CWCT TN-006 - The Principle of Pressure-Equalisation PDF

Technical Note No. 6 THE PRINCIPLE OF PRESSURE-EQUALISATION This Technical Note is one of five on wind loading for the window and cladding industry. The series comprises: TN 2 Introduction to wind loading on cladding TN 3 Wind loading on wall cladding and windows of low-...

Technical Note No. 6 THE PRINCIPLE OF PRESSURE-EQUALISATION This Technical Note is one of five on wind loading for the window and cladding industry. The series comprises: TN 2 Introduction to wind loading on cladding TN 3 Wind loading on wall cladding and windows of low-rise buildings TN 4 Issues relating to wind loading on tall buildings TN 5 Wind tunnel testing TN 6 Pressure equalisation Introduction achieved in practice. Reference should be also be made to Standard for walls with ventilated Water is forced through openings and joints in rainscreens and Standard for testing walls and windows by several mechanisms. ventilated rainscreens (CWCT, 1998). Two significant mechanisms are: Cladding constructions 1 Pressure difference acting across a sealed or closed joint may force water through; Cladding that is designed and constructed on the principle of pressure-equalisation 2 Kinetic energy of air-borne droplets in a comprises: fast moving air flow may carry water through an open joint. 1 An outer rainscreen, which intercepts raindrops and drains them safely away, The principle of pressure-equalisation is to contains large, shielded openings and reduce the pressure difference across a wall transfers the wind load to the framing and any consequent air flows by creating a members, and thence the backing wall or pressure on the rear of the joint or opening building frame; that matches, as closely as possible, the pressure on the outer face. It offers 2 An inner air-tight barrier, which carries advantages in terms of: wind loading; 3 A cavity between, which is pressurised
Weathertightness (elimination of the most and provides a means of collecting and significant leakage mechanisms, achieved draining to the outside any water which without relying on correctly installed penetrates the rainscreen. sealants);
Structural requirements (pressure Traditional cladding difference across panels less than peak The rainscreen or two-stage approach to wind pressure); weathertightness is not new; for many years
Ease of construction (minor imperfections walls have been constructed by hanging a in the size and fit of components are less ‘rainscreen’ of vertical tiles in front of a critical). However, more complicated structural wall (Figure 1). In this case the detailing of openings, and ‘rainscreen’ provides the major barrier to compartmentation is required. water and the brick wall is the structural component and the air barrier. In practice, the This technical note describes the principle of effective air barrier is probably the inner, pressure-equalisation and how it may be plastered finish to the wall. © CWCT 2000 January 2000 This document has been printed from the CWCT ‘Cladding Forum’, access to which is restricted to subscribing Members of the Centre for Window & Cladding Technology. Information about the availability of CWCT publications and membership is available at our website – www.cwct.co.uk – or from the address at the end of this note. The principle of pressure-equalisation TN 6 Pressure-equalisation is a development of the more traditional drained-and-ventilated cladding systems. Pressure-equalised walls have larger openings. In addition to allowing drainage these openings permit a greater movement of air so that the pressure within the cavity undergoes greater and more rapid change. Clearly, for both forms of wall there is a varying pressure within the cavity which follows the external pressure but never reaches the same peak levels. In fact, the pressures are only equal for an instant when the cavity pressure is a maximum or minimum and the external pressure is rising or falling (as shown Figure 1 Tile hung wall later). For this reason it may be more appropriate to talk of pressure-moderation Although the tiled screen is very permeable to rather than pressure-equalisation. air because of the large number of gaps between the tiles, little air penetration occurs Nature of the wind because the cavity has a limited capacity to accept air and air cannot move through the air- The UK wind loading code BS 6399: Part 2, tight barrier behind. This prevents the first like other national wind loading codes mechanism of water penetration (see considers the wind pressure to be constant for Introduction above); the second is minimised the purpose of structural design. In practice by the overlapping joints between the tiles. the wind pressure varies with time as shown in The effectiveness could be further improved Figure 3. The wind pressure on the external by properly sealing the top, bottom and sides face of the building varies about a mean value of the air cavity (but allowing any water to and may go from a minimum value to a drain out at the bottom). maximum value in a period of just 0.1s or less. The analysis of this complex movement of air A similar consideration applies to the use of into and out of a cavity as a result of these tiled roofs where the roofing felt beneath the pressure variations is possible. However, it is tiles acts to stop gross air flow between the more normal to make working assumptions tiles. about the range of pressure that may be experienced in the cavity. Modern rainscreen facades Modern rainscreen facades (Figure 2) are Pressure-moderation designed to be either drained-and-ventilated or pressure-equalised. To simplify the explanation of pressure- moderation it is sufficient to consider a Drained-and-ventilated walls have openings to sinusoidal variation of wind pressure about a allow any water that passes the rainscreen and mean value, as shown by the solid line in enters the cavity to drain out. The drainage Figure 4. The pressure in the cavity will rise openings also allow air to enter the cavity and as the external pressure rises but a pressure any change in external pressure on the difference is needed to drive air into the cavity rainscreen will lead to some change of and the two pressures are never equal. pressure in the cavity. 2/9 The principle of pressure-equalisation TN 6 Figure 2 Modern rainscreen facade Figure 3 Variation of wind pressure with time 3/9 The principle of pressure-equalisation TN 6 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, are too permeable (because air escapes from the cavity). Considering a sustained and uniform pressure, the pressure difference across an obstruction is Figure 4 Variation of wind pressure with time given by: P = Cd  V2 However, when the external pressure starts to where: fall it will at some point equal the cavity pressure.
