Understanding Thermal Bridging: How Insulation Fixings Impact Energy Efficiency

What Is Thermal Bridging?

Thermal bridging is when heat moves through a part of a building that conducts heat better than the materials around it. That path for heat to escape is called a thermal bridge. It also goes by the name cold bridging.

Think of insulation like a warm coat. If you put a metal zip through the coat, heat can travel out along the metal. The zip creates a path that bypasses the insulation. That is what a thermal bridge does in a wall.

Thermal bridges reduce how well your insulation works. They can also cause cold spots on internal surfaces. Cold spots attract condensation. Condensation can lead to damp and mould if left untreated.

There are two main types of thermal bridge in buildings:

• Repeating thermal bridges occur at regular intervals. Insulation fixings are a good example. Each fixing creates a small bridge across the insulation layer.
• Non-repeating thermal bridges occur at junctions. Examples include corners, window reveals, and roof-to-wall joints.

This guide focuses on repeating thermal bridges caused by fixings. These are the bridges you can directly control by choosing the right product.

How Insulation Fixings Cause Thermal Bridging

Insulation fixings hold insulation boards against a wall. They pass through the insulation layer to anchor into the substrate behind. That is the job they need to do.

The problem is that a fixing must cross the insulation layer to do its job. If the fixing is made from a material that conducts heat, it creates a direct path from the cold outside to the warm inside. The insulation sits around the fixing. But the fixing itself bypasses it.

Steel fixings are the biggest concern. Steel has a very high thermal conductivity. It moves heat far more easily than insulation materials. Even a small steel fixing can create a noticeable bridge.

The more fixings you use, the more bridges you create. A typical EWI installation uses multiple fixings per square metre. Each one is a small thermal bridge. Together they add up.

Plastic vs Metal Fixings: Which Conducts More Heat?

The material of a fixing has a direct effect on how much heat it carries through the insulation layer. Plastic and metal behave very differently when it comes to thermal conductivity.

Plastic fixings are not always suitable for every application. Pull-out resistance, fire classification, and board thickness all affect which product is right. But for thermal performance, plastic-bodied fixings consistently outperform metal ones.

What Are Thermal Head Fixings?

Thermal head fixings are designed to reduce the impact of any metal elements at the face of the fixing. They use a disc or cap made from a low-conductivity material over the head of the anchor. This sits on the outer face of the insulation board.

The head of a standard metal fixing sits at or near the surface of the insulation. It is the point where the fixing meets the render or outer finish. Without a thermal break here, this junction becomes a cold spot. Render may crack or fail around it over time.

A thermal cap or disc over the head serves two purposes. It spreads the load from the fixing across a wider area. And it reduces the direct thermal connection at the surface of the insulation.

Some fixings use a fully plastic body with only a steel nail inside. The nail provides pull-out resistance in the substrate. The plastic body means the shaft running through the insulation layer carries far less heat than a fully metal fixing would.

The Real Impact on Your Energy Bills

Thermal bridging through fixings adds to the total heat loss of a building. This is measured as part of the overall U-value calculation for a wall. More thermal bridges mean a higher effective U-value. A higher U-value means worse thermal performance.

In practice, the effect of individual fixings on a whole-house energy bill may be small. But across a fully insulated building with many fixings, it adds up. And in buildings where good U-values are required to meet Part L of the Building Regulations, thermal bridging through fixings can push performance below the required level.

Beyond energy bills, there is the matter of surface temperature. Thermal bridges lower the temperature of the internal wall surface at the bridge point. Cold surfaces can reach the dew point. At the dew point, moisture in the air condenses on the surface. This can lead to mould growth over time.

Mould is a health risk. It is also a sign that moisture is accumulating inside the building fabric. Choosing fixings that reduce thermal bridging is part of getting the full benefit from your insulation installation.

Choosing the Right Fixing to Reduce Bridging

Reducing thermal bridging through fixings comes down to three things: fixing material, head design, and fixing density.

Common Mistakes That Make Bridging Worse

Most thermal bridging problems caused by fixings come from one of a small number of avoidable errors. Here are the ones that come up most often on site.

• ⚠️
  Using standard steel anchors in place of insulation fixings A standard masonry anchor is not designed for use through insulation. It has no thermal break. The steel shaft runs directly from the outer face to the substrate. This creates a much larger thermal bridge than a purpose-made insulation fixing.
• ⚠️
  Over-driving fixings so the head sinks into the board Sinking the fixing head below the board surface can damage the insulation around it. It can also compress the board and reduce its effectiveness at that point. Drive the fixing so the disc or head sits flush with the board face.
• ⚠️
  Not using a thermal cap or disc where one is required Some systems supply fixings without caps and rely on the installer to fit the cap separately. Missing this step leaves a metal surface exposed at the insulation face. This creates a cold spot and can cause render failure around the fixing head over time.
• ⚠️
  Using too many fixings per board Adding extra fixings beyond the recommended pattern does not improve holding strength in a meaningful way once the correct number is in place. It does add more thermal bridges. Follow the system manufacturer's fixing layout.
• ⚠️
  Choosing a fixing based on price alone without checking conductivity Cheaper fixings are sometimes made from materials with higher thermal conductivity. The price saving per fixing is small. The performance impact over the whole wall can be significant. Check the material specification before purchasing.

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