Window installation does not end with fixing and sealing. Around every window (inside and out) there are always gaps, joints and imperfections: the render may not reach the right position, the frame might not sit perfectly flush with the wall, expanding PU foam remains visible.
This is why special finishing accessories are used: frame extensions, trim covers (cover strips) and sill extensions. These small components are not luxuries - they are construction necessities. Let us see what they do and why you should always request them.
A standard thermally broken aluminium frame is approximately 7-8 cm wide. If your wall (including ETICS insulation) is 30 cm thick, more than 20 cm of wall "remains." This means the frame "disappears" inside the wall.
Extensions (also known as Fardy profiles) are aluminium strips that "click" onto the external (or internal) face of the frame, increasing its visible width. Thanks to these, the frame reaches the render (or ETICS surface). Without extensions, an open gap would remain between the frame and the finished surface, exposing the PU foam.
Available in widths of 2, 3, 4 and 5 cm (or larger depending on the manufacturer). The fabricator calculates precisely how many centimetres are missing and orders the correct extension. Important: extensions must be ordered together with the windows, not after the fact. Retrofitting is more difficult, though not impossible.
Trim covers (or cover strips / mouldings) are aluminium or PVC strips that are adhered or clipped onto the external face of the frame. Their purpose: to cover the joint between frame and finished render, providing a clean, minimalist line.
A window without trim covers looks "unfinished": exposed foam, silicone and raw materials remain visible. With trim covers, the window gains a perfect line, like a picture frame. The strips feature a dedicated "lip" that hides the joint, while their outer face can be painted to match the aluminium colour or the render.
Beyond aesthetics, trim covers protect the sealing joint (Compriband tape or silicone) from sun (UV), rain and impacts. UV radiation destroys elastic tapes within a few years if left exposed. The trim acts as a "UV shield."
An extremely common mistake when fitting both metal sills and trim covers: sealing the joint completely around with silicone. This looks correct - "no water gets in!" - but it creates a perfect trap.
Atmospheric moisture, micro-condensation behind the trim or water entering above the seal cannot escape! It becomes trapped behind the strip, creates mould, rots any timber surrounds and, worst of all, runs into the wall instead of draining freely.
Trim covers on the bottom edge must remain open (no silicone) or have dedicated drainage openings. The principle: "seal top and sides, leave the bottom free." Silicone closes a thermal path but opens a moisture path if it allows no drainage escape.
The ideal seal around a trim cover or metal sill follows a P shape (inverted U): seal with silicone or sealant along the top edge and both sides, leaving the bottom edge explicitly open as a water "escape".
Rain hitting the trim cover is deflected left and right thanks to the silicone. If any water penetrates, it runs downward inside the joint and exits freely through the open bottom edge before it can reach the wall. The rule "seal above, drain below" applies to every external sealing point.
RAL philosophy follows exactly this logic: 3 zones. The interior zone is airtight (membrane), the middle zone is insulating (foam) and the exterior zone is open for breathability and drainage (Compriband). Trim covers belong to the exterior zone - they must not seal hermetically.
Trim covers and frame extensions are not luxury accessories. They are essential components for the aesthetics, waterproofing and longevity of your windows. Request them in the quote, verify they are rated to the correct dimension and ensure the bottom edge remains open for drainage. The difference is immediately visible - and silently protects your home for decades.
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