First published in the June 2019 issue of Energy Matters
The evidence is continuing to grow – thermal bridging can have a big impact on your building’s energy use and its ability to withstand outdoor temperature changes. Knowing what thermal bridging is and how to prevent it can reduce your building’s heating and cooling energy use.
What is Thermal Bridging?
Heat will always move along the path of least resistance. Thermal bridging happens when a material transfers heat at a faster rate than surrounding components. For instance, we use insulation in our building envelopes to help stop heat loss in winter and heat gain in summer. But insulation is often installed in parallel with more conductive materials, like wood, metal, and concrete. These materials conduct heat more readily than the insulation they surround and act as “bridges” for heat to flow across.
Where do Thermal Bridges occur?
Thermal bridges typically occur at breaks in insulation in walls, roofs, and other building envelope components. They often occur at interaction points or “interfaces” between different components, such as slabs, parapets, and glazing, or components in a wall assembly, as well as corners, wall intersections, cladding attachments, and structural beams. They can also happen where insulation is lacking.
Why do Thermal Bridges matter?
Thermal bridging greatly affects the thermal performance of the building envelope. When thermal bridges allow heat to bypass the insulation layer, the effectiveness of the insulation is reduced so that more energy is needed to replace the lost heat in winter and cool a building in summer. Thermal bridges can also increase the risk of condensation and mold and reduce resident comfort.
How is Thermal Bridging prevented?
You may think that adding more insulation might reduce the impact of the thermal bridging, but this is usually not the case. This is because thermal bridges allow heat to bypass the insulation.
To prevent thermal bridging, thermal breaks are needed. Usually a thermal break is comprised of a material that does not conduct heat well and is placed in an assembly between more conductive materials. For example, in aluminum windows, rigid polyurethane, which has low heat conductivity, can be inserted as a barrier between the more heat-conductive metal inner and outer frames.
Similarly, inserting structural thermal breaks between the balcony and floor slab will also reduce heat flow between a building’s interior and exterior. A structural thermal break is load-bearing as well, and will help maintain structural integrity by transferring loads between the balcony and floor.
In new buildings, thermal breaks should be included in the design to prevent thermal bridging. For example, wall and roof assemblies, windows, and basement slabs should be designed to minimize heat transfer. Techniques as simple as sealing building envelope penetrations with foam, caulking, and tape can dramatically improve air tightness. In fact, certain energy efficiency building standards, such as LEED and Passive House, emphasize the importance of addressing thermal bridging as part of the overall quality of the design. Costs to address thermal bridging are minimal when considered at the design stage of a new building.
Thermal bridges are likely numerous in existing buildings, especially those with balconies and single-glazed, metal window frames. Although addressing thermal bridging in existing buildings can be costly and often cost prohibitive, window replacements present an ideal opportunity to address a key area for thermal bridging. Replacing old windows with triple-pane, vinyl frame windows or at least metal frame with thermal breaks, will help to reduce heat transfer in and out of windows each season. For information on choosing windows, see our article in the Q1 2019 issue of Energy Matters. Installing insulating cladding can also help reduce thermal bridging in multi-residential buildings, while improving insulation can mitigate them in houses.
Taking even the smallest steps to reduce thermal bridging can have big impacts on your energy consumption. Whether dealing with a new building or improving an existing structure, consider what steps you can take to prevent thermal bridging from negatively impacting your energy costs and resident comfort.
Want to learn more?
BC Hydro – Building Envelope Thermal Bridging Guide
Passive House elearning – Thermal Bridges
Zero Carbon Hub – Thermal Bridging Guide
University of Toronto – Resilience Planning Guide, p. 18-21