Pure wooden windows must be protected from the weather on the outside by regular care and coating of the surface. Wooden windows with an outer cover shell made of aluminium, so-called wood-aluminium windows, are increasingly being used. Thanks to the aluminium cover, which offers optimal and durable weather protection and additional design options, the wooden profile is permanently protected from sun and rain. This means that wood species that would be unsuitable for direct weathering can also be used.

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Image: Wood-aluminium windows - bauwion

More about wooden windows and wood-aluminium windows

Wooden windows and wood-aluminium windows have always been produced by smaller companies and medium-sized companies, so that there are sometimes significant differences in the exact profile design. From softly rounded profiles to angular rectilinear, a wide variety of window elements are manufactured. Window profiles can be awarded the so-called RAL quality mark. This means that they are monitored in accordance with the RAL-GZ 695 guideline of the Quality Association for Windows and Front Doors e.V. with regard to weather resistance, appearance, dimensional accuracy, behaviour in cold/heat, impact resistance and fire behaviour. This gives the user the certainty that all window components that have been awarded the RAL quality mark are subject to regular quality monitoring.

Notes on planning

thermal insulation: The thermal insulation value of a window uw(w for window) is calculated in accordance with DIN EN ISO 10077-1 from the following three individual values:

uf= heat transfer coefficient for the window frame(f for frame)
ug = heat transfer coefficient for glazing in the middle area (g for glazing)
ψg = coefficient for the edge bond of the glass (pronounced: psi)

The values given by the manufacturers usually refer to a standard window of size 123 cm x 148 cm. However, since the frame and glass with their different insulation values are in different ratios depending on the proportion and size of a specific window, the actual values deviate from the standard values, so that the Uw value can be calculated individually for each window on the property. The manufacturers take care of this service on request. As a rule, small windows have a higher and thus worse uw value overall due to the relatively worse insulation value of the frame . Windows with a small proportion of frames, i.e. large windows, have a better overall insulation value. Important parameters that influence the insulation value of the window frame (uf) are above all the construction depth and the types of wood used. Spruce, fir and pine have a lower, and therefore better, Uf value than, for example, meranti, larch or oak.

Sound insulation: The protection of a room against external noise is becoming more and more important in our living environment. The sound-insulating quality of the outer wall of a room depends largely on the sound insulation level of the windows, as these are usually significantly weaker than the surrounding wall, regardless of the wall construction. There are different sets of rules for the assessment of the required assessed sound insulation level R'w of windows, some of which also lead to different results. The planner must therefore decide according to which of the following regulations he works or whether he makes several assessments:

VDI 2719, 1987-08, Sound insulation of windows and their additional equipment
DIN 4109, Sound insulation in building construction
24th BImSchV, 24th Ordinance on the Implementation of the Federal Immission Control Act
DIN-EN 12354-3, Building acoustics - Calculation of the acoustic properties of buildings from the component properties

In practice, VDI 2719 and DIN 4109 are usually used.

Wind resistance, driving rain tightness and air permeability: Windows are also differentiated according to the following properties, which the planner must evaluate or specify:

Resistance to wind load (in accordance with DIN EN 12210)
Driving rain tightness (in accordance with DIN EN 12208)
Air permeability (in accordance with DIN EN 12207)

These three quality characteristics depend on each other and can also be determined on the basis of the guideline FE-05/2 prepared by ift-Rosenheim. The classification is based on the wind load zone, the installation height and the terrain category.

Burglary protection: The classification and evaluation of the burglary resistance of door elements, windows, curtain walls, lattice elements and closures is regulated in the European standard DIN EN 1627, which distinguishes seven different RC classes (RC for resistance-class). In addition to the actual constructions (windows, doors, etc.), their locks, locking cylinders, security fittings and fillings (e.g. glazing) are also tested for the classification into the various classes. This means that a component only corresponds to a certain RC class if all these associated components have been tested accordingly. As can be seen from the RC classes described, the production of completely burglar-proof components is not possible. The aim of the security features is therefore to increase the duration, risk and resistance of burglary in such a way that a potential perpetrator refrains from attempting to break in. In the event of increased security requirements, additional measures should therefore be taken, e.g. alarm protection systems, motion detectors, etc.

Installation situation: The connection of the windows to the shell wall should be carried out in accordance with the specifications of the thermal bridge catalogue of DIN 4108 Supplement 2. If a different installation situation is planned, either a certificate of equivalence in accordance with DIN 4108 Supplement 2 or a thermal bridge calculation in accordance with DIN EN ISO 10211-1 must be prepared to ensure that no weak points in terms of humidity and heat technology arise.

The correct position of a window in relation to the reveal depth, also known as window recess, depends on the wall structure. Among other things, the isothermal course is decisive for the assessment. This can be calculated (also by the manufacturers) and ensures that no moisture and heat defects occur. If arranged incorrectly, condensation can occur on the inner window reveal. The following window recesses have proven their worth:

Monolithic masonry: According to DIN 4108 Supplement 2, window installation in the middle third of the wall is possible without additional mathematical proof (= reveal installation).
Multi-layer exterior wall with external thermal insulation: This case applies, for example, to exterior walls with an external thermal insulation composite system (ETICS) or walls with a rear-ventilated façade (VHF). In these cases, the windows should be at least flush on the outside of the load-bearing wall layer, so that the external insulation can partially cover the frame. DIN 4108 axe blade 2 allows a maximum distance of the outer edge of the window from the insulation level of no more than 25 mm. If there is a greater distance to the insulation level, a mathematical proof must be provided. The most favourable isothermal course and thus the best installation situation in terms of building physics results from the installation of the windows in the insulation level (pre-wall installation). However, this type of assembly is more complex and expensive.

