You're in the middle of building your new home or undertaking a major renovation. The bricklayers have finished the walls and the plasterers are about to move in. That's when your contractor utters the classic phrase: "Get the aluminium installer to come and fit the subframes so we can plaster."
In Greek construction practice, this phrase is almost reflexively followed by ordering a few metres of galvanised sheet metal (cold-formed steel). It was the norm for decades. However, under today's energy-building standards, using a metal subframe is one of the most destructive mistakes you can make.
A subframe (or blind frame) is a frame fitted into the bare wall opening (the raw brickwork), long before the final aluminium window is installed. It acts as a guide and mould for the other trades, holding the position of the future window while the plasterer and marble mason complete their work all around it.
The subframe shows the plasterer exactly where to stop the render and the mason where to bring the marble sill. It creates a perfectly levelled, square opening so the final window fits faultlessly. Without it, each trade works "approximately", producing crooked openings and irregularities that cost dearly later on.
If we bolted the final, expensive window directly onto the brickwork before plastering, the site crews would cover it in mortar, paint splatters and scratches. The subframe "reserves the spot" for the window, which goes in pristine at the end, once the house is painted and ready for handover. This protection alone saves thousands of euros in site-damage replacements.
The final aluminium frame is screwed onto the subframe. This means the subframe material must be capable of carrying tens or even hundreds of kilograms of glazing and must hold screws for decades without yielding. Strength, corrosion resistance and thermal conductivity become critical factors - precisely where galvanised sheet metal fails catastrophically.
Historically, we used galvanised steel because it was cheap, rigid and crews knew how to work with it fast. Today, however, when you're paying thousands of euros for thermally broken aluminium and energy-rated glazing, the sheet metal cancels out your entire investment. Here are the 3 reasons:
You buy a top-of-the-range aluminium profile with oversized polyamide thermal breaks to stop heat transfer. Then the installer screws it onto a perimeter steel strip (the sheet metal) that directly connects the freezing outdoor air with the warm interior wall of your living room. The thermal conductivity of steel is 50 W/(m·K), while that of an insulating material is just 0.035 W/(m·K) - a difference of over 1,400 times! Your home's heat races out through the steel, completely bypassing the window's insulation.
When cold outdoor air chills the metal subframe, it also chills the plaster surrounding it on the inside. Indoor moisture touches the freezing plaster (right next to your otherwise flawless window) and turns into water. This is classic dew-point condensation. Within months, you'll see a black "ring" of mould encircling your brand-new window - a problem that costs both health and money.
Galvanised sheet metal has a zinc coating to prevent rust. However, when the installer cuts it at the corners to assemble it, or screws it into the wall, this coating is locally destroyed. At those points, bare steel comes into contact with the alkaline moisture of the cement.
At every cut, drill hole or bend, the protective zinc layer breaks. The bare steel is exposed to the alkaline moisture of the render and an uncontrolled electrochemical oxidation begins. Rust never stops on its own - it merely slows down. Within 5 to 10 years, the volume of oxidised surface can triple, pressing against the render until it cracks.
Over the years, rust swells, blows out the plaster and creates unsightly brown stains around the window. These stains cannot be covered by any coat of paint because the rust continues to "bleed" through every layer. The only solution is cleaning with anti-corrosion treatment, priming with aluminium primer and then repainting - an expensive, time-consuming process that must be repeated every few years.
In modern, energy-efficient building design, the rule is ruthless: Never use a thermally conductive material (steel) as a bridge between the exterior and interior of the building. Even the Greek Energy Performance of Buildings Regulation (KENAK) now explicitly requires the reduction of thermal bridges around window perimeters.
Installing a top-of-the-range energy window on a galvanised sheet-metal subframe costing €20 is literally like buying a brand-new Ferrari and fitting it with worn-out tractor tyres. The engine runs like a dream, but the road grip (the building-envelope insulation) fails completely. The installation base must match the technology of the window.
Fortunately, the modern industry now offers perfect (and affordable) materials to replace steel: Purenit (recycled high-density polyurethane) and high-density rigid EPS/Neopor. These materials deliver enormous mechanical strength, zero thermal conductivity, 100% moisture resistance and zero corrosion. The extra cost starts at around €15–25 per running metre - a fraction of the budget for an energy-rated window that exceeds €400/m².
The subframe is the "foundation" of your window. Build on the wrong foundation and the window will under-perform energetically for the next 30 years, with the risk of mould lurking every cold winter. A proper insulating subframe (Purenit or high-density EPS) eliminates the linear thermal bridge, removes all corrosion risk, and ensures your energy investment performs at 100% of its capacity. The extra expense pays for itself within 2–3 winters from lower heating bills alone. If you're building or renovating, ask your installer: "What subframe are you using?" If the answer includes the word "sheet metal", it's time to find a new installer.
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