CWCT FACETS

01.02 Joints

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Categories: Envelope Sealing

The function of joints in the building envelope
Joints are required where components or assemblies of a wall meet. The need for a joint may arise because dissimilar forms of construction meet, a component such as a window is inserted into a wall or multiple panels or components (each of limited size) have to be joined together.

Joints are an integral part of the wall and may be required to provide a seal against water leakage, air permeability, sound transmission, fire spread and so on. In multi-layered walls only one layer may be required to provide the barrier and the function of a single joint will depend on its position in the wall as well as the performance required of the wall as a whole.

Joints may be required to allow movement of the facade. Movement of the structure or the facade may be accommodated by allowing panels and components to move relative to each other. Joints may also be required to transmit loads and provide fixity of components. However this section does not cover adhesive joints or welding.

Some joints are provided to allow the separation of components. This may be the every day operation of an opening window or door. However it may also be the rare opening of a smoke vent or simply to allow the removal and replacement of components as part of the repair process.

The provision of joints and their detailed design will determine the ease with which a facade can be constructed. The design of the facade and its joints should be undertaken with a clear understanding of the construction method and sequence.

Joints will affect the appearance of the wall and may be provided for just that purpose. Certainly panel sizes may be selected on the basis of aesthetics.
 


Joint leakage
Air and water leakage through a joint in a facade may lead to several problems, ranging from occupant discomfort in draughts, to damage to property by contact with water, or even damage to structural elements of the building envelope.

The diffusion of water vapour through a joint may be a significant benefit in terms of reducing the risk of condensation within the facade, or may be a significant factor in causing condensation within the facade, depending on where the joint is. Section 05.05 covers moisture movement.

The main issues are watertightness and air leakage.  Water leakage will depend on the location of the joint within the building envelope.  Wind driven rain wets the building differently across its surface, image, and flows across the wall or roof to affect some joints more than others, image.
 


Water leakage mechanisms
Water leakage through a joint may occur by several different mechanisms.  These are:

  • Gravity flow
  • Pressure flow
  • Kinetic flow
  • Surface tension effects
  • Pumped flow
  • Air-supported droplets


Gravity flow is where a drop of water moves under the influence of its own weight, image.  The physics of gravity flow demand that there is sufficient weight to overcome surface tension forces.  Gravity flows can be discouraged by providing steps and upward slopes in the path of the flow, although care should be taken not to create regions where water can collect.

The solutions are relatively simple and require only good workmanship and a clear understanding of drainage paths, in three dimensions, at the design stage.

Pressure flow can occur as the result of a pressure difference across a body of water - it is usually necessary that the water form a continuous film across the joint, otherwise a sufficient air pressure differential cannot be generated to move the water, image.  Pressure flows can be discouraged by preventing a continuous film of water from forming, and this requires that the joint is sufficiently wide that any film of water collapses under its own weight.  However, if water cascades over the joint opening then a continuous film may be generated and blown into the joint by air pressure fluctuations.  This can be discouraged by distancing the stream of water from the joint opening with a suitable overhang (a rain-screen with a drip).

Water leakage by this mechanism may be prevented by pressure equalisation, the provision of well drained joints or a high upstand in the joint. Note; a wind pressure of 300Pa will support a 30mm column of water.

Kinetic flow can occur if water droplets approach the joint at a sufficiently high speed that they are carried through the joint by their own momentum, image.  Suitable arrangements of baffles, preventing a direct passage through the joint, or a rain-screen with a drip, preventing access to the joint opening, will stop kinetic flows.

Surface tension attracts water to surfaces.  This allows a droplet of water to run along a horizontal surface, unless some step is provided in the path of the water to encourage the water to separate from the surface, image.  A ‘drip’ is often used on the edge of any overhanging feature to prevent water from running back along the underside of the overhang.  With a timber sill a groove is cut in the underside.  With an aluminium or plastic extrusion a nib is included on the underside of a sill.

Capillary flow may occur due to surface tension, but this requires a narrow joint, so that the mass of water is sufficiently small that it can move under the influence of the (low) surface tension force, image.  If the joint is wide enough to prevent the formation of a continuous liquid film then capillary flow cannot occur.  Where a film of water does occur then pressure flow may occur.

