01.04 Layered Construction

Categories: Envelope Sealing

Introduction
Building envelope sealing may rely on a single set of seals such that there is a single layer acting as a barrier to provide the required performance of the wall.  Alternatively multiple seals may be used.  In a rainscreen there are two layers of wall. An outer rainscreen prevents water entering the wall and an inner air barrier prevents excess air leakage.  In this case the joints in the rainscreen are only required to prevent water from passing.  The seals in the air barrier then provide an air seal to control air permeability. Components such as windows and panels may also contain dual seals or multiple seals, for instance outer and inner glazing gaskets.
 

Drained walls
Where multiple layers are used for a wall water may intentionally, or otherwise, enter the cavity between the layers or dual seals.  This water has to be drained back to the outer face of the wall and drainage holes (co-ordinated with the internal drainage paths) are provided to allow water to drain from the cavity.
 

Face sealed walls
Any wall that is not drained is presumed to be face sealed and the outer joints have to be designed so that no water passes them during the life of the facade.  Any water passing the face seals will either

• leak to the inside

or
• remain in the wall cavities and
• reduce thermal performance
• lead to corrosion

Drained and ventilated walls
If drainage holes are provided to allow water out then air may enter these holes.  If additional holes are provided remote from the drainage holes a circulation of air can be achieved.  This will dry any residual moisture in the cavity and remove water vapour.
 

Pressure equalisation
As a further measure walls and glazing frames may be pressure equalised.  For more information on pressure equalisation of glazing and curtain walling frames see Section 01.05.  Package 02 covers this and other aspects of ventilated rainscreen walls.
 

Joints
Joints in the outer layer
Joints in the outer layer of the wall or the outermost joints in a dual sealed system may be:

• open
• baffled
• labyrinth
• closed
• sealed

All but sealed joints may allow a limited amount of water to enter the wall.  Even sealed joints may allow water to pass as a result of poor construction or deterioration with age.  Joint types are described in more detail in Section 01.02.
 

Joints in the inner layer
Joints in the air barrier (inner layer) of a wall and the inner most joints of a dual sealed system are required to provide an air seal and have to be sealed joints using either:

• wet-applied sealants
• gaskets

Even if sealed joints are provided in the outer most layer it should be presumed that the inner joints are the air seal.  This is essential if the outer layer contains drainage holes and allows air to pass.  It is also the case that the outer joints are more exposed to the weather and the environment and will break down first leaving the inner seals to control air permeability in the aged wall
 

Drainage
Cavity drainage
There should be identifiable drainage paths from all potential leakage points to return water to the outer face of the wall.

Water may drain from the cavity by;

• falling to be intercepted by a flashing or internal gutter
• running along an internal gutter or rebate
• running along a glazing rebate.

Water will flow along a horizontal surface provided the water surface is sloped.  Water will only flow when sufficient depth of water exists to overcome surface tension effects.  This may be as much as 6mm depth of water.  Water should not be expected to travel more than a metre along a horizontal surface within a wall unless water depths in excess of 6mm can be accommodated.  This would normally require a purpose designed internal gutter or channel.  Water will not run uphill and account should be taken of the deflected shape of the transoms and other framing members.
 

Windows
Drainage paths must not be obstructed by setting blocks or swarf.  Drainage holes should have the minimum dimensions shown in Section 01.05 and holes should be free of burrs that will inhibit flow.

Drainage from a window frame is by drain holes from the glazing rebate to the outer face or this may be through the bottom of the frame.  All cavities should be drained including those:

• in frames of opening lights
• in fixed frames
• between window frame and wall

Water passing the perimeter seal of a window will get into the cavity between the wall and the window frame and drain down to the sill.  Provision must be made to drain water out at sill level.
 

Stick curtain wall
Walls may be drained by allowing water to run out at each transom as for window frame drainage.  Alternatively the glazing rebate of the transoms may carry the water to the mullions where it is drained downwards.  This can only be done over a few storeys before the flow of water in each mullion becomes too great.  Water is normally drained from the mullions at every third storey.

Water should be drained clear at the foot of a curtain wall and not allowed to flow into inwards.  At the end of a run of curtain walling,  the curtain wall should be sealed to the adjacent cladding to give continuous air and water seals.
 

Water barrier
In any wall there should be an identifiable water barrier.  In deed this should cover the whole building.  The water barrier should comprise water resistant components and joints that prevent unacceptable quantities of water passing.  The water barrier need not be continuous and may include breaks where one component is lapped over another to leave a protected opening.  Where water drains down a cavity then the outer layer is not a full water barrier and the cavity flashings must intercept and drain that water.

Where the components of a wall are porous (brick, concrete, etc.) then water will soak in to the wall and percolate down through it.  With these materials face seals to joints will be bypassed by water within the leaf of the wall.  In all cases water must be properly drained from permeable portions of the wall at the heads of windows, doors and similar penetrations.  This is usually achieved by the use of a d.p.m..
 

Air barrier
For all walls it is important that an air barrier is identified throughout the building envelope.  For most walls this is the inner layer of the wall. For a blockwork wall with an inner plastered surface the plaster layer provides the air barrier.  With some forms of construction the air barrier is the internal plaster or dry-lining system.

The air barrier has to be continuous with all components such as windows and doors sealed to it.  All penetrations of the wall for pipes, vents, etc. also have to be air sealed.  Particular attention is required at eaves details and all too often window sills are not adequately sealed to the window or the wall.
 

Construction details
Joints in curtain wall frames for example at mullion/transom intersections and splice joints in mullions are potential weak points for water leakage and require careful design to ensure all the requirements of the joint are met as described in Section 01.02.  Joints in the building envelope. Continuity of drainage at joints must also be considered.

Construction interfaces, for example, between different cladding types or building elements, are areas of unusual complexity and may be designed and constructed by different people or companies.  Problems on complex construction projects become concentrated around the interfaces, which contractual arrangements exacerbate by virtue of the number of separate contractors who must co-ordinate with unfamiliar trades having different cultures, standards, responsibilities and requirements.Unless interfaces can be ‘designed out’ they should be tested.

Concentrating efforts on construction interfaces during the design development and testing phases should reduce the number of problems experienced during the construction phase.

CIRIA Report R178 gives many examples of how various construction interfaces can be detailed to give weathertight joints.