02.05 Pressure Equalisation

Categories: Rainscreens

Introduction
Water is forced through openings and joints in walls and windows by several mechanisms, Section 01.02.  Two significant mechanisms are:

• Pressure difference acting across a sealed or closed joint may force water through;
• Kinetic energy of air-borne droplets in a fast moving air flow may carry water through an open joint.

The principle of pressure-equalisation is to reduce the pressure difference across a wall and any consequent air flows by creating a pressure on the rear of the joint or opening that matches, as closely as possible, the pressure on the outer face.

The principle of pressure equalisation offers advantages in terms of:

• Weathertightness (elimination of the most significant leakage mechanisms, achieved without relying on correctly installed sealants);
• Structural requirements (pressure difference across panels less than peak wind pressure);
• Ease of construction (minor imperfections in the size and fit of components are less critical).  However, more complicated detailing of openings, and compartmentation is required.

Pressure equalisation can be achieved by adding less than two percent by volume of air to the cavity.  However, adequate air has to enter the cavity and be trapped in separate compartments, image.

This Section describes the principle of pressure-equalisation and how it may be achieved in practice.  Reference should be also be made to Standard for walls with ventilated rainscreens and Standard for testing ventilated rainscreens (CWCT, 1998).
 

Pressure-moderation
The external wind pressure on any cladding system varies rapidly with time.  This image shows a typical wind pressure measured over a two minute period.

To simplify the explanation of pressure-moderation it is sufficient to consider a sinusoidal variation of wind pressure about a mean value, as shown by the solid line in this, image.

The pressure in the cavity will rise as the external pressure rises, image, but a pressure difference is needed to drive air into the cavity and the two pressures are never equal.  However, when the external pressure starts to fall it will at some point equal the cavity pressure.  At this time the cavity pressure is a maximum and it falls as air is drawn out of the cavity, image.  Again the two pressures are not equal.

This process continues as air is repeatedly drawn into, and expelled from, the cavity, image.  The principle of pressure-equalisation is to make the cavity pressure as nearly equal to the external pressure as possible at any time.  In practice there is a degree of pressure-moderation where the pressures are sufficiently close that it is, for the purpose of design, regarded as pressure-equalisation.
 

Factors determining cavity pressures
The pressures in the cavity will depend on:

• Air permeability of the rainscreen;
• Air permeability of the air barrier;
• Air movement within the cavity;
• Volume of the cavity;
• Flexibility of the cavity.

Air permeability
Air permeability of the rainscreen and the air barrier affect the cavity air pressure.  The cavity air pressure required for pressure-equalisation will not be achieved if:

• The rainscreen has insufficient area of openings (because air cannot enter the cavity fast enough);
• The other faces of the cavity, principally the air barrier but also the cavity barriers, are too permeable (because air escapes from the cavity).

Considering a sustained and uniform pressure, the pressure difference across an obstruction is given by:

DP = C_d  x  r  x  V²

where:

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

C_d and r are the same for both the air and water barriers and it follows that for each, the flow (Q) is:

Q = A  x  V = A  x  ( DP )^0.5

and that

A_i²  x  ( P_c - P_i )  =  A_o²  x  ( P_e - P_c )

where, image:

A_i is the area of openings in the air barrier;
A_o is the area of openings in the rainscreen;
P_c is the pressure in the cavity;
P_i is the pressure inside the building;
P_e is the external pressure on the building.

The degree of pressure-equalisation corresponding to different ratios of rainscreen permeability to air barrier permeability is shown below.
 

The CWCT Standard states that a wall may be called a pressure-equalised wall if the air permeability of the rainscreen is ten times greater than that of the air barrier and the cavity is adequately compartmented.  The CEN wind loading code requires a ratio of only 3.33 for a rainscreen to be described as 'permeable' but includes only to the airtightness of the backing wall and makes no mention of the cavity barriers.
 

Compartmentalisation
It is important for all forms of rainscreen to place a barrier/cavity closer at intervals along the cavity to:

• Minimise air flow within the cavity;
• Minimise the pressure drop across the rainscreen.

Cavity air movement
Adjacent faces of a building experience positive and negative external wind pressures, image, at the same time and air will be drawn through the cavity at high velocity to low pressure areas if it is non-compartmentalised.  This occurs on the face, image, as well as at the corners, image.

This air flow could move large amounts of water into the cavity, with the risk of water penetration.  Therefore, the air cavity must be closed off (at intervals corresponding to the spatial variation and rate of change of wind pressure) to limit air movement, and, moreover, for each cavity to achieve the appropriate cavity pressure (see below).
 

Cavity volume and dimensions
External wind pressures on a building vary in magnitude, from a peak positive pressure at the centre of the windward face caused by stagnation of the wind, to suction of far greater magnitude near the corners of adjacent sides; wind pressures also vary in the vertical direction.

It is important that pressure-equalisation is rapid, since for as long as there is a pressure difference across the joint water can be carried through and into the cavity.  Cavity volume has an important effect on the cavity pressures (as do the four other factors of the wall listed above):

• If the cavity compartments are small enough the air pressure within them will be virtually equal to the external wind pressure, preventing water from being drawn through the openings in the rainscreen.

