02.01 Principle

Categories: Rainscreens

Introduction
Rainscreen is a form of layered construction.  An outer layer, the rainscreen, protects the inner layers from most of the water incident on the wall.  Walls that include a rainscreen are known as rainscreen walls.

Traditional forms of rainscreen wall are tiles or slates hung on a wall, timber shingles and lap boarding.

Modern rainscreens generally comprise panels larger than traditional tiles.  They may be up to a storey in height but are typically of dimension 600mm to 1200mm on any side.

Layered wall construction is described in Section 01.04.  A rainscreen wall generally comprises windows and doors as well as opaque areas of wall.  The sealing of walls is covered in Section 01.04.

Materials used in rainscreens include:

• Aluminium panels
• Thin natural stone
• Terracotta units
• GRP panels
• GRC panels
• High pressure laminates
• Composite panels

Rainscreen walls may be constructed as:

• Overcladding, Section 02.02,
• Curtain walling, Section 02.03.

Rainscreen walls may be constructed to keep the water out by being a:

• Drained and ventilated wall, Section 02.04,
• Pressure-equalised wall, Section 02.05.

Both methods of keeping water out of the building may be used with equal success for rainscreens that have closed, baffled or labyrinth joints.  However, for any given geometry of panels and joints pressure equalisation will reduce the risk of water penetration.

Rainscreens with open joints, Section 01.02, should be constructed as pressure equalised walls.
 

Principal features of rainscreens
The key features of a rainscreen system are, image:

• An outer skin of panels, the rainscreen, which may range from aluminium sheets a few millimetres thick to stone slabs several centimetres thick
• An air gap, at least 25 mm deep
• An insulated airtight backing wall

The air gap is the primary feature of a rainscreen system.  Many other types of cladding system either do not have an extensive air gap (for example many stick-system curtain walls only have an air gap at the perimeter of glazing units and infill panels), or rely on an air gap only to vent moisture from the wall and reduce condensation risk (e.g. profiled sheet metal cladding).  In a rainscreen system the air gap is designed as a pressure cushion to prevent water from reaching the insulation or backing wall.

As its name suggests, the purpose of the outer rainscreen panels is to shield the majority of the wall from direct rain.  However, depending upon the nature of the joints between the panels some water penetration may occur, and the air gap and airtight backing wall combine to limit this penetration.  The performance of rainscreen systems thus depends upon the particular way in which joints, air gap and backing wall are constructed.
 

Panel joints
These may be any combination of:

• Open joints
• Baffled open joints
• Labyrinth joints.
• Closed
• Sealed joints

Note that open, baffled open and labyrinth joints can be designed to limit water penetration by all known mechanisms with the exception of water droplets carried through by a fast moving air flow.  It is the air gap and the backing wall that are used to reduce or eliminate water penetration by entrainment.  Joints are described in greater detail in Section 01.02.

It is a feature of rainscreen systems that joints do not need to be sealed, because water penetration is managed by the combination of the behaviour of the air gap and the backing wall.
 

Air gap
The air gap may be treated in one of two ways:

• The air gap is left continuous, perhaps running up through several storeys, image, and across several panel widths, although artificial cavity barriers may be introduced and the air gap is often limited by firestops or closers around penetrations.  With this approach water penetration is allowed to drain downwards and outwards, and air movement is encouraged to dry out any water which enters the air gap.  To enable this ventilation the rainscreen may have all joints left unsealed or open, or may only have unsealed joints at the top and bottom of the air gap.  The term 'drained-and-ventilated' is applied to this technique, which is described in Section 02.04.

 
• The air gap is compartmented into cavities which may be no more than one panel width and height in extent, image, or which may encompass several panels.  At least one joint at the perimeter of the panel is left unsealed or open, to enable pressurisation of the air gap, which then limits water penetration by virtue of the reduced pressure difference across the panel.  The term 'pressure-equalised' may be applied to this technique, which is illustrated in the image.

In the past the term 'pressure-equalised' has been loosely used.  The degree of pressure-equalisation that is achieved depends upon the ratio of the vent aperture area to the cavity volume, and to the degree of airtightness that is achieved for the cavity dividers and the backing wall.  The conditions necessary to achieve pressure-equalisation are given in Section 02.05.
 

Backing wall
The backing wall is critical to the performance of rainscreen systems.  If air movement through the backing wall is too great then the risk of water penetration past the rainscreen is increased; air leakage through the backing wall also represents an energy loss, and so must be limited for this reason.

The backing wall must therefore be reasonably airtight, but there are four main forms that the backing wall can take:

• A masonry or blockwork support wall.  This type of wall may be an existing wall, or may be new build, Section 02.02,
• A dry lining system, Section 02.03,
• A prefabricated support wall.  Many systems which appear to be panellised or unitised curtain walling systems (factory built) are actually rainscreen systems in which the rainscreen panels hang onto the outer face of the prefabricated element, Section 02.03,

A stick system backing wall.  In this type of wall, which might be termed an 'integral' construction, the insulation is in the form of prefabricated panels held in a stick system curtain walling frame, with the rainscreen panel fixed to the outside of the stick system, Section 02.03.

Air barrier
It is not necessary for the backing wall itself to be airtight - there may be a layer associated with the backing wall which presents the most resistance to air flow.  The layer which provides the maximum resistance to air flow is termed the air barrier.  Note that the air barrier may be a layer of plaster or render on the inner face of a blockwork wall, a continuous sheet affixed to the outer face of a plasterboard dry lining, or the insulation trays in an integral wall construction.

The air barrier needs to be continuous, although it need not lie in a single plane.
 

Cavity
The space between the air barrier and the inner face of the rainscreen panels is called the cavity.  Insulation normally fills part of the cavity and the cavity width is not the same as the air gap, image.

It may, however, be necessary to make an allowance for the volume of air trapped within porous insulation and backing wall materials as part of the total air volume.

Minimum cavity widths and air gaps are given in Section 02.04.