What is a thermal bridge and what damage can it do in a passive house?
Thermal bridges are one of the most important concepts in passive house design, and one of the least understood outside the building performance community. They’re invisible once a home is built, they’re rarely mentioned in conventional construction conversations and their consequences can range from mildly annoying to damaging. Understanding what they are and why passive house design takes them so seriously is one of the more useful pieces of technical knowledge you can bring to a new build project.
What is a thermal bridge?
A thermal bridge is a weak point in a building’s insulation – a spot where heat finds an easier path through the building fabric than the insulation around it would normally allow. It’s a pathway of lower thermal resistance through which heat flows preferentially because the surrounding materials resist it more effectively.
The most common thermal bridges in conventional Australian construction are structural elements that connect the inside and outside of the building fabric. A steel beam that runs through an insulated wall. A concrete floor slab that extends through the building envelope to form a balcony. A window frame that connects the interior and exterior faces of the wall. Roof battens fixed through an insulation layer. All of these create pathways through which heat moves more readily than through the insulated assembly around them.
The word “bridge” is appropriate because just as a bridge allows people to cross a barrier that would otherwise stop them, a thermal bridge allows heat to cross an insulation barrier that would otherwise stop it. And just as a bridge concentrates foot traffic at a single point, a thermal bridge concentrates heat flow at a specific location in the building fabric, producing effects that are disproportionate to the area it occupies.
Why does a thermal bridge matter in a passive house?
In a conventional Australian home, thermal bridges are a known issue but an accepted one. The building fabric is already performing poorly enough that the additional heat loss through a thermal bridge is a relatively small addition to a large total.
In a passive house design, the calculation changes entirely. The building envelope has been carefully optimised to achieve very low levels of heat loss through its insulated fabric. In that context, a thermal bridge that would be a minor issue in a conventional home becomes proportionally much more significant because it’s a high-conductivity pathway through a fabric that’s otherwise performing very well. Passive house designers in Australia treat thermal bridge elimination as one of the primary design objectives.
What are the consequences of thermal bridges in a passive house?
Thermal bridges produce three distinct physical consequences, all of which matter in a passive house context.
The first is heat loss. A thermal bridge conducts heat from the warm interior to the cold exterior in winter – or from the hot exterior to the cool interior in summer – at a rate higher than the surrounding insulated assembly. This additional heat flow adds to the building’s heating or cooling demand, undermining the performance that the insulation elsewhere in the envelope is delivering.
The second is surface temperature reduction. A thermal bridge creates a cold spot on the interior surface of the building fabric at the point where heat is being conducted away. In winter, this cold spot can be significantly cooler than the surrounding wall surface. In some cases, it can be cool enough to feel noticeably cold to the touch or to create a sensation of radiant coldness in the room nearby.
The third – and potentially most damaging – consequence is condensation and mould. When the interior surface of a wall, floor or ceiling drops below the dew point of the indoor air, moisture from the air condenses on that surface. Over time, repeated condensation at a thermal bridge location creates the conditions for mould growth on the interior surface where it’s visible, or within the building fabric where it isn’t.
Where do thermal bridges typically occur?
You tend to find thermal bridges wherever there’s a break in the insulation layer – a point where a higher-conductivity material connects the inside and outside of the building fabric. In Australian residential construction, the most common locations are:
Window and door frames, where the frame connects the interior and exterior faces of the wall and – in aluminium frames particularly – conducts heat very readily. This is why passive house windows use thermally broken frames that interrupt the conductive pathway through the frame material.
Slab edges, where the concrete floor slab meets the external wall. In a conventional home, the slab edge is often uninsulated or poorly insulated, creating a significant linear thermal bridge around the perimeter of the building at floor level. In passive house design in Australia, the slab edge is one of the most carefully detailed thermal bridge locations, typically insulated with a continuous layer of rigid insulation that wraps around the perimeter of the slab to interrupt the conductive pathway.
Structural elements that penetrate the insulation layer, such as steel columns, concrete beams or timber posts, where the structural material conducts heat through or around the insulation. In passive house design, structural elements are either kept within the insulated envelope or separated from the external environment by a continuous layer of external insulation.
Balconies and cantilevered elements, where a structural connection between the interior floor and an external balcony slab creates a powerful thermal bridge at the junction.
Roof-to-wall junctions, eave details and other building envelope connections where the continuity of the insulation layer needs to be maintained around a geometric change in the building form.
How does passive house design address thermal bridges?
Passive house design addresses thermal bridges through two complementary strategies: elimination and calculation.
Elimination involves designing the building envelope so that thermal bridges don’t exist or are reduced to the minimum achievable. This means keeping structural elements within the insulated envelope, using thermally broken window frames and balcony connectors, wrapping the slab edge in continuous insulation and detailing all junctions and connections to maintain the continuity of the insulation layer.
Calculation involves quantifying the thermal bridging that can’t be eliminated and accounting for it in the energy model. Every thermal bridge in a passive house design is assessed using specialist thermal modelling software that calculates its heat loss contribution. These values feed into the overall energy model, allowing the design team to verify that the building’s total heat loss still meets the Passive House Standard’s performance criteria.
This combination of elimination and calculation is what distinguishes passive house designs in Australia from conventional construction, where thermal bridges are neither eliminated nor calculated – they’re simply accepted as an inherent feature of how buildings are built.