 
• If the cavity compartments are too large then the time taken to pressurise them will be too great and the cavity pressure shown in will lag too far behind the external pressure, image.  Clearly large openings in the rainscreen will mitigate the effect of cavity volume and the important parameter is the ratio of cavity volume to vent area in the rainscreen.

Measurements made by the National Research Council of Canada (Ganguli, U & Quiroette, R L, 1987) show the pressure difference (and by inference the degree of pressure-equalisation) achieved for different cavity volumes and vent areas as follows.
 

Taking a ratio of cavity volume to vent area less than 80m gives a high degree of pressure-moderation.  This is a prerequisite for a pressure-equalised wall as stated in the CWCT Standard.  Note that the volme is measured in m³ and the vent area is measured in m².

To ensure a high degree of pressure-moderation the cavity should be compartmented as follows:

• At each floor level;
• At 6.0 m horizontal spacing;
• At 1.5 m centres within 6.0 m of a corner, image;
• At all corners of the building, or within 300mm of every corner.

Partial compartmentation can be beneficial.  It will not give pressure-equalisation but will moderate the cavity pressures.  Even if a wall is not intended to be pressure equalised there should be cavity closers at the corners, image, to prevent air movement from areas of high positive pressure to areas of high negative pressure, image.

Detailed calculation may show that larger compartments can be used to achieve pressure-equalisation.
 

Cavity barriers
Barriers/closers must be of sufficient strength and stiffness to resist the air-pressure differentials which act across them.

Particular attention should be given to parapets where the external pressure varies with position and the rainscreen may not have an air barrier behind it.  In fact, the rear face of the wall may also be an external surface.
 

Rigidity of the cavity
Rigidity of the cavity is not normally a concern.  Most forms of construction are sufficiently rigid that the cavity volume does not change more than one or two per cent under wind loading.  However, problems may be experienced if a flexible membrane is used that is not rigidly supported.

The rigidity of the inner barrier affects the time response to pressure-equalisation; as its rigidity increases, less time is required to equalise the wind and cavity pressure.
 

Pressure-equalised walls
Walls that meet all of the criteria set out above to give high levels of pressure-moderation may be called pressure-equalised walls.

Walls may not necessarily be designed to be pressure-equalised but it is worthwhile applying any of the measures described, in part or as a whole, to produce some moderation of pressure differential and reduce the risk of water penetration.
 

Glazing frames
Cavities in glazing frames are often ventilated and drained to manage any water that passes the primary outer seals of the frame.  Ventilated cavities experience varying cavity pressure and may be designed on the principle of pressure-equalisation.

The cavities in the frame fall into two categories: glazing cavities and the cavities that occur between a fixed framing member and the frame of an opening light, image.  In either case the cavity may be ventilated direct to the front face or it may be ventilated from, and drain through, another drained-and-ventilated cavity.

The presence of ventilation openings in the cavity means the only barrier against air leakage into the building is the inner glazing gasket, or the inner seal at a parting frame.  These seals have to be effective to prevent unacceptable air leakage into the building and achieve a high degree of pressure-moderation.  In addition, openings must be of sufficient size to rapidly allow air into the cavity, and possibly an adjacent cavity.

With window frames the cavity volume is small and the vents do not have to be very large to achieve effective pressure-equalisation.  Of more importance with glazing frames is the positioning of the vent openings within the frame.  Openings are positioned at the bottom of the frame to provide a drainage route.  For larger windows the cavity pressure will respond more quickly if openings are also provided at the head of the frame.  This reduces the lag of the cavity pressure and will increase the effect of pressure-moderation.  Providing openings at the head of the frame also aids ventilation of the cavity.
 

Stick curtain walling
The glazing cavity of a stick system curtain wall may be pressure-equalised in the same way as for a window frame.  However, it is necessary to compartment the wall just as a rainscreen wall is compartmented.

In the case of a stick wall it may be relatively easy to compartment the wall as the mullions and transoms provide natural barriers to compartment the glazing cavity.  For designs where drainage occurs at every glazing or infill cavity it is only necessary to ensure that no air leaks from one glazing cavity around the perimeter of the glass unit/infill panel to the next.  This is done by sealing the transoms to the mullions, image.

A moulded frame gasket (i.e. where the corners are an integral part of the gasket) is recommended for the inner (and outer) seals because of its greater air and water tightness reliability.

For designs where the drainage route is down the mullions, or the drainage routes link different glazing cavities in some other way, it is difficult to achieve effective pressure-equalisation.  Such systems are constructed as drained-and-ventilated systems and adequate drainage alone is used to reduce the risk of water penetration.
 

Pressure-equalised or drained-and-ventilated
The degree of pressure-moderation achieved within a cavity in a glazing frame may be measured under test by using pressure tappings.  When measuring cavity and external pressures it is essential that the external pressure varies at a realistic rate.  The cavity pressure will always follow the external pressure if the rate of pressure change is slow enough.

Leakage of water and the effectiveness of any drainage can be observed by using an endoscope to see inside the cavity.