The amount of water that can pass a joint in this way is limited by the narrowness of the joint.  However, water ingress into the joint may cause local damage such as mould growth within the joint.

Pumped flow occurs if a film of water is trapped between two surfaces which undergo a relative motion.  Pumping is prevented if the film of water cannot form in the first place.  Again the joint should be sufficiently wide that a film of water cannot form.

Air-supported droplets can be carried by a fast moving flow of air through a joint (this is different to kinetic flow in that the droplets may be very small and possess little momentum), image.  This phenomenon can be prevented by slowing the air, such that the water droplets fall under gravity and can be drained away, or by stopping the flow of air by using some form of draught-strip.  The air movements may be slowed by adopting a pressure equalised design.  Alternatively the joint can be shaped as a labyrinth such that the droplets impinge on some convenient surface from where they can be drained.
 


Joint design for watertightness
Most water movement through a joint can be minimised by sensible joint design.  The use of rain-screens, baffles, steps, drips and wide joints can limit the available mechanisms for water transport through an opening.  However, the issue of air-borne water droplets passing through a joint does require that the air flow through the joint is either stopped or slowed.  This may necessitate the use of a draught-strip within the joint, and this draught-strip can be placed at any position within the joint.

Herbert and Harrison (1974) examined a number of open joints with differing degrees of baffling within the joints.  It was found that overlapping baffles were sufficient to prevent water penetration, and that directing water droplets into regions of low air velocity was a successful strategy for draining the water out of the joint.  An important finding of this study was that there is a critical air velocity through an open joint, above which water droplets are entrained into the air flow and carried through the joint.  A value of about 5 m/s at the joint opening was suggested for this critical velocity.  However, water could be forced through the joint at lower velocities if a ‘plug’ of water formed across the opening of the joint.  It is recommended by Herbert and Harrison that the joint opening should not narrow too rapidly, and that the smallest gap within a joint should be at the back of the joint, presumably to minimise air velocities near the entrance of the joint.  Guidance is also given on the design of joints which rely on an overlap (a step in the joint) to reduce water penetration.

Bassett, Bishop and Brown (1991) give very good case studies of water penetration through joints, and remedial actions that were required to correct the problem.  The study is a good demonstration of how easily water will find weaknesses in constructions, particularly if there is an air pressure difference encouraging water flow.  The need for proper detailing at corners and junctions of seals is also emphasised.

It should be noted that any cavity within a facade may become filled with water, and so all cavities should either be linked and drained to the exterior, or drained individually.  Joints around the cavity should all be carefully sealed, particularly if the cavity is intended to be pressure moderated, but care must be taken not to impede drainage routes.  Components such as aluminium window frames may also be at risk from water entering joins in the frame, unless those joins are sealed and protected from distortions that would open up the join.  PVC-U window frames may also be at risk from water penetration if welded corner joins fail - note that water can pass through the smallest gaps by capillary action, often wind-assisted.  If water leakage occurs through a poor join in a framing system then the weather-stripping or glazing gaskets may be unreasonably blamed for the failure.

A joint can be designed to function with only one seal, usually an air-seal.  However, a common design option in many UK windows is to locate a gasket at the external opening of a joint, either to act as a rain-screen (as in the opening light of a casement window) or to retain a component (as in the case of glazing gaskets).  However, this exposed position is not conducive to the long-life of many gasket materials.  The advantages of being able to use a single gasket in a joint, away from the joint opening, are best understood in conjunction with an appreciation of the behaviour of typical gasket materials.  This is discussed in Section 01.06.

BS 6093 (1981) gives guidance relating to the design of joints, and shows how a joint may be designed to avoid water penetration with the minimum amount of sealing.  Where gaskets are considered it is noted that the gasket manufacturer should be consulted early in the design stage, because the gasket interacts with the joint and it is the combination of joint and gasket that provides weather-tightness.  Furthermore, the need to achieve close tolerancing is emphasised, as is the need for continuity at junctions between gaskets.  The need for drainage is emphasised wherever gaskets must be joined on site and are more likely to be improperly joined, allowing some water penetration to occur.  For the most part however this standard considers sealant joints.  However, it is uncertain whether many joint designers even consider this standard, preferring to work on the basis of previous experience, usually ‘that’s the way we’ve always done it’, without any knowledge of whether the last set of joints actually worked!