 is the density of air;
V is the air velocity; At this time the cavity pressure is a maximum and it falls as air is drawn out of the cavity.
Cd is a coefficient. Again the two pressures are not equal. This Cd and  are the same for both the air and process continues as air is repeatedly drawn water barriers and it follows that for each, the into, and expelled from, the cavity. The flow (Q) is: principle of pressure-equalisation is to make the cavity pressure as nearly equal to the Q = A  V = A  P0.5 external pressure as possible at any time. In practice there is a degree of pressure- and that moderation where the pressures are sufficiently close that it is, for the purpose of Ai2 (Pc - Pi) = Ao2 (Pe - Pc) design, regarded as pressure-equalisation. where: Factors determining cavity pressures
Ai is the area of openings in the air barrier; The pressures in the cavity will depend on:
Ao is the area of openings in the rainscreen;
Pc is the pressure in the cavity; 1 Air permeability of the rainscreen;
Pi is the pressure inside the building; 2 Air permeability of the air barrier;
Pe is the external pressure on the building. 3 Air movement within the cavity; 4 Volume of the cavity; The degree of pressure-equalisation corresponding to different ratios of rainscreen 5 Flexibility of the cavity. permeability to air barrier permeability is shown in Table 1. Air permeability Air permeability of the rainscreen and the air barrier affect the cavity air pressure. The 4/9 The principle of pressure-equalisation TN 6 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. Rainscreen opening area/ Cavity pressure Air barrier opening area External pressure 10.0 99.0% 8.0 98.4% 6.0 97.3% 4.0 94.1% 2.0 80.0% Figure 5 Air flow within a rainscreen cavity at the corner of a building due to Table 1 Degree of pressure-equalisation for ratios of rainscreen permeability to This air flow could move large amounts of air barrier permeability water into the cavity, with the risk of water penetration. Therefore, the air cavity must be Compartmentalisation closed off (at intervals corresponding to the spatial variation and rate of change of wind It is important for all forms of rainscreen to pressure) to limit air movement, and, place a barrier/cavity closer (items 9/10, moreover, for each cavity to achieve the Figure 2) at intervals along the cavity to: appropriate cavity pressure (see below). 1 Minimise air flow within the cavity; Cavity volume and dimensions 1. Minimise the pressure drop across the External wind pressures on a building vary in rainscreen. magnitude, from a peak positive pressure at the centre of the windward face caused by Cavity air movement stagnation of the wind, to suction of far Adjacent faces of a building experience greater magnitude near the corners of adjacent positive and negative external wind pressures sides; wind pressures also vary in the vertical at the same time and air will be drawn through direction. the cavity at high velocity to low pressure areas if it is non-compartmentalised (Figure 5). 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. 5/9 The principle of pressure-equalisation TN 6
If the cavity compartments are too large then the time taken to pressurise them will be too great and the cavity pressure shown in Figure 4 will lag too far behind the external pressure. Clearly large openings Ratio of cavity Pressure Degree of in the rainscreen will mitigate the effect of volume to vent difference pressure- cavity volume and the important parameter area across panel equalisation is the ratio of cavity volume to vent area in 661m 40-45% 55-60% the rainscreen. 459m 28-34% 66-72% Measurements made by the National Research 165m 16-22% 78-84% Council of Canada (Ganguli, U & Quiroette, R 118m 9-18% 82-91% L, 1987) show the pressure difference (and by inference the degree of pressure-equalisation) Table 2 Degree of pressure-equalisation achieved for different cavity volumes and vent achieved for different ratios of areas (Table 2). Taking a ratio of cavity cavity volume and vent area volume to vent area less than 80m gives a high degree of pressure-moderation. This is also a prerequisite for a pressure-equalised wall. Barriers/closers must be of sufficient strength and stiffness to resist the air-pressure To ensure a high degree of pressure- differentials which act across them. moderation the cavity should be Particular attention should be given to compartmented as follows (Figure 6): parapets where the external pressure varies 1 At each floor level; with position and the rainscreen may not have an air barrier behind it. In fact, the rear face 2 At 6.0 m horizontal spacing; of the wall may also be an external surface. 3 At 1.5 m centres within 6.0 m of a corner; Rigidity of the cavity 4 At all corners of the building. Rigidity of the cavity (in relation to a flexible member) 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. 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 Figure 6 Plan showing horizontal spacing part or as a whole, to produce some of compartment barriers 6/9 The principle of pressure-equalisation TN 6 moderation of pressure differential and reduce although this cannot always be assumed to be the risk of water penetration. the case. Glazing frames Structural loading Cavities in glazing frames are often ventilated and drained to manage any water that passes Walls are conventionally designed to the primary outer seals of the frame. withstand a wind load defined as the Ventilated cavities experience varying cavity difference between the external wind pressure pressure and may be designed on the principle and the pressure inside the building that of pressure-equalisation. occurs as a result of infiltration or significant openings in the building envelope. The cavities in the frame fall into two categories: glazing cavities and the cavities P = q(Cpe - CPi) that occur between a fixed framing member and the frame of an opening light (Figure 7). where: In either case the cavity may be ventilated direct to the front face or it may be ventilated
P is the loading on the wall; from, and drain through, another drained-and-
q is the design wind loading; ventilated cavity.
