Cladding - a extended notes by Dr. H. Li

(Word-to-Word copy-and-paste of (home.bre.polyu.edu.hk/~bshengli/notes6.html)) By Dr. Heng Li

1. References" Edward Allen, 1990, Fundamental of Building Construction ¨C Materials and Methods, 2nd Edition, John Wily & Sons, USA

2. Learning Objectives: "An understanding of the basic concept of claddings."

3. Functions of Cladding

3.1 Primary Functions

The major purpose of cladding is to separate the indoor environment of a building from the outdoors in such a way that the indoor environment can be maintained at levels suitable for the intended use.

Keeping out of water

Cladding must prevent the entry of water, e.g. rain, snow and ice into a building, especially when water on the face of a building is often driven by wind at high velocities and high air pressures, in every direction. The water must be drained away from a wind-ward building face during a heavy rainstorm, and the water pushed by wind, will readily penetrate the smallest crack or hole and enter the building.

Preventing air leakage

The cladding must prevent the unintended passage of air between indoors and outdoors. Smaller air leaks are harmful because they waste conditioned air, carry water through the wall, allow moisture vapour to condense inside the wall, and allow noise to penetrate the building.

Controlling light

The cladding must control the passage of light, especially sunlight. Sunlight is visible light, useful for illumination but bothersome if it causes glare. Sunlight includes destructive ultraviolet wavelengths that must be kept off human skin and away from interior material that will fade or disintegrate.

Controlling the radiation of heat

The role of cladding is to regulate the flow of radiant heat from sun, it should present interior surfaces that are at temperatures that will not cause radiant discomfort. (not too cold and not too hot)

Controlling the conduction of heat

It must resist to the required degree of the conduction of heat into and out of the building. It should avoid thermal bridges, wall component such as metal framing members that are highly conductive of heat and therefore likely to cause localized condensation on interior surfaces.

Controlling water vapour

It must retard the passage of water vapour. Vapour moving through a wall assembly is likely to condense inside the assembly in cold weather and cause problems of staining, lost insulating value, corrosion and freeze-thaw deterioration.

Controlling sound

It should isolate the interior of a building from noise outside, or vice versa. Noise isolation is best achieved by walls that are airtight, massive and resilient.

3.2 Secondary Functions

Resisting wind forces

The cladding of a building must be adequately strong and stiff to sustain the pressure and suctions that will placed upon it by wind. The upper reaches of taller buildings are buffeted by mush faster winds, and wind directions and velocities are greater.

Adjusting to movement

Several kinds of forces are always at work throughout a building, tugging and pushing both the frame and the cladding: thermal expansion and contraction, moisture expansion and contraction, structural deflections.

Thermal expansion and contraction ¨C Indoor/outdoor temperature differences can cause warping of cladding panels due to differential expansion and contraction of their inside and outside faces. The building frame itself will expand and contract to some extent, especially between the time the cladding is installed and the building is first occupied.

Moisture expansion and contraction ¨C Bricks and stone expand slightly after they're installed. Concrete blocks shrink slightly as its curing is completed and excess moisture is given off.

Structural movements ¨C Building foundations may settle unevenly, causing distortion of the frame.

Resisting Fire

According to the building regulations.

Weathering gracefully

To maintain the visual quality of a building.

4. Conceptual Approaches to Watertightness in Cladding

Keep water completely away from the wall, however, this is impossible as it requires a very broad overhang.

Eliminate openings from a wall, sealing every seam in the wall.

Eliminate or neutralize all the forces that can move water through the wall.

4.1 Forces that can move water through the wall

a. Gravity

It is a factor in pulling water through a wall only if the wall contains an inclined plane that slopes into the building.

b. Momentum

It is the horizontal component of the energy of a raindrop falling at an angle toward the face of a building. Momentum is easily neutralized by applying a cover to each joint in the wall.

c. Surface tension

It causes water to adhere to the underside of a cladding component, it can allow water to be drawn into the building. The provision of a simple ˇ®drip' on any underside surface to which water might adhere will eliminate the problem.

d. Capillary action

It is the surface tension effect that pulls water through any opening that can be bridged by a water drop. This action can be eliminated by providing a concealed ˇ®capillary break' somewhere inside the opening.

e. Wind currents

The generic solution to the wind current problem is to let wind pressure differences between the outside and inside of the cladding neutralize themselves through a concept know as the rainscreen principle.

5. Sealant Joints in Cladding

All cladding systems require sealant joints. The role of a sealant is to fill the joints between cladding components, preventing the flow of air and/or water, while still allowing reasonable dimensional tolerances for assembly and reasonable amounts of subsequent movement between the components.

Sealants are typically used to seal joints between panels of stone or precast concrete in a curtain wall, to seal the joint beneath the shelf angle in a brick curtain wall, and to seal joints between dissimilar materials, such as where a metal and glass curtain wall ends against a masonry wall.

5.1 Sealant Joint Design

The time of year when the sealant is to be installed must be taken into account when specifying the size of the joint and the type of sealant.

In cold weather - sealant will have to stretch very little during its lifetime but will have to compress a great deal in summer.

In hot weather - as the materials around it expand and crowd together. Sealant will have to compress very little but will be greatly stretched in winter.

6. Curtain Wall

Curtain wall is an exterior cladding supported at each story by the steel frame, rather than bearing its own load to the foundations. The principal advantage of the curtain wall is that because it bears no vertical load, it can be thin and lightweight regardless of the height of the building.

The name of "curtain wall" derives from the idea that the wall is thin and "hangs" like a curtain on the structural frame. The walls are supported from the bottom at each floor level.

The curtain wall must fullfil the same functional requirements as any other system of external walling. The main problem in the design of curtain walls lies in the framework which holds the panels and it is normally metal or timber. Curtain wall may be faced outside with any non-combustible material suitable for exposure to the weather. It can be constructed in place or prefabricated.

6.1 Connection

Their fixing must be designed accordingly. Fixings should be of stainless steel or non-ferrous metal and so designed, that should one fail the remaining fixings are capable of taking all the loading on the walling. This provides a margin of safety and prevents progressive failure of a number of fixings.

Fixing devices must be capable of adjustment in any direction to provide for inaccuracies in the structural surfaces to which the framing is attached. Cast-in anchor channels are commonly used in concrete frames to provide the horizontal adjustment.