PVC-aluminium window Kömmerling 76 AluClip

A significant increase in the window's stability is achieved by installing steel reinforcements all the way around the outer frame and the sash (right into the mitred corners). Only this detail allows the corner bearing and the restrictor stay to be screwed into the steel core with all four screws. This continuous screw fixing is required to provide a load-bearing capacity of over 100 kg. The dimensional stability of the profile construction makes it possible to produce large elements.

In window construction, hardware refers to the components that enable the window to be opened and closed. It therefore comprises the locking mechanism on the opening side of the window sash and the corresponding keeps in the frame, as well as the hinges on the opposite side of the window sash. All of the mechanics that allow a window sash to be opened, and in the case of a tilt-and-turn window to be turned or tilted, are also part of the hardware. In professional terminology, the window handle used to operate the hardware is not counted as part of the hardware itself, but is often also "meant" when hardware is spoken of.

The term spacer refers to the window components that form the transition between the glass surface and the frame profile. They are also responsible for the distance between the individual glass panes of a multiple glazing unit, hence the name. In order to achieve a good U-value and to significantly minimise heat losses at the edge seal, modern spacers are no longer made of aluminium, but of flexible structural foam, composite plastic or plastic-coated stainless steel. These are used when a window is to be produced with what is known as a "warm edge". Passive house standard windows are only possible with such modern spacers.

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.

Windows and doors should protect against rain, wind, noise and also against burglars. All these objectives are served by the seals in the rebate, where a distinction is made between rebate seals and centre seals. Most windows and balcony or patio doors are fitted with both types of seal. The seals are made of an elastic and hard-wearing plastic. A rebate seal is fitted on the inside and/or outside at the rebate of the window profile. The seal closes the gap between the window sash and the window frame seamlessly, preventing cold outside air from entering and stopping the warmth from escaping the room. The rebate seal also improves the sound-insulation values of the windows and doors. Thanks to the special construction of the rebate seals, water – whether condensation or rainwater that has penetrated – is effectively channelled to the outside.

Modern windows and doors are constructed to keep the greater part of street noise outside. The sound insulation values (the sound reduction index) indicate how effectively the sound damping of a building component works.

The standard DIN 4109 "Sound insulation in building construction" sets out the requirements for sound insulation and defines the sound insulation classes. The sound insulation value is given in decibels (dB).

Example: the sound insulation value, or sound reduction index, of a window with insulating glazing is 32 dB Rw,P (or Rw p). The designation Rw denotes the reference value and P stands for the test value (tested in the laboratory). The sound reduction index of 32 dB Rw,P indicates that, in the laboratory measurements, the tested building component reduced the noise level by 32 dB.

In construction, the rule applies that the test value of a building component (Rw,P) should be 5 dB higher than the actual value (Rw). On average, a modern window with standard insulating glazing lets through only about 10% of the noise.

Sound insulation classes and the associated sound reduction index:

• Class 1 25-29 dB
• Class 2 30-34 dB
• Class 3 35-39 dB
• Class 4 40-44 dB
• Class 5 45-49 dB
• Class 6 over 50 dB

For living and working spaces, sound insulation class 1 is the standard; for schools, classes 2 and 3; for hospitals, 3 and 4.

Multi-sash windows divide a large window area into individual sashes and can be implemented with various types of hardware. The window area can be subdivided either by a fixed central post (also known as a fixed mullion or transom) or by a loose false mullion (Stulp).

The Stulp (also referred to as a loose post) is the vertical profile strip on the false-mullion sash and, as an integral part of the window sash, opens together with the window. This creates an undivided window opening when the sash is open. When the false-mullion sash (also called the secondary sash) is closed, the Stulp profile serves as a rebate for the other sash.

A false-mullion window consists of two sashes joined in the centre. The main sash has a standard window handle, while the other has a hand lever (a release located in the frame).

The advantage of this window is that it can be opened "completely" without a central post remaining in place. False-mullion windows also provide a slim window construction and are particularly suitable for narrow window openings.

One disadvantage is that only the main sash of the window can be tilted. To open the secondary sash, you first have to open the main sash.

For wide windows (from approx. 150 cm), it is advisable to fit a post (transom) to improve the stability for the sashes to close against.

There are six resistance classes with increasing protective effect from RC1N to RC6 in accordance with the European standard DIN EN 1627 (the old designation, used until September 2011, was WK 1 to WK 6). RC stands for Resistance Class.

RC1N - Attempted break-in using physical force such as kicking, body-slamming, jumping against, levering up and the like. Elements of this resistance class are therefore mainly suitable as protection against vandalism. Windows of resistance class 1 offer only very low burglary protection.

RC2 - The opportunist offender additionally attempts to force the window open using simple tools such as a screwdriver, pliers or wedges. Windows of resistance class 2 offer low to average burglary protection.

RC3 - The offender additionally attempts to gain entry using a second screwdriver and/or a crowbar. WK3 windows offer high burglary protection. In a private household, protection to resistance class RC 2 and RC 3 is usually sufficient.

RC4 - The experienced offender additionally uses sawing and striking tools, e.g. an axe, crowbar, hammer and chisel, as well as a cordless drill. Windows of resistance class 4 offer very high burglary protection.

RC5 - The very experienced offender additionally deploys power tools, e.g. a drill, jigsaw or reciprocating saw and an angle grinder with a maximum disc diameter of 125 mm. Windows of resistance class 5 offer very, very high burglary protection. The resistance time is at least 15 minutes. 

RC6 - The very experienced offender additionally uses power tools such as a drill, jigsaw or reciprocating saw and an angle grinder with a maximum disc diameter of 230 mm. Windows of resistance class 6 offer extremely high burglary protection, and the resistance time is at least 20 minutes.  

The SWISSPACER warm edge spacer has become a byword for the effective prevention of thermal bridges on windows. It provides a special insulating effect at the edge of the insulating glass unit and determines the spacing between the panes. This prevents heat from escaping to the outside via the edge of the glass in winter and reduces the formation of condensation.

Windows have to meet ever more requirements, and development has advanced considerably over recent decades, particularly in the field of thermal insulation. Multi-pane insulating glass is one example of this. However, it can only deliver its full efficiency when a warm edge is built into the window by means of spacers. Triple glazing in particular benefits from an effective edge seal, which reduces energy loss to a minimum and supports the window's thermal insulation.

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.