You've just taken delivery of your brand-new, energy-rated windows. The profiles are solid, the glazing impressive and the certificates promise top-tier thermal insulation. The installation crew arrives: they screw the frame in, fill the joint with cheap, yellow expanding PU foam, run a bead of silicone inside and out to "seal it" and hand over the job.
In Greece, 90% of installations are still carried out this way. In central and northern Europe, however, this practice is now considered illegal and unfit for modern buildings. The reason? No matter how perfect your window is, if the joint around it isn't insulated properly, moisture will find its way in, the foam will degrade, the silicone will crack and mould will appear. The solution is the undisputed "gospel" of modern installers: RAL-Standard Installation.
RAL is neither a brand of materials nor a specific company. It stands for RAL Deutsches Institut für Gütesicherung und Kennzeichnung (the German Institute for Quality Assurance and Labelling). In the window industry, "RAL-Standard Installation" describes a strict, certified installation guide built on a single golden rule of building physics:
"More airtight inside, more open (vapour-permeable) outside." This means the joint must prevent indoor moisture from entering the wall cavity, yet if any moisture does get trapped, it must be able to "breathe" and escape to the outdoors - without letting rain water in. Plain silicone cannot do this (it blindly seals both sides). The RAL method achieves it by using 3 distinct Sealing Levels.
RAL installation is not a manufacturer's claim. It is a documented, certified standard backed by air-pressure tests (Blower Door Test), thermographic inspections and condensation measurements. A building that doesn't follow the RAL method cannot be certified as a Passive House, since the joint will exhibit air leakage that destroys the airtightness score.
Although Greece does not (yet) have legislation mandating the RAL method, the Energy Performance of Buildings Regulation (KENAK) demands ever stricter thermal insulation and airtightness criteria. Within a few years, RAL installation will effectively become the only option for any new construction or renovation aiming to meet Europe's energy-efficiency standards.
This is the first and most critical level: the side of the joint facing your living room. The goal is clear - no water vapour from the room should enter the wall. If it does, it will reach the "cold core" of the wall, condense at the dew point and breed mould inside the joint, right where nobody can see it.
Specialist elastic EPDM or aluminium membranes, acrylic mastics or vapour-barrier tapes are used. These materials create an airtight seal between the window frame and the interior plaster, forming an impenetrable barrier against water vapour. The wide membrane (7–15 cm) permanently bridges aluminium and wall, even if micro-cracks appear due to earthquakes or thermal movement.
The classic silicone "bead" that installers apply indoors is not a reliable vapour barrier. After 5–6 years under the Greek sun and thermal movement, the silicone hardens, detaches from the plaster and opens microscopic gaps. A membrane, by contrast, is wide, elastic and gets plastered over - it doesn't rely on a thin glue line but on an entire bonding surface.
This is the main cavity (the depth of the joint) between the aluminium and the masonry. The goal is twofold: to maintain temperature (no cold-air draughts) and to block noise from the street. In this zone, standard rigid PU foam (the cheap yellow kind) is strictly prohibited, as it cracks after the first cycle of thermal expansion and contraction.
Closed-cell elastic PU foams (Flex PU Foams) are used; they can compress and return to their original shape without cracking - even after decades of service. Their sound attenuation reaches 60 dB and their thermal conductivity stays steadily below 0.035 W/(m·K). They cost about €10–12 per can (versus €4–5 for standard foam).
Alternatively, self-expanding polyurethane tapes (Compriband) are used; they completely fill the cavity, keeping it dry and insulated. The tape is stuck to the frame before the window is placed in the opening and expands on its own over several hours, hermetically embracing the wall. Multifunctional "3-in-1" tapes simultaneously replace both the foam and the membrane.
This is the side facing the balcony or the street. Here the joint takes relentless punishment from rain, wind and UV radiation. The goal is twofold: to keep rain out, yet allow any trapped moisture to evaporate outward. This is precisely where the RAL method vastly outperforms silicone.
Forget plain silicone! Vapour-permeable (breathable) membranes or specialist self-expanding exterior tapes are used instead. They work exactly like waterproof/breathable jackets (Gore-Tex): they keep rain out but let the wall "sweat" outward. They withstand water pressures above 600 Pa (equivalent to driving rain at hurricane wind speeds).
If you seal the wall "blindly" on both sides with the same impermeable materials (e.g. plain silicone everywhere), moisture gets trapped in the middle. Within 2–3 winters, the plaster will rot, the foam will turn black with mould and the paint will blister around the window. Remediation requires window removal, cleaning, new insulation and repainting - a cost of €400–600 per window that far exceeds the €15–25 that proper RAL materials cost.
The RAL method guarantees the joint stays dry on the inside. It is the only absolute solution against the black stains around the frame. Without an interior vapour barrier, moisture cannot get trapped inside the joint - so there is no "food" for mould. This prevents both structural damage (rotting plaster) and respiratory health risks from fungal spores.
The home seals 100%. RAL installation is a prerequisite for passing the Blower Door Test and achieving Passive House certification. Without this level of airtightness, cold-air draughts negate the window's insulation, increasing heating bills by 15–25% compared to a properly sealed building.
While plain silicone and standard foam polymerise (harden and crumble) after 5–6 years under the Greek sun, RAL materials are engineered to follow the building's thermal movements, retaining their elasticity and seal for decades. A RAL-sealed joint has a design lifespan of 25–30+ years - matching the window itself.
The best window in the world is cancelled out by poor installation. The RAL method is not a "luxury extra" but the only scientifically proven way to integrate a modern window into your building's envelope. The specialist membranes, tapes and elastic foams may add roughly €15–30 per window to the installation cost, but the benefits are non-negotiable: zero mould, zero draughts, zero joint failures for decades. Next time you request quotes for new aluminium windows, your first question should be: "Does your crew perform certified RAL-standard installation?" If the answer is "foam and silicone", it's time to look elsewhere.
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