01.05 Framing technology

Categories: Envelope Sealing

Introduction
This section shows how the principles of sealing a building envelope may be applied to the design of a rebated framing system containing infill panels such as glazing units or thermally insulated spandrel panels.

The principles which may be used to create a weather proof wall of panel or monolithic construction such as a blockwork or concrete wall or a rainscreen wall are set out in Section 01.01.

Methods of sealing windows into walls and sealing the interface between a glazing screen and adjacent walling are described in Section 01.04 which should be read in conjunction with this Section.
 

Curtain walling/windows
In curtain walling and windows it is necessary to seal the joint between the glass or infill panel and the surrounding frame.  Opening lights and doors have the added complication that the opening joint between the casement or door and the surrounding frame also has to be sealed. Four basic techniques may be used:

• Fully bedded (applies to windows only);
• Single, front-sealed;
• Drained-and-ventilated;
• Pressure-equalised.

Fully bedded
Fully bedded glazing is a ‘sealed system’ relying on the glazing rebate being completely filled with glazing compound to prevent the passage of water.  No drainage is incorporated since the seal is assumed to be 100 per cent effective at excluding water.

Any voids in the bedding compound will attract water, which may penetrate to the edge of the glass unit, become trapped and degrade the unit edge seal if insulated glazing units have been used.

Fully bedded glazing systems are not recommended due to the difficulties of ensuring that a full bedding is achieved and due to the risk of water penetration if regular inspection and maintenance by the building user are not sufficiently rigorous.  Fully bedded systems may be used for single glazed constructions such as shop windows and external glazing screens.
 

Single front-sealed systems
Face-sealed glazing/sealing systems rely on a weatherproof outer seal to prevent water penetrating into the rebate and thence the interior.  This seal must remain completely free of defects to prevent leakage paths occurring.  Any water that bypasses the outer seal can drain away only within the framing system.

Face sealed glazing systems may let some water through into the glazing rebate from time to time.  Therefore, provision must be incorporated in the design to allow this water to either evaporate or drain away.
 

Drained-and-ventilated systems
In drained-and-ventilated systems, the front gaskets provide an initial barrier.  The rebates and cavities are drained and ventilated to the exterior to prevent the accumulation of any water that bypasses the outer seals.  Drainage is usually via small holes or slots in the underside of transoms or via sloping glazing platforms that drain water to the mullion and down to the exterior every two to three floors.  Any obstruction of the glazing rebates should be avoided since this may reduce the drainage performance.

Ventilation of the cavity behind the outer glazing gasket allows the passage of air behind the outer gasket and the inner glazing gasket is the only seal preventing excess air leakage through the glass/panel to frame joint.

With any drained or pressure-equalised system (see below), water may not always flow through the drainage outlet because the volume of water is not sufficient to overcome surface tension.  Therefore, such systems often incorporate ventilation holes/slots in the underside of the upper transom to assist ventilation so that moisture can evaporate away helping to maintain the glazing rebate and seals in a dry condition.
 

Pressure-equalised systems
The ventilation openings of pressure-equalised systems are of an increased size to permit rapid equalisation of pressure in the glazing cavities with the external pressure, thereby preventing water penetration of the outer face.  Effective pressure-equalisation requires the following features:

• Outer face is sealed as tight as possible against rainwater;
• Inner face is sealed as tight as possible against air flow;
• Cavities are made into compartments by sealing mullion/transom joints;
• Each compartment is connected to the exterior by protected openings.

As with a drained and ventilated cavity.  Ventilation of the cavity behind the outer glazing gasket allows the passage of air behind the outer gasket and the inner glazing gasket is the only seal preventing excess air leakage through the glass/panel to frame joint. To ensure that the inner gasket is airtight, all corners must be diligently sealed. However, this is not a fundamental requirement for the outer gasket.
 

