The window opening package

The brief

An aperture in a wall, floor or roof is known as an opening. Openings join spaces for functional and/or visual reasons and thus establish a relationship between them. In the following we shall restrict our observations to openings in vertical external walls. The surfaces within the depth of a wall created by forming an opening are known as reveal (vertical), sill and head/lintel (horizontal).

The window is a building component for closing off an opening. It consists of outer and sash frames plus the glazing and is fitted into the structural opening. Together, window and opening therefore form an indispensable constructional package. The window is both an element of the package and the divider between interior and exterior.

The light permeability of the glazing promotes visual links between inside and outside, and also admits daylight into the interior. Consequently, the position and size of the opening is a key element in the design of the interior. Furthermore, if the incoming light - divided into direct sunlight and diffuse daylight - is also directed and regulated, this has a particular influence on the design concept.

In terms of the performance of the building, the window must provide a viable separation between the interior and exterior climates, and to do this it must exhibit certain thermal I nsulation characteristics. The main load on a window construction is that due to water and moisture in all their states, both from inside (moisture in the air, vapour diffusion) and from outside (rainwater, snow, meltwater). Essentially, the window design should prevent water from entering, but if it does enter it should be able

Spandrel panel

to drain away in a controlled fashion (waterproofing). The airtightness of the window-opening package also needs to be given attention. After all, the window assembly must guarantee comfortable conditions inside the building, and that involves thermal and sound insulation issues.

When preparing the working drawings the tolerances must be taken into account. As windows can be produced with considerably tighter tolerances than, for example, masonry, it must be possible to accommodate the tolerances when fitting the window into its structural opening. But the window manufacturer can use the as-built dimensions and hence construct a window to the exact size required.

At the window head it is necessary to leave space for a sunshading system, which will have an effect on the window head and lintel design.

The principle of the opening rebate

The opening rebate is a peripheral step or shoulder in the structural opening and thus forms the contact face between outer frame and structural opening. The window is fitted up against this step, fixed with screws and sealed. To avoid stresses caused by temperature-related movements, the frame must be built in with minimum tolerance. All fixings must be protected against corrosion.

The principle of the frame rebate (see full-size details)

The biggest problem with the window is keeping out water and wind. The rebate in the structural opening and the rebates in the frame members are therefore the most important elements in this battle. Special attention must be paid to the tightness of the joints between outer frame and opening, and outer frame and sash frames. The weatherstripping between outer frame and sash frames remains in the same position around the entire periphery and is sealed at the corners. There are two different sealing positions in a window element: Outer frame-opening

- water and wind

- accommodation of climate-related movements in the masonry

Outer frame-sash frames

The rebate is intrinsic to the design of windows with opening lights, i.e., opening windows:

- j oint permeability for controlled air change rate between sash frames and outer frame

- protection against driving rain, water and wind

Fig. 23: Isometric view of opening rebates

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