In the previous article we made the case crystal clear: embedding a freezing piece of steel (galvanised sheet metal) inside the wall to support your new energy-rated window is a construction crime. It destroys insulation, promotes mould and, sooner or later, rusts.
But what do we use instead of sheet metal? If you're building a modern nZEB (Nearly Zero-Energy Building) or a Passive House, engineers and architects now have specialised, high-tech materials at their disposal. Insulating subframes are the "invisible hero" bridging the gap between heavy masonry and elegant aluminium.
Purenit is arguably the most revolutionary material to enter construction sites in recent years. Visually it resembles chipboard or compressed wood, but it has absolutely nothing in common with either. It's produced from recycled rigid polyurethane foam (PUR/PIR) at extremely high density.
A modern lift-and-slide window with triple glazing can weigh over 300 kg. Purenit doesn't flex, doesn't compress and holds fixing screws as firmly as solid wood. Its compressive strength exceeds 30 N/mm², while screw pull-out strength reaches 3,500 N - numbers that surpass some soft-wood species.
Unlike wood, Purenit doesn't swell, doesn't rot and never grows mould, even if submerged in water for years. Water absorption is practically zero (<3% by volume). Even in extreme moisture conditions - such as freshly poured concrete or ground-floor units near the sea - the subframe never deteriorates.
Despite its hardness, it remains an insulating material. It cuts off heat transfer from the external concrete to the interior like a knife, eliminating the linear thermal bridge. Its thermal conductivity (λ) is around 0.06–0.08 W/(m·K) - extraordinarily low compared to steel's 50 W/(m·K).
Installers can cut it with a standard circular saw, drill it and bond it with polyurethane adhesives on site, adapting it to any opening. No specialist equipment or training is needed - any crew familiar with woodworking tools can work with Purenit.
For the sides and top of the opening (where there's no massive vertical glazing load), installers often use special, rigid profiles made from high-density Expanded Polystyrene (EPS) (above 150 kg/m³). These are the familiar grey Neopor blocks seen on external insulation systems - but in a much denser, harder form.
EPS profiles are lighter than Purenit and often more affordable. Indicatively, costs start from €8–12 per running metre for the sides. Their mechanical strength is still sufficient for parts of the opening that bear no vertical load - only lateral wind pressures.
EPS/Neopor "embraces" the aluminium profile, creating a continuous insulating envelope. Its λ value reaches 0.031–0.035 W/(m·K) - even lower than Purenit. This means the joint around the window insulates just as well as the rest of the wall.
They work seamlessly with the building's External Thermal Insulation Composite System (ETICS), creating a single "warm" envelope with no gaps. The ETICS insulation overlaps onto the EPS subframe, eliminating every linear thermal bridge at the head and jambs.
The biggest problem in construction is the junction between different materials. When you use insulating subframes, you solve the plasterer's and installer's biggest headache.
Modern installation techniques (such as the RAL method) require the use of specialised sealing tapes and membranes bonded onto the subframe. If the subframe were sheet metal, the tape wouldn't adhere properly and would dry out from the heat of the metal. On Purenit, the membranes bond perfectly and airtightness remains unquestioned for decades.
Render and ETICS materials bond excellently to these surfaces, preventing the familiar hairline cracks that typically appear around windows after a few years. This is because the thermal expansion of Purenit is similar to that of render, whereas sheet metal expands dramatically more.
Yes, an insulating subframe made of Purenit costs more than a plain, rusty sheet-metal strip. But that cost is negligible compared to your total window budget.
Galvanised sheet metal costs €3–5 per running metre. A Purenit insulating subframe costs €15–25/rm, while high-density EPS ranges around €8–15/rm. For a typical 1.20 × 1.40 m window (perimeter ≈ 5 m), the extra outlay is just €50–100. That represents less than 3% of the cost of an energy-rated window.
The energy savings from eliminating the linear thermal bridge pay back the extra expense within 2–3 winters. After that, every winter is pure profit - lower heating bills, zero mould, zero rust. For a typical home with 10 windows, the total extra spend on Purenit subframes ranges from €500–1,000 - negligible against a window budget of €15,000–25,000.
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. With Purenit at the sill and high-density EPS on the sides, you guarantee: zero thermal bridges, zero rust, zero mould - and a window performing at 100% of its rated capacity, just as the manufacturer guarantees. Ask your installer today: "What subframe are you using?" If the answer doesn't include "Purenit" or "EPS", it's time to look into more modern solutions.
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