Drainage
Drainage provision is incorporated within a glazing system as a precautionary measure, based on the principle that some water will penetrate into the glazing space.  Drainage is normally achieved by machining holes at a suitable low point in the system to allow water to flow out under gravity. The action of a number of forces will, however, conspire to inhibit drainage under normal conditions:

• Surface tension will tend to keep the water in contact with the metal surfaces and the perimeter of the drainage hole.  The sooner this hold is broken the more readily will the water drain away.  An increase in volume and thus weight of water will eventually overcome surface tension, but increasing the size of drainage hole is equally important.  A good workable minimum drainage slot is 20mm or 25mm by 5mm.  For circular drainage holes a minimum diameter of 8mm is recommended.
• External wind pressure will hold water back until either the wind pressure falls or the water builds up to a level sufficient to overcome the pressure.  Drainage slots at the sill, which are covered by a film of water, cannot assist in the process of pressure-equalisation.  Indeed, a number of systems, which purport to be pressure-equalised, fail to perform owing to their reliance on a single level of drainage holes.  Holes should, ideally, be introduced at other locations in order to assist in maintaining an equalisation of pressure and drainage of water.
• Obstructions to the drainage paths will also prevent the escape of water from within the system.  These obstructions can occur through the accumulation of debris leading to the blockage of drainage holes or the use of unsuitable setting blocks placed across the drain slot.  Weep hole covers will help to avoid the accumulation of debris and will act as a baffle to limit the effects of external wind pressure.
• Insufficient clearance between the edge of the glass and the glazing frame will encourage the retention of water by surface tension forces.  A minimum clearance of 5mm in general, and 6mm at the sill, will be sufficient to prevent bridging by water.
• Insufficient slope of the glazing sill/platform can prevent water from flowing to the drainage holes or to the ends of the transom and down the mullion.  The dead load deflections of transoms should be limited so as not to impair drainage of the system.

Window construction
Fully bedded and single sealed systems
Many single glazed windows were constructed using fully bedded glazing systems. These were constructed using a bedding compound and clips to retain the glass or a combination of a glazing bead and bedding compound.  In some cases a separate bedding system and seal were used as shown in this image.  These systems are totally dependent on the performance of the outer seal to avoid leakage of water into the glazing rebate where it will be retained due to the lack of drainage.

A development from fully bedded systems was the use of gaskets to replace the bedding compound, image.  Again these systems are totally dependent on the outer gasket to prevent water entering the glazing rebate which is not drained.

Some windows were constructed using a channel gasket that provided both the inner and outer gaskets as a single profile that fitted closely over the edge of the glazing or infill panel.  In this case water leaking past the gasket will be retained in either the glazing rebate or the rebate of the gasket where it is held in contact with the glass or infill panel. Channel gaskets and channel frames are currently only used for glazing of shop fronts.
 

Drained systems
Drained window frames will be either:

• Drained,
• Drained and ventilated,
• Pressure-equalised.

Window frames are of drained construction if there are drainage openings that allow any water entering the glazing rebate to flow back to the outer face of the framing system.

Window frames with opening for ventilation, at a different level from the drainage holes, will permit a flow of air through the glazing rebate and be drained and ventilated.

Window frames that are designed to be pressure-equalised generally require larger openings and a tighter inner (air) seal.

A typical drained frame is shown in this image.  The lower framing member of a window is the same whether the widow is simply drained or drained and ventilated.  Water is able to drain from the glazing cavity through protected drainage holes or slots.  It may be possible to drain the glazing cavity through the base of the framing profile but note that any drainage water must then be intercepted by a sill or flashing and then flow freely to the outer face of the facade.

There is no need for the specifier/client to distinguish between drained and ventilated  and pressure-equalised framing systems.  Both are capable of delivering a window that meets the requirements for water penetration resistance.  The important point is that the framing system should be drained.  It should, preferably, have ventilation openings which may be part of a drained and ventilated or pressure-equalised scheme.
 

Opening lights
Where an opening light comprises a opening casement and a fixed frame, image, there is a cavity between the casement and the frame.  Water may enter this cavity and it should be drained in a similar way to the drainage of the glazing cavity.