Frame insulation: Even in the most modern design, windows are a thermal weak point in the building. Within the window element, the frames and sash profiles usually have a worse u-value than the glazing. By insulating the frames, heat losses are reduced. However, the installation of roller shutter rails or parapet railings must be carefully planned in detail, these may have to be installed before the insulation is carried out.

If the overinsulation is on the wooden frame, it is more effective than with an over-insulated aluminium shell, as the latter greatly reduces the effect due to the material- and construction-related thermal conductivity. From a thermal point of view, it therefore makes sense not to have the aluminium shells applied to the full width of the frame, but to reduce them to the weathered or structurally necessary width.

Dimensional specifications: Unlike doors, windows are not prefabricated in standard dimensions, but are manufactured according to plans or measurements for each object. However, the maximum normative requirements according to DIN 68121-1, Section 3.3.2 must be observed. For example, patio doors with the IV68/78 profile can be produced with a maximum width of 1.15 m and a height of 2.40 m. Alternatively, manufacturers can also approve other maximum dimensions. The assumption of the warranty must then be specially agreed.

Profile shapes: Most manufacturers offer their windows with different profile shapes, regardless of their quality. The most common profiles are offset with rounded or straight edges. A more modern variant is the flush profile, in which the frame and the opening sash are in one plane. There are also window profiles in which the sash profile is completely covered by the frame profile. The visual advantage of this variant is that from the outside there is no difference between fixed glazing and an opening sash. In addition, this profile has a lower frame share and can also be very far insulated with opening sashes, both of which have a positive effect on the Uw value.

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Notes on construction

Installation: Windows can be attached to the reveal in different ways. In the case of solid walls, windows are usually screwed directly through the frame into the masonry. Alternatively, chattering can be used for assembly. In the case of the so-called pre-wall installation, brackets must be attached to the outside of the shell wall. After the windows have been fastened and before the waterproofing has been applied, the connection joint must be insulated. This is usually done by foaming or by using pre-compressed, heat- and sound-insulating sealing tapes.

Sealing: Windows must be installed and sealed in accordance with the "Generally Accepted Rules of Technology". The implementation of these installation specifications is also incorrectly called RAL installation and is described in the "Guidelines for the Planning and Execution of the Installation of Windows and Front Doors (March 2010)", published by the Quality Association for Windows and Front Doors e.V. anchored in various detail variants. With regard to waterproofing, the principle applies that the connection joint must be tighter on the inside than on the outside, so that moisture entering through vapour diffusion can dry to the outside. One of the most common variants is sealing with sealing tapes or compression tapes. However, sealants such as silicone or acrylic are also permissible for waterproofing under certain circumstances and are often used in renovation measures. In the case of windows in the splash water area, a seal must also be installed on the outside to prevent direct use of splash water. This is often done with EPDM tapes or liquid waterproofing.

Preparation of the reveals: The window sealing in new buildings is usually made with sealing tapes. Before the windows are installed, a dust-free and smooth design of the reveal must be ensured in order to ensure the adhesion of the sealing tapes. In the case of masonry, a smooth line of cement slurry must be produced for this purpose or care must be taken to ensure that only smooth masonry stones (beginner's stones) are used in the reveal.

Weather conditions: The installation of windows is generally possible even at sub-zero temperatures, but clear specifications for installation apply to waterproofing materials such as sealing tapes or sealants. As a rule, sealing tapes cannot be installed below +5°C. However, special products are also available for processing down to -5°C if required.

Blower door test: To check the tightness, a blower door test is strongly recommended before completion of the building or is often specified by the thermal insulation certificate.

Standards and literature

DIN 4108, Supplement 2, Thermal insulation and energy saving in buildings, thermal bridges, planning and execution examples

DIN 18055, windows; Joint permeability, driving rain tightness and mechanical stress

DIN 18355, VOB Contract Regulations for Construction Services — Part C: General Technical Contract Conditions for Construction Services (ATV) — Carpentry DIN

68121-1, Wooden Profiles for Windows and French Doors, Part 1: Dimensions, Quality Requirements

DIN 68121-2, Wooden Profiles for Windows and French Doors, Part 2: General Principles

DIN 68800-3, Wood Preservation; Preventive chemical wood protection

DIN EN ISO 10077-1, Thermal behaviour of windows, doors and closures Calculation of the heat transfer coefficient

DIN EN 12207, Windows and doors; Air permeability

DIN EN 12208, windows and doors; Driving rain tightness

DIN EN 12210, windows and doors; Resistance to wind load

DIN EN 14351-1, Windows and doors - Product standard, performance characteristics - Part 1: Windows and external doors without properties in terms of fire protection and/or smoke tightness.

EnEV - Energy Saving Ordinance for Buildings, Ordinance on Energy-Saving Thermal Insulation and Energy-Saving System Technology for Buildings

VFF Guideline HM.01, Guideline for Wood-Metal Window and Exterior Door Constructions,

Association of Window and Façade Manufacturers e.V.

Guideline TBDK of the Gütegemeinschaft Schlösser und Beschläge e.V. Fastening of load-bearing fittings of Tilt&Turn and Tilt Fittings with definitions of Tilt&Turn Fittings.

► Bundesverband ProHolzfenster e.V.

► ift Rosenheim, Institut für Fenstertechnik e.V.

► Verband Fenster + Fassade

► Gütegemeinschaft Fenster und Türtüren eV.

► The required sound insulation level of soundproof windows: A comparison of different regulations, Bavarian State Office for the Environment (pdf)

► Guidelines for the use and tendering of windows and external doors according to the product standard DIN EN 14351-1 – CE marking/ System testing/ minimum requirements, issued by the Test Center for Components (PfB). (PDF)

Source: bauwion