The SFTC (1986) have also published guidance on weather-sealing, This image is taken directly from their publication.  It is interesting to note the statement on the lower figure (‘covered’ joint) that ‘the weatherseal must not be fixed here’, indicating the location where UK windows almost always have a weather-strip!  It is explained that putting the seal in such an exposed location will cause the seal to get wet, lose efficiency and possibly pump water into the joint.  Guidance is given on the design of the joint at all locations around an opening window, and there are also recommendations for the dimensions of drips to separate water from the underside of a frame.
 


Air leakage
This Section covers air leakage through joints.  Air permeability at the scale of components and complete facades are covered in Section 01.01.

Air leakage through a joint is governed by the size of opening and pressure difference across the joint.  Air leakage may be limited by reducing the pressure difference (seldom an option) or reducing the size of any openings in the joint.  Some gaskets will seal better under a positive pressure whilst a negative pressure will reduce the seal of the gasket.  For other gaskets the reverse applies depending on the design of the gasket and for opening joints the direction of opening.

Although some air movement is required through the facade, for ventilation purposes, a modern view is that the air movement should be controlled.  Uncontrolled air ‘leakage’ is generally the result of poor joint design, and may cause discomfort, result in poor energy efficiency, and lead to a failure of performance in pressure-moderated or pressure-equalised systems.

The presence of significant air movements may lead to occupier discomfort in two ways - either the occupier of a room is in the direct line of the moving air and feels cold as a result (the draught air is generally below body temperature even without allowing for ‘wind chill’), or some other lightweight item is in the direct path of the air jet and moves as a result (papers blowing off a desk, curtains billowing over a window or slatted blinds vibrating).  The joint and gasket designers should aim to eliminate jets of air moving through a joint.

Draughts are discussed in CIRIA SP87A (1992), which gives a simple chart relating acceptable air velocity (in terms of occupant comfort) to air temperature.  The acceptable velocity ranges from about 0.1 m/s for air at 17oC, to about 0.5 m/s for air at 30oC.

Air leakage through a facade also affects the energy efficiency, Package 05, and comfort levels, Package 06. It is apparent that the energy efficiency of a building will be greatly reduced if the leakage air flow-rate Q is not controlled.  However, an air-tight building must have some ventilation if the occupants are to be comfortable (ventilation also helps to reduce the risk of condensation and is now a requirement of the Building Regulations Approved Document F (1995)).  This apparent conflict of interest in requiring that leakage must be stopped and ventilation then introduced is not a problem if it is remembered that the aim is to provide sufficient ventilation - not a surfeit.  Issues of air-tightness are also important if the building facade is in some way pressure moderated or pressure equalised:
 


Small gaps
Air flow thorough narrow gaps takes the form of laminar flow.  For the laminar flow of air through a uniform gap between two parallel plane surfaces the mean air velocity is related to the pressure differential across the joint by

v = t2 x Dp / 12 x m x l

where

v is the mean velocity, in m/s
t is the width of the gap between the surfaces, in m
m is the dynamic viscosity of the air, (about 1.8x10-5 Ns/m2)
Dp is the air pressure differential across the joint, in Pa
l is the length of the joint, in m

For a gap 0.1 mm wide and 50 mm long a mean air velocity of 0.1 m/s only requires a pressure difference of 108 Pa.  This is the static pressure difference that will occur if a wind of 13 m/s blows against the outside surface of the building.  Although this wind velocity may appear to be high it is classed on the Beaufort scale as a fresh to strong breeze.  A larger joint width will obviously give a higher air velocity for the same pressure differential.

It should be noted that the air flow-rate through a 1 metre length of this joint is

Q= A x v = 10-5   m3/s

or

q = 3600 A x v = 0.036  m3/h per metre of joint

The requirements of BS 6375 Part 1 (1989) state that for a 108 Pa pressure differential the acceptable rate of air leakage through a window with an opening light is 2.1 m3/h per metre of opening joint - significantly greater than for the example above.
 


Larger gaps
Air flow through most gaps in the facade is of a more turbulent nature.

For fully turbulent flow the pressure difference across an obstruction is given by:

DP = Cd x r x v2

where:

    r is the density of air;
    V is the air velocity;
    Cd is a coefficient.

