Window Installation to RAL Standards: Why Fitting Matters as Much as the Window

Many homeowners compare profiles, glass and U-values in great detail – and underestimate the installation. Yet it is the fitting that determines whether a window actually achieves its insulation values in everyday use. Leaky joints cause draughts, heat loss and, in the worst case, mould. This is exactly what RAL installation is designed to prevent.

What does RAL installation mean?

The term comes from the installation guideline for windows and entrance doors published by the German RAL quality association for windows, façades and entrance doors (RAL-Gütegemeinschaft Fenster, Fassaden und Haustüren). It describes how the joint between window and masonry should be planned and carried out in line with recognised good building practice. At its heart is the three-layer principle.

The three layers in detail

1. Inside: airtight

On the room side, the joint is sealed airtight and vapour-retarding, for example with a special membrane or sealing tape. This prevents warm, moist indoor air from entering the joint and condensing there.

2. Middle: insulated

In the middle, insulation material or expanding foam provides thermal and acoustic insulation. This functional layer must stay permanently dry – only then does it insulate reliably.

3. Outside: weathertight

On the outside, a tape protects the joint against driving rain and wind. At the same time it is vapour-permeable, so that any moisture that has got in can escape to the outside. The basic rule is therefore: tighter on the inside than on the outside.

Fixing: foam is not a fastener

The window must be mechanically anchored in the masonry – with frame plugs, screws or fixing anchors – and set on load-bearing blocks that transfer its weight into the structure. Expanding foam only insulates; it does not hold the window in place. Secure fixing is particularly important for heavy units such as lift-and-slide doors.

Special case: installation within the insulation layer

On houses with external wall insulation (ETICS), the window can be positioned in front of the masonry, within the insulation layer. Special mounting brackets and consoles are available for this. This type of installation reduces thermal bridges at the reveals, but it requires careful planning.

How to recognise a good installation

• The fitter explains which materials are used on the inside and on the outside.
• The joints are not just filled with foam but sealed on both the inside and the outside.
• The window is fixed with plugs or anchors and set on load-bearing blocks.
• Window sills and roller shutter boxes are included in the sealing.
• After installation, all sashes open and close smoothly and tightly.

At Aristofenster, professional installation by experienced fitters is part of our complete service – from a free survey once you place your order to after-sales service. Find out how ordering works in our article Ordering windows step by step. You will find our windows under PVC windows, Aluminium windows and Wooden windows.