What is thermal bridging in curtain walling?

Thermal bridging in curtain walling can lead to heat loss. Discover the key measures we take to avoid it.

Thermal bridges – also known as "cold bridges", "heat bridges" and "thermal bypasses" – aren't unique to curtain walls. They apply to any object where one area has higher thermal conductivity than the surrounding materials. This lets heat pass through, reducing the object's thermal resistance.

In curtain walling, this happens when heat passes through conductive materials such as aluminium and steel. Heat penetrates the insulation layer and gets lost.

When heat bypasses insulation in this way, more energy is required to heat and cool a space. What's more, condensation can build up in the building envelope, leading to mould – not to mention discomfort for the people using the building.

The construction industry knows the importance of insulation. Yet thermal bridging in building envelopes is more common than you might think and is often a nasty surprise to clients.

Here at Wrightstyle, our curtain wall systems are thermally broken, meaning they provide thermal breaks that eliminate thermal bridging. It's one example of why our glazing systems are the best in the business.

The importance of conduction in understanding thermal bridging

Heat transfer happens in three ways: convection, radiation and conduction. In thermal bridging, whether in a curtain wall or other architectural component, heat transfers through conduction.

The rate at which the heat is transferred depends on two things:

The thermal conductivity of the material on each side of the thermal bridge

The difference in temperature between them

Heat follows the path of least resistance through the material with the highest thermal conductivity and the lowest resistance. In the case of curtain walls, this path or thermal bridge can take place in three places:

Structural connections, such as metal fixings and brackets that anchor the curtain wall to the building's main structure

The frame itself – steel or aluminium mullions and transoms that haven't been fitted with non-conductive thermal break material

Junction points, located where the curtain wall connects to the floor slab, roof or cladding. If the junction is inadequately sealed, thermal bridging can occur.

What are the key causes of thermal bridging in curtain walling?

Curtain walls form part of a building's thermal envelope. This is the physical barrier that separates a building's conditioned interior – whether heated or cooled – from the unconditioned outdoors. It includes walls, roofs, floors, windows and doors as well as curtain walls.

Curtain wall frames are often made from aluminium. This is a highly conductive material with a typical thermal conductivity of more than 200 W/mK (watts per meter-Kelvin). In most cases, the frame extends from the building's exterior to its interior, making it prone to thermal bridges.

Here at Wrightstyle, our curtain wall systems – and indeed all our glazing systems – are made from steel. This is for a number of reasons, including steel's superior strength. It's also, however, because steel is far less thermally conductive than aluminium.

Mild carbon steel has a thermal conductivity of around 50 W/mK, while stainless steel performs better with a thermal conductivity of around 14-16 W/mK. This makes it an ideal choice of material for a curtain wall glazing system.

What are the impacts of thermal bridging?

There are no positive outcomes from thermal bridging. It brings only heat loss, condensation and increased energy costs.

Thermal bridging makes buildings less energy-efficient – hotter in summer and colder in winter. This can lead building owners and users to rely more heavily on cooling and heating systems.

These come at a price, of course. Thermal bridging has a direct negative impact on energy bills, as well as the building's environmental footprint.

Thermal bridging also incurs unwanted costs by leading to mould growth and damage to interiors. This is because the cold spots on the inside of the frames or glass cause moisture to accumulate.

Thermal bridging is a problem. So, what are the solutions?

How can thermal bridging be prevented?

One solution for thermal bridging is to include thermal breaks. These are plastic or composite elements placed within the frame.

They're typically a quarter of an inch to one inch thick and made from a low-conductivity material, such as polyamide or nylon. Placed between the exterior faceplate and the structural mullion, they break the conductive path and stop heat from being transferred.

Other solutions include:

• Limiting the number of building components that span from the inside to the outside
• Ensuring continuous insulation around the fixings and at slab edges
• Investing in so-called "warm-frame" designs
  
  

Thermal breaks are among the most effective ways to reduce thermal bridging in curtain wall systems. For this reason, the curtain walls in our SR series are all thermally broken.

Where else is thermal bridging found in construction?

Thermal bridging typically occurs at junctions between two or more building elements. This can include:

• Areas with poor insulation
• Door-to-wall junctions
• Floor-to-wall junctions
• Balcony-to-wall junctions
• Metal ties in cavity walls
• Roof-to-wall junctions
• Wall-to-wall junctions
• Window-to-wall junctions
• Window and doors frames
• Studs and joists in exterior walls, ceilings or roofs
  
  

What else affects the thermal efficiency of a curtain wall?

Thermal bridging is a common cause of thermal inefficiency in curtain wall systems – but it's not the only one.

The type of glass used also plays its part. This is why thermally efficient glazed curtain walls will typically use double or triple glazing.

There's also the question of installation. If a curtain wall is poorly installed, thermal bridging may follow. It's essential that installers ensure proper sealing and minimise bridging at fasteners and plates.

When sourcing a curtain wall glazing system, make sure to look for markers of quality. These will minimise your chances of thermal bridging and other thermal inefficiencies when the product is installed.

Wrightstyle is a trusted UK curtain wall company and supplier of specialised glazing systems. Don't hesitate to get in touch with our experts for comprehensive product and technical advice.