What is cavity wall core insulation?

It is the technique of placing insulation material (EPS, XPS or rock wool) between two rows of bricks - an inner and an outer leaf - creating a 'sandwich'.

Why does the cavity wall have a thermal bridge problem?

Because insulation only goes between the bricks. Concrete columns and beams remain uninsulated, leaking energy like open windows.

What is the real U-Value of a cavity wall with uninsulated concrete?

While the brick section can reach 0.55 W/m²K, the average shoots up to 1.15 W/m²K due to thermal bridges from the structural frame.

Cavity Wall Core Insulation - How It Works & Why It Dominated Greek Construction

If you've ever watched a Greek building being constructed in recent decades, the image is familiar: masons raise a row of bricks, place blue or white polystyrene slabs in between, and then build another row on the outside.

This technique is called Cavity Wall Core Insulation. From the 1980s until the KENAK (Building Energy Performance Regulation) came into force in 2010, 90% of homes in Greece were built this way. Although external ETICS insulation has now taken the lead, the cavity wall remains a huge chapter in construction, either because your house is already built this way, or because certain new builds choose it for specific architectural reasons (e.g. exposed stone or brick facades). Let's dissect this wall and see exactly how it works.

1. The Anatomy of the Cavity Wall: A Resistance "Sandwich"

A properly built cavity wall (also known as a double-leaf wall with gap) resembles a sandwich. It consists of these layers, from inside (room) to outside:

Cavity wall cross-section - layers from interior plaster to outer masonry

🧱 Internal Plaster & Masonry

The first layer is the interior plaster, followed by the inner masonry leaf - typically a single brick. This wall faces the room.

🛡️ The Core (Insulation Material)

This is where the insulation boards go - usually Extruded Polystyrene (XPS), Expanded Polystyrene (EPS) or Rock Wool. This is the "heart" of the system.

💨 Air Gap (Optional)

A small gap of 2-3 centimetres between the insulation and the outer brick, which helps the wall breathe and prevents moisture transfer. Ideally, this should always be present.

🏗️ Outer Masonry & Plaster

The external brick exposed to the environment, followed by the exterior plaster. This forms the last line of defence against rain and wind.

2. Core Insulation Advantages: Why We Built This Way

Why did we (and sometimes still do) build this way? The cavity wall offers significant advantages that cannot be ignored.

Cavity wall advantages - protection, thermal mass, sound insulation

🛡️ Total Insulation Protection

The insulation is encased and protected between two walls. It is safe from impacts, weather, birds or rodents. Its service life is essentially equal to the building's.

🌡️ Thermal Capacity on Both Sides

The inner wall absorbs your radiator's warmth and retains it, while the outer wall absorbs summer heat before it even reaches the insulation (Time Lag).

🏠 Free External Appearance

You can leave the outer wall bare - exposed brick or stone - something that is impossible with ETICS, which "wraps" everything in render.

🔇 Excellent Sound Insulation

The alternation of materials (plaster, heavy brick, light insulation, air gap, heavy brick) works as a perfect sound trap, offering superior acoustic comfort.

3. The Big Trap: Structural Thermal Bridges

If core insulation is so good, why did we abandon it for new builds? The key word is: The Structural Frame (Concrete).

In traditional cavity wall construction, insulation goes only between the bricks. However, the house also has columns, beams and slabs made of reinforced concrete (ring beams). These elements interrupt the wall.

Thermal bridges at uninsulated concrete columns and beams

❌ Uninsulated Concrete = Open Motorway

If the beams and columns remain uninsulated (as was usual in the past), heat finds its way out through them. Your house is insulated at the bricks but "bleeds" energy through the concrete.

4. The 10x10 Model Experiment

Let's take the 10x10 Model and assume we build its walls as a cavity wall with 5cm EPS in the core.

10x10 Model - U-Value of cavity wall with thermal bridges

✅ Brick Only

If we measure only the brick section, the U-Value is excellent, approximately 0.55 W/(m²K).

❌ Uninsulated Concrete

Yet the wall also contains columns/beams (roughly 25% of the façade area), which we left uninsulated. There the U-Value is 3.00 W/(m²K)!

📊 The Result

The average (overall wall U-Value) shoots up to about 1.15 W/(m²K). We paid for double masonry and insulation, but because of thermal bridges we lose almost double the energy compared to an ETICS system of the same thickness that would wrap everything.

💡 Conclusion: Core insulation is an excellent and durable system. However, to be considered energy-efficient by today's standards, it requires careful design so that all columns and beams are also insulated (externally), ensuring continuity with the core insulation without leaving any gaps.

Insufflation Method: Insulating Existing Wall Cavities Placing Extruded Polystyrene (XPS) Between the Bricks Thermal Bridges in Cavity Walls: Insulating Columns & Ring Beams External vs Internal Insulation: Comparative Selection Guide

Insulation Systems: Complete Solutions for Every Building

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