Standard rain-screen façade systems rely on two lines of defence. The first line of defence – the outermost surface of the façade – is the primary barrier against air and moisture, designed to stop all of the air and water. The second line of defence is intended to stop small amounts of air and water vapour that may penetrate the first line.
The rain-screen concept utilises an outermost building envelope layer differently, as it is not designed to be impervious to air and water. Instead, it acts as a barrier against the rain but relies on a waterproof inner layer to block air and moisture penetration. Water entry resulting from air pressure difference can be controlled by the introduction of air space in the wall. Between the outer and inner layers is a ventilated air cavity that drains water to the outside. The cavity (including the inner surface cavity) acts as the primary line of defence against air and water penetration.
Cladding material is selected primarily for its appearance. The cladding is usually panelised and can be made of a variety of materials, such as stone, precast concrete, terra cotta, glazing, metal, etc. Because the inner layer is not visible in the completed construction, it is not designed for its visual qualities. Instead, it must be designed to withstand wind and seismic loads. To thermally and acoustically insulate the building and to prevent air and water from entering.
A variation of the rain-screen concept is the pressure-equalized rain-screen (PER). In a PER, the air pressure within the inner air cavity is made equal, or close to equal, to the air pressure at the exterior face of the building envelope, as seen in Figure 2-33. This prevents air and water from being pulled into the cavity. To achieve pressure equalisation, openings acting as vents must be designed into the outer building envelope surface.
The larger the openings, the more equal the inner and outer pressures will be. If the PER is designed well, the combination of equalized pressures and gravity will force rainwater to drain to the exterior.
Curtain walls are generally defined as thin, usually aluminium-framed walls, containing in-fills of glazing, metal panels, or other cladding materials. The framing is attached to the building structure and does not carry the floor or roof loads of the building while the wind and gravity loads of the curtain wall are transferred to the building structure, typically at the slab line.
Systems range from manufacturer's standard ‘off the shelf’ systems to custom walls provided by specialist fabricators. Custom curtain walls for commercial buildings are very common and cost-competitive with standard systems as the wall area increases. Façade specialist is often engaged with an expertise in custom curtain wall design for projects that incorporate these systems.
Installed on-site, an aluminium frame is fixed to the building prior to glazing being installed. With its named derived from the appearance of the building during construction, which has framing exposed that look like sticks externally, stick system curtain walls are typically installed according to the following sequence.
- Anchorages are made around the perimeter of the building.
- Vertical mullions are fixed to the slab edge, often extending vertically over numerous floors.
- Horizontal transoms are site fixed to the mullions to form a grid of members.
- Glazing, back pans, insulation etc. are then installed to complete the curtain wall.
Generally, the lowest cost and lowest quality curtain wall system, stick systems as quality control over site installation and glazing are lower than factory assembled systems.
Semi – Unitised Systems
Partially site assembled and partially factory assembled, semi-unitised curtain walls are similar to stick systems where mullions and transoms are normally site assembled. However, factory glazed cassette frames are then fixed onto the frame to complete the install.
Semi-unitised cassette frames are generally either 2 or 4 sided. 2-sided cassette frame systems are normally structurally glazed along the vertical joints to the mullions while the horizontal joint is site glazed. In 4-sided systems, they are factory glazed on all sides and fixed to the mullions and transoms. The seal between the cassettes and the mainframe are either via gaskets or via sealant applied on site. Gasket systems are preferable as they offer superior movement capability, long term performance and are not reliant on quality control over site sealing.
Semi-unitised curtain walls have similar quality control challenges as stick systems.
Entirely factory assembled and glazed, unitised curtain walls are fully glazed units in a factory, shipped to the site and installed. With enhanced quality control, higher quality of assembly and glazing can be achieved.
When installing on-site, interlocking split mullions and split transoms provide a composite, weatherproofing the curtain wall frame. The split mullions and split transoms are normally sealed by compressed gaskets which allow the curtain wall to accommodate building movements without stressing the glazing, cladding and weather seals.
Spandrel frame curtain walls comprise a frame at floor level, which covers the zone between the ceiling and the sill of the window to the floor above. This frame is fixed in position along the edge of each floor slab, and short mullions and the vision glazing are installed on-site between the sill and head of the window on each floor.
This system is common and only suitable for curtain walls which have horizontal banding between the spandrel zone and the vision zone. The curtain wall can be either full site assembled, or the spandrel frame can be factory installed, and the vision glazing can be installed on-site.
One of the main benefits of this type of curtain wall is a very high provision for accommodating building movements. However, due to the large ratio of the frame area, they can be very poor performing when considering for their total thermal resistance.