full-height wall assemblies by up to 60%” RDH 2017 study 2. They reduce the risk of condensation developing, which can result in mould, structural decay of timber and damage to interior finishes. Cold spots = mould spots. 3. They improve thermal comfort for building occupants at locations near thermal bridges.
Why have I never heard of a Structural Thermal Break?
Thermal bridges are not new concepts and Structural Thermal Breaks are widely used internationally. For example, BRANZ published a great paper back in 2005 on the subject (https://www.buildmagazine.org.nz/assets/PDF/B87-22-Condensation-and-thermal-bridges.pdf). However, in NZ, they are often overlooked because: • The impact of thermal bridging is underestimated. Because thermal bridges do not take up large areas of the building enclosure, their impact on energy performance is often discounted. However, various studies estimate the impact of thermal bridging on the heat loss across a thermal envelope to be 20% - 70% depending on detailing. • The impact of introducting structural thermal breaks is hard to calculate and therefore difficult to justiry • There is a lack of data available to enable analysis of thermal bridges. The lack of standardised industry data means that individual details are required to be calculated typically via numerical modelling, which is often cost prohibitive. The availability of standard details is via the like of the High Performance Construction Details Handbook is improving this. • It can be difficult to mitigate thermal breaks either due to practical reasons like lack of local product availability or because there is a lack of acceptable compliance pathways. • Mitigation of thermal bridging is not specifically required by H1 and so often it ends up in the ‘too hard basket’. There are requirements in the Passive House Standard (PHPP 10) and Homestar Version 5 but obviously there are only a limited handful of projects where these apply.
How is the impact of a thermal bridge calculated?
The only formal requirements to address thermal bridges in the NZ industry relate to Passive House and Homestar Version 5 which both contain criteria relating to surface temperature factor (fRsi) and Energy Loss. Surface temperature factor or fRsi is an index (ranging from 0 to 1). It is referred to as the hygiene criteria as it indicates the risk of mould occurring at a thermal bridge. Homestar adopts the PHPP approach and sets fRsi requirements for various details which vary for different climate zones. fRsi values for a catalogue of details are freely available in the High-Performance Construction Details Handbook. Energy Loss is represented as either a Psi (2 dimensional) or Chi (3 dimensional) value which can be calculated in a modelling software and applied to the length or number of points where it occurs. If it is a linear thermal bridge, or a psi value we multiply by the length of that thermal bridge to calculate the total energy loss of that junction. Again, the High-Performance Construction Details Handbook includes psi and chi values for a whole range of common details.
Can you avoid thermal bridges in the first place?
Yes, definitely! The best way to reduce the impact of thermal bridges is of course to get rid of them. This is not always possible, but it should be the first port of call. Common places to find thermal bridges are at the junctions between walls, roofs, floors, any change in geometry that affects the insulation layer. A good place to look for thermal bridges is on the structural engineer’s drawings. To remove thermal bridges from a design consider: • Simplifying the form factor • Introducing double & staggered wall assemblies • reducing framing factors (delete the nogs!!) • considered detailing at junctions • introducing raised heel trusses at eave junctions to maintain insulation depth • support balconies/canopies via external structure rather than extending internal structure to the exterior of a building.
Do I always need to address thermal bridges?
While all thermal bridges drive energy loss and increase the risk of condensation, some have more impact than others and when resources are limited (i.e. always!) these are the areas to address. Consider the following factors when evaluating the potential impact of a thermal bridge and the associated cost effectiveness of elimination or mitigation of that bridge: ● Is it suitable, durable or resilient for the climate and conditions? For example a high occupancy residential building in a humid climate needs greater attention to avoid mould growth at thermal bridges. This is reflected by the latest fRsi criteria in PHPP10 ● What Impact will it have on a building’s energy efficiency? Significant heat losses typically occur at the slab edge or at changes in geometry where the building interacts with the ground. Other linear elements, again such as slab edges, but also mid-floor junctions, balcony connections, parapets can have significant impact if the length of the element is significant. ● How many bridges are there? 3d point thermal bridges can have a significant impact on heat loss if they are repeating elements such as steel or concrete foundations or facade connection points.
More information
Find out more about Structural Thermal Breaks on these external websites which are great resources.
