Materials, components, types of construction
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Special highly insulating frames combining various materials with solid core insulation are also composite frames. The following material combinations are currently available: • Plastic-aluminium (with a growing market share) • Wood-bronze • Wood-GRP (Fig. B 2.58) Ventilation components integrated into frames
Ventilation openings can be integrated into or around windows to more efficiently use natural ventilation or be combined with mechanical ventilation (e.g. in exhaust air systems). They have either a certain permanent ventilation cross-section or can be regulated or self-regu lating using integrated flaps (see “Technical building components in and around windows”, p. 198ff.). Connection between glass and frame
The structural connection between a window’s glass and frame takes on a range of functions during the service life of opening elements: • Mechanical securing of the window in posi tion and fixing • Seals against wind and rain • Absorbs movement and deformation • Protects the pane’s edge bond • Enables the pane to be replaced The following paragraphs will deal with current common ways of ensuring these in detail. Blocking glazing units Whatever the frame’s material, properly stor ing glass panes is essential in glazing work [19]. Setting blocks ensure that frame and glazing do not directly touch at any point by providing clearly defined support points. Blocks are set after the rebate is prepared and before glazing is sealed, with the aim of distributing the pane’s weight and transferring it into the frame to prevent additional loads being imposed on the pane, e.g. due to changes in temperature or movement of the sash. To a certain extent, blocks can also be used to ensure the frames are aligned at a right angle (“upright blocks”). 108
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The pane itself should not be load-bearing unless that is a certified feature of the product, e.g. glued glass and frame elements. Wood or plastic blocks must be secured against slipping. They are available in various thick nesses ranging from 1 to 6 mm and are uni formly identified by means of specific colours. They range from 60 to 100 mm in length and are typically 2 mm wider than glazing. Various types of blocks can be differentiated according to their function. • Bearing blocks have a load-bearing function. • Spacer blocks ensure a minimum distance between the rebate base and pane, e.g. in case elements slip or to absorb particular loads in certain types of openings. Spacer blocks may be elastic in such cases. • The bridge form of glazing bridge packers in a smooth rebate base ensures a continuous spatial join so that pressure can be equalised all along the frame and any water can drain away. This is especially important in woodframe windows. The distance between a block and the edge of the pane should be one block length, so at 60 –100 mm, at least 20 mm and at most 250 mm for very wide panes. Openings for pressure equalisation must be kept free. To reliably prevent “droplet bridges” the glass rebate base and glazing should be at least 5 mm apart. Special arrangements of spacer blocks and bearing blocks designed to absorb various loads must be used for different kinds of sashes. Blocks that can be adjusted during the window’s service life are sometimes set into wooden-framed windows, and the spaces for these must remain more or less easily accessible. Bearing and spacer blocks are also used in attaching frames to buildings and to differen tiate them may be referred to as supporting blocks (see “Load transfer”, p. 124f.). Glazing rebates, rebate drainage and pressure equalisation A glazing rebate is a usually grooved space, including blocks and sealing, that holds a pane in a frame. It is designed to protect the pane’s
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edge bond from UV radiation by covering it and to securely and durably hold the pane in place while meeting all the various other requirements (e.g. ensuring that the rebate is ventilated and airtight) as well as reducing thermal bridges around edge bonds. The height of a glazing rebate depends on the longest edge of the pane (0.5 – 3.5 m / > 3.5 m). DIN 18 545-1 specifies 18 or 20 mm for multi-pane insulating glass (for a pane edge of < 0.5 m, at least 14 mm). The glass inset is typically about 2/3 the height of the glazing rebate. Voids in frames, such as sealant-free glazing rebates, in which moisture from condensation or infiltrating water can collect must remain permanently open towards the outside without allowing more water to permeate. Moisture collecting in such voids can directly damage wooden frames, but whatever the frame mater ial, water vapour pressure can impair the edge bonds of insulating glazing. Openings that prevent this are referred to as vapour pressure equalisation openings. They must be open towards the outside and be installed in front of the middle seal or in rebate gasket systems (Fig. B 2.59). Typical openings are round holes with a diam eter of 8 mm or slits 8 ≈ 20 mm [20]. There must be at least three in the lower horizontal rebate and at least one in the upper corner (Fig. B 2.60). Glass sealing
High loads caused by direct exposure to weather and variations in thermal expansion make permanently sealing joints between panes and frame materials a major tech nical challenge. [21] Malleable window putty has been used as a sealant in this area for centuries. Permanently elastic sealants and sealing profiles have only become available fairly recently. Windows sealed with sealants are described as ‘wet-glazed’ in contrast to dry glazing, which uses sealing profiles. DIN 18 545 defines various groups of loads on glazing, and information on various aspects of these are shown in Fig. B 2.61 (p. 110). Glazing with a filled rebate (Va) is now only used in special cases in single-pane windows.