Exterior panel door Schüco Royal 95

Windows with an add-on shell

The add-on shell, made from a high-quality aluminium alloy, is clipped onto the outside of the windows and wraps around the sash and outer frame. It gives the window element a durable, weather-resistant surface, a wide choice of colours and easy maintenance. Beneath the shell lies a different core in each case: uPVC or timber.

Wall design with an add-on shell

An add-on shell is a wall cladding that is placed in front of a load-bearing masonry structure and connected to it. This may be done for aesthetic or insulation-related reasons. Common examples of add-on shells are brick facings or timber, sheet-metal or plastic cladding on external walls. Insulation materials applied to the outside, which are then rendered or otherwise clad, also count as an add-on shell.

On internal walls, plasterboard or timber-panel cladding, composite panels or coarse chipboard are used, for example.

A RAL colour refers to standardised colours from a colour collection that has been authoritative for colour selection for over 85 years and is distributed by RAL GmbH under the name RAL Classic.

Each colour in the colour catalogue is assigned a four-digit colour number. The first digit is systematic: 1 - yellow, 2 - orange, 3 - red, 4 - violet, 5 - blue, 6 - green, 7 - grey, 8 - brown, 9 - white/black shades, and the remaining three digits are chosen sequentially.

For a surcharge, almost all products, such as timber windows, timber-aluminium windows, aluminium windows, roller shutters and external window sills, can be produced in a RAL colour of your choice.

The thermal transmittance coefficient, abbreviated U for unit of heat transfer (the unit of heat transfer), replaced the k-value in the course of European standardisation. Measuring the U-value determines the heat loss from the inside to the outside, specifically per square metre and per hour at a 1° temperature difference.

A window's thermal insulation depends on the frame material, the glazing and its airtightness. A window's insulating properties are determined by the U-value. The lower this figure, the better the thermal insulation.

For windows, three U-values are generally distinguished. The thermal transmittance coefficient Uw refers to the entire window. This value incorporates the U-values for the glazing Ug and the frame Uf.

• Uf = U frame – is the U-value of the window frame.
  The Uf-value is determined by measurement for the frame-and-sash combination. The visible width of the profile is taken as the area for calculating the Uw-value.

• Ug = U glazing – is the U-value of the glazing.
  The Ug-value depends on the type of gas filling in the cavity between the panes, the distance between the panes and the number of panes.

• Uw = U window – is the U-value of the entire window.

The thermal transmittance coefficient for windows is generally calculated for the standard window size of 1.23 m x 1.48 m. Please note that larger glass areas achieve better values, since glazing allows better U-values than the frame material.

Under OIB Guideline 6 of 2011, the Uw-value for standard glazing must not exceed 1.4 W/m2K. The German Energy Saving Ordinance (EnEV) of 2014 limits the thermal transmittance coefficient to 1.3 W/m2K for an existing building. Windows with a Uw-value no greater than 0.8 W/m2K are considered suitable for passive houses.