For most facade constructions it is assumed that:

Q = A x v

Where A is contant and

v = Const x DP2/3

This is the basis for the air leakage curves given in BS 6375 and the CWCT Standards.
 


Water vapour diffusion
The presence of water vapour within a cavity in a facade may give rise to the formation of condensation within the facade, which may then lead to damage of the facade structure.  Whilst fully sealing the facade could help to prevent the diffusion of water vapour into the facade this is a dangerous practice because the integrity of the seals cannot be guaranteed against poor workmanship or premature failure.  An alternative is to ensure that one face of the facade (that which is usually exposed to the environment with the lowest moisture content) is deliberately left unsealed so that water vapour can diffuse out of the facade.  This may require the use of a seal which prevents water penetration but allows water vapour to diffuse through the seal.

The obvious solution is to use a baffle.  As an alternative to a rigid baffle however, open-cell polymer foams may be impregnated with non-setting sealant materials which enable them to repel water whilst allowing water vapour to diffuse through.  The structure of the foam then behaves like a complex baffle.  An advantage of an impregnated foam is that it can be easily compressed and pushed through a small joint opening but will then recover to near its original size and shape, thereby filling the joint.  The sealant causes the foam to adhere to the joint surfaces, allowing the joint to expand (the degree of  movement required  may limit the use of a gasket - a gasket which is still in compression at the greatest joint width could place an excessive force on the joint surfaces at the smallest joint width), but the foam can be easily removed should it need to be replaced.  Moisture movement is covered in greater detail in Section 05.05.
 


Location of joints in the building envelope
Joint positions are determined by the general layout of the wall and its appearance. The position of joints such as glazing gaskets and window perimeter seals is totally dictated by the general arrangement of the wall. The number and location of site made construction joints will depend on the degree of pre-assembly (factory made joints), the maximum size of component that can be transported to site, the interfaces between different forms of construction and the aesthetic demands of the architect and client. Joints serve two principal functions that govern their spacing, location and size. During construction they accommodate tolerances and for the rest of the building life they accommodate cladding and structural movement.
 


Location for tolerance
Larger components or assemblies will in general have larger tolerances associated with their manufacture and perimeter joints of the assembly or component to component joints will have to be wider to allow fit of the components and subsequent sealing. Open joints are in general better able to accommodate tolerance.

For sealed joints it is often possible at the design stage to select either a large number of narrower joints or a few wide joints. In general factory made joints are of higher quality than site made joints and the use of factory made assemblies with a few wider and well constructed site joints will provide the lowest risk of joint/seal failure. (for information on tolerances see sections 03.07 Cladding tolerances and 03.08 Building tolerances).
 


Location for movement accommodation
Cladding panels and components have to either change shape or move relative to each other to accommodate movement of the structural system and the cladding panels. For sealed joints it is possible to accommodate movement at a few wide joints or at a larger number of narrower joints. The required spacing/joint width will depend on the magnitude of the movement and the joint design. In general open joints will accommodate greater movements, (for information on movements see sections 01.09 building movements and 01.10 cladding movements).
 


Types of joint
Joints can be described by their geometry,construction and operation.

When described by their operating characteristics joints are:

  • Fixed joints

    • These are between components that are mechanically fixed to prevent movement of the two edges of the joint.
     
  • Movement joints

    • These are between unrestrained edges of components and have to allow relative movement of the joint edges.
  • Opening joints

    • These joints are constructed so that they may be opened. For instance the seal around a door or opening light.

Joints may be classified under the following construction types and geometries:

Open joints (fixed/movement/opening)
Joint containing no sealant, gasket or baffle and with a clear line of sight through the joint image.

Baffled joints (fixed/movement/opening)
Joints in which some component (a baffle) is used to block the direct line through the joint, whilst not closing the joint image.

Labyrinth joints (fixed/movement)
Joints in which the edges of adjacent panels are shaped to interlock and create a tortuous path through the joint, whist not closing the joint image.

Closed joints (fixed/movement)
Joints that are not sealed but where there may be a small gap between components (less than 0.5mm) image.

Sealed joints (fixed/movement)
Joints where a gasket or wet-applied sealant is used to form an airtight seal between components image.

Gasket joints (movement/opening)
Joints containing a gasket that allow the joint to part and re-seal on closing.