You've just purchased the most expensive, premium thermally broken aluminium window on the market. The manufacturer gave you certificates showing an excellent Uw. The first cold snap comes, you sit next to the closed window and... you feel a subtle chill. You touch the wall around the frame and it is freezing cold.
The window is doing its job perfectly. The problem lies where the aluminium ends and the wall begins. Welcome to every engineer's "nightmare": Linear Thermal Bridges.
Heat always seeks the easiest escape route. When an insulated window is placed on a wall or marble sill, the junction creates a vulnerable point: a Thermal Bridge. Unlike U (which measures losses per m² of surface), the Ψ (Psi) Coefficient measures losses per running metre of length, in W/(m·K).
The Ψ index answers: "How much extra energy is lost along the window's perimeter because of a poor junction with the wall?" A window with a perfect Uw of 1.0 may lose more energy around its perimeter (from a high Ψ) than through the glass itself. That is why many people feel cold despite "top-of-the-line" windows.
Ψ < 0.04 W/(m·K): Excellent RAL-type installation.
Ψ = 0.10 – 0.20: Average, with simple PU foam. Acceptable
but not ideal.
Ψ > 0.30: Poor junction - continuous marble sill or
open joint. Massive energy loss.
Where exactly are these problems found in a typical Greek building? Three points are responsible for the overwhelming majority of thermal bridges around windows.
The most classic construction error in Greece. The builder installs a single, unbroken piece of marble at the threshold (inside to outside). Marble is an excellent heat conductor. Even if you mount premium aluminium on top, cold will "travel" through the continuous marble, pass under the window and freeze your living room floor.
Many installers screw the frame to the wall, fill the gap with standard PU foam and apply a line of silicone. Standard foam, however, polymerises over time, absorbs moisture and loses its insulating ability. Result: a perimeter thermal bridge that lets air and moisture through.
Excellent glass, perfect aluminium, but the roller shutter box (above the window) is plain sheet metal with no internal insulation. Cold air enters the box from outside and freezes the wall right above your head, creating black mould stains.
If you are an engineer supervising the project or a homeowner who wants the job done right, demand the following during installation.
The marble must be cut in the middle, right under the frame. Between the outer and inner marble pieces, insulation material is inserted (e.g. extruded polystyrene XPS, 3–5 cm thick). This "breaks" the bridge, reducing Ψ to below 0.05 W/(m·K).
Modern installation requires: closed-cell elastic foams (instead of standard PU foam), self-expanding tapes (compriband) around the frame perimeter, and EPDM membranes (vapour barrier inside, vapour-permeable outside). These materials seal the joint hermetically and drive the Ψ value close to zero.
The third - and often forgotten - thermal bridge sits above the window: the roller shutter box. The solution is choosing boxes with internal Neopor or polyurethane lining, which have a thermal transmittance 3–5 times lower than standard boxes.
(1) Shutter box with internal insulation at least 30 mm thick. (2) Aluminium cold bridges: the box should not directly touch the wall without a thermal break. (3) Proper sealing between box and wall with self-expanding tape, not just silicone. (4) Consider motorised shutters with sound-dampening insulation if you are in a noisy area.
A building's thermal shield is like a chain: it is only as strong as its weakest link. If the window's Uw is 1.0 but the installation Ψ is huge, you've effectively wasted your money, and mould (condensation) on the walls will soon follow. Ask for the installation method in writing before you sign.
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