Cpe is the external pressure coefficient; The presence of ventilation openings in the
Cpi is the internal pressure coefficient. cavity mean the only barrier against air leakage into the building is the inner glazing In the case of a wall with ventilated cavities it gasket, or the inner seal at a parting frame. is not clear what peak pressures act across the These seals have to be effective to prevent air barrier and the rainscreen respectively. In unacceptable air leakage into the building and the absence of information it should be achieve a high degree of pressure-moderation. assumed that the full wind loading may act on In addition, openings must be of sufficient size either the air barrier or the rainscreen. to rapidly allow air into the cavity, and possibly an adjacent cavity. For a pressure-equalised wall it may be assumed that the peak pressure difference With window frames the cavity volume is across the rainscreen is limited to two-thirds small and the vents do not have to be very of the peak external pressure, i.e. large to achieve effective pressure- equalisation. Of more importance with P = 2/3 q  CPe glazing frames is the positioning of the vent openings within the frame. Openings are However the pressure difference acting across positioned at the bottom of the frame to the air barrier should still be taken as the full provide a drainage route. For larger windows pressure difference acting across the wall. the cavity pressure will respond more quickly if openings are also provided at the head of the Detailed calculation or measurement may well frame. This reduces the lag of the cavity show that the peak pressure difference across pressure and will increase the effect of the rainscreen is considerably less than two- pressure-moderation. Providing openings at thirds of the peak external pressure. the head of the frame also aids ventilation of the cavity. Fire stops Cavity barriers can serve as fire stops to prevent the spread of fire, and vice versa, 7/9 The principle of pressure-equalisation TN 6 Stick curtain walling Summary The glazing cavity of a stick system curtain The key aspects of pressure-equalisation are: wall may be pressure-equalised in the same way as for a window frame. However, it is 1. An inner barrier highly impermeable to air; necessary to compartment the wall just as a 2. A rainscreen so vented as to achieve a rainscreen wall is compartmented. permeability far greater than that of the air barrier; In the case of a stick wall it may be relatively easy to compartment the wall as the mullions 3. Compartmentation of the cavity to achieve and transoms provide natural barriers to an adequate ratio of cavity volume to compartment the glazing cavity. For designs opening area and so minimise air where drainage occurs at every glazing or movement within the cavity and rapidly infill cavity it is only necessary to ensure that equalise the cavity and wind pressures. 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 (Figure 8). References and Bibliography A moulded frame gasket (i.e. where the Anderson, J M & Gill, J R, 1988, Rainscreen corners are an integral part of the gasket) is Cladding, a guide to design principles and recommended for the inner (and outer) seals practice, Construction Industry Research and because of its greater air and water tightness Information Association (CIRIA). reliability. For designs where the drainage route is down CWCT, 1998, Standard for walls with the mullions, or the drainage routes link ventilated rainscreens, Centre for Window different glazing cavities in some other way, it and Cladding Technology. is difficult to achieve effective pressure- equalisation. Such systems are constructed as CWCT, 1998, Standard for testing ventilated drained-and-ventilated systems and adequate rainscreens, Centre for Window and Cladding drainage alone is used to reduce the risk of Technology. water penetration. Ganguli, U & Quiroette, R L, 1987, Pressure Pressure-equalised or drained-and- Equalization Performance of a Metal and ventilated Glass Curtain Wall, National Research Council of Canada. 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. 8/9 The principle of pressure-equalisation TN 6 Figure 7 Horizontal section (plan) through a curtain walling frame (mullion with glazed in outward opening window) showing cavities © CWCT 2000 CWCT Technical Notes 1 – 30 have been part- funded by the DETR under research contract Figure 8 Curtain wall mullion/transom 39/3/338 (CI 1354) joint closed/sealed off to achieve University of Bath, Claverton Down, compartmentation Bath, BA2 7AY Tel: 01225 826541; Fax: 01225 826556; email: [email protected]; website: www.cwct.co.uk 9/9