Drainage of the cavity between the frames can be achieved by draining it directly to the outer face.  Alternatively, and often conveniently, it may be drained in to the cavity of the the lower framing member and then drained to the outer face.  Note that drainage of the casement caviity can now be achieved by draining it into the cavity between the two frames.  Sensible consideration of the drainage paths for each and every cavity can minimise the number of drainage openings and protective covers required on a window.  It is sometimes possible to avoid the use of any drainage holes and protective covers by arranging for drainage to occur through a full length slot between the lowest framing member and the sill.

The opening joint around a casement may contain inner and outer gaskets and also an intermediate seal, image.  This seal is the more flexible of the three gaskets and is the primary air seal.  In this case the space between the casement and fixed frame is divided in to two cavities.  The outer cavity has to be drained and ventilated as described above.  The inner cavity cannot be drained as any drainage openings would bypass the primary air seal.  For this reason no water should be drained into the inner cavity.

Some styles of opening light are more difficult to seal against water penetration than others.  Particular problems can arise with horizontal and vertical pivot windows as the gaskets of the opening joint cannot be made continuous.

Air leakage through opening lights will normally be higher than that through fixed lights of equivalent area.  However, it should be noted that not all opening lights are capable of providing the same control of air leakage.  Vertical and horizontal sliding windows are likely to allow more air leakage than casements and  not all window styles are capable of delivering the air sealing performance required on high quality buildings.  Of course there may be other, overiding, considerations that determine the style of window.
 

Curtain wall construction
Face sealing
Curtain walling has in the past been constructed using fully bedded glazing systems and today existing walls may be encountered that are fully bedded as for windows, image, or dry glazed without drainage, image.

Face sealing techniques are used with particular types of facade construction.  Face sealing is generally restricted to:

• Glazing systems in which the glass is secured by a structural gasket, image,
• Frameless glazing systems.

Drained systems
Curtain wall framing systems that have a glazing rebate to retain glass or infill panels are almost universally constructed with drained rebates.  The principle is the same as that applied to windows so walls may be:

• Drained,
• Drained and Ventilated,
• Pressure-equalised.

Drainage of the glazing rebate can be achieved by either:

• Transom drainage,
• Mullion drainage.

Note that some framing profiles can be machined and assembled to achieve either method of drainage.
 

Transom drainage
Transom drainage is achieved by draining the glazing cavity through holes in the pressure plates and cover caps, image.  Water is drained from each and every glazing bay local to the point of leakage in to the wall.  The drainage path is down the glazing cavity at each side and along the glazing platform of the transom.  It is important that the drainage path is not blocked by the setting blocks, swarf or any excess sealant.  The guidance on drainage given above is equally applicable to curtain walling and windows.

With transom drainage it is important to ensure that water does not drain from one glazing bay to another.  The transoms are sealed to the mullion to prevent this, image, image and image.  Drainage openings are provided in each mullion to drain the glazing bay above it, image.

To construct a drained and ventilated wall additional ventilation openings are provided in the transom above each and every glazing bay, image.

To achieve pressure equalisation of a curtain wall frame it is necessary to compartment the wall as described in Section 02.05.  This is achieved by sealing every transom to its adjacent mullions, image. This provides a continuous airtight seal at the back and edges of the compartment.
 

Mullion drainage
Mullion drainage is achieved by by draining water along the transoms to their ends where the water is able to drain down the glazing cavity in the mullion, image. Some systems are more complex with a drainage chamber incorporated in to the transom profile, image.  With these it is important to that the transoms are correctly sealed to the mullion with either a wet applied sealant or an injection moulded gasket depending on the system design.

Mullion drainage is only effective over two to three storeys unless water is drained out of the mullion at two to three storey intervals.  This is necessary because of the flow of water that can be carried in the mullion glazing cavity or mullion drainage channel.

Mullion drained walls cannot be pressure-equalised because of the requirement to link one glazing bay to another.