As we saw in the first article of this section, core insulation in the cavity wall does an excellent job of insulating the… bricks. But what about the remaining 25% to 30% of a building's façade, which is made of reinforced concrete (columns, beams, slabs and ring beams)?
In older buildings, these elements were left completely bare. The result? Heat from the house found an open "motorway" to escape outside. These leakage points are called Thermal Bridges. If you're building with a cavity wall today, eliminating these bridges is legally (per KENAK) and practically mandatory. Let's see how it's done correctly.
In a typical Greek house, the structural frame (concrete skeleton) accounts for 20-30% of the external wall area. The reason it creates thermal bridges is physical:
Concrete is an excellent heat conductor - it cools and heats far faster than brick. Reinforced concrete has a thermal conductivity of λ ≈ 2.0 W/mK, whereas a hollow brick is only 0.4 W/mK. Beams and columns conduct heat 5 times faster than the rest of the wall.
When you have a cavity wall with insulation in the middle, but the column at the edge of the wall is uninsulated, the radiator works overtime to compensate for the heat escaping through the concrete.
In winter, the inner surface of an uninsulated column can drop to 8-10°C, while the adjacent wall maintains 18°C. Room moisture condenses on the cold surface - which is why in older homes you see black mould high up in the corners of the ceiling (where beam meets column).
The solution must be designed before the concrete is poured. The structural engineer and architect must collaborate so that every column, beam and ring beam is insulated.
Columns cannot be "split" down the middle (they'd lose strength). They must be externally clad with special XPS boards (5cm) with a scored (rough) or grooved surface. These are placed inside the formwork before the concrete is poured. When the concrete sets, it "embraces" the rough XPS surface and they become one body.
Ring beams (horizontal concrete strips that tie the brickwork) in older buildings were poured across the full width of the cavity wall, cutting the insulation in two. Today, a 5cm XPS strip is placed in the middle of the formwork. This way, the ring beam "splits" into two parts while the insulation remains continuous from floor to ceiling.
The biggest challenge is not insulating the column or wall in isolation. It's joining the two insulations without gaps.
The most critical point in construction is the joint. The column's external insulation must overlap (lap over) the insulation inside the cavity wall. Not a single millimetre of gap should remain between the column's XPS and the brickwork XPS. Every joint is sealed with special foam tape or polyurethane foam.
If the column's external face is to be plastered, the XPS board must have a scored (grooved) surface so the render bonds well. If instead the outer wall is exposed brick, the column insulation will be "hidden" behind it.
Let's take the 10x10 Model again (cavity wall, 5cm EPS in the core):
Average wall U-Value: 1.15 W/m²K. Mould in corners, enormous thermal losses.
Columns/beams insulated with 5cm scored XPS. Average U-Value: 0.45 W/m²K. Consumption reduction: 40%.
Column inner surface temperature rises from 8°C to 17°C. No more mould, uniform temperature at every point in the house.
💡 Conclusion: If you're building a new home with a cavity wall, insulating the core is only half the job. The other half - insulating columns and ring beams - is what determines whether you'll have a truly warm, mould-free, energy-efficient home.
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