CWCT FACETS

01.01 Weathertightness

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Categories: Envelope Sealing

Introduction
This Section describes requirements for weathertight construction and how this can be achieved in the design of windows and cladding systems. In this context weathertightness is concerned with the penetration of the building envelope by air and water. Thermal transmission, either in the form of heat gain resulting from solar radiation or heat loss in cold weather is covered in Package 05.
 

Effects of weather penetration
Water ingress through a cladding system and into a building can lead to:

  • Damage to internal finishes;
  • Degradation of seals (e.g. at the perimeter of glass units or between mullions and transoms);
  • Corrosion of elements (e.g. fixings or steelwork),
  • Reduced levels of thermal insulation.

High rates of air leakage will increase heat loss and lead to discomfort of the occupants of the building.
 

Rainwater wetting
Designing against water penetration requires an understanding of the theory of rainwater wetting and subsequent leakage mechanisms.

Rain and wind usually occur concurrently so that raindrops fall at an angle, which depends on the size of drops and speed of the wind.  Due to water’s greater inertia, abrupt changes in wind direction caused, for example, by the flow over the top and around the sides of a building, leads to the rainwater separating from the air stream and being deposited near that part of the facade.  If rainfall continues for long enough, rainwater will begin to flow across the whole facade, causing heavy wetting of the mid regions of the windward face, particularly on walls of impervious materials.  Impervious cladding materials, such as metals and glass, encourage water to run off, rather than be absorbed, so that much more water accumulates and covers the façade.  Any projections (e.g. framing members of curtain walling) may serve to direct water towards joints which are points of vulnerability, requiring good joint design, construction and sealing to prevent water penetration.

Shelter from any adjacent buildings or an overhang will affect the pattern and degree of wetting.
 

Ways of achieving Weathertightness
Porous materials
For permeable cladding materials water can flow through the bulk of the material under the action of capillarity and air pressure.

The most common form of construction using permeable material is masonry. Buildings with solid masonry walls keep the rain out by virtue of the wall’s thickness and low permeability.  Rain driven onto the wall during rainstorms is absorbed into the pores of the brick, concrete or stone units but subsequently dries out. Provided that the wall is sufficiently thick and the permeability is low water will not penetrate to the inner surface.

The presence of internal plaster may improve the resistance to air penetration, particularly where there are voids and cracks in the masonry and hence improve resistance to rain penetration. The wall’s resistance to weather can be further enhanced by the use of external renders. Strong renders have a lower permeability than weaker materials however there is a danger that the render will crack allowing water to penetrate the crack and become trapped. Weaker more porous renders may perform better as there will be less immediate run-off and less cracking.

To be watertight, solid masonry walls need to be thick and they are not suitable in severe exposure conditions. In modern construction, a cavity is normally incorporated into masonry walls to provide an additional barrier to the passage of water. Provision should also be made for drainage of any water passing through the external masonry leaf.  Water will drain down the cavity and down through the pores of the masonry in the outer leaf.  If the flow is arrested it leads to a build up of damp within the wall, for instance above impermeable barriers such as flashings.
 


Sealed construction
For impermeable materials, water penetration can only occur through joints and weathertightness is dependent on controlling water and air movement through the joints.

The use of an impermeable cladding material together with sealed joints may appear to be a simple solution to providing weathertight construction however obtaining joints that will remain watertight when exposed to weather and the inevitable movements of the joint is very difficult.  Sealed joints may serve to retain any water that enters the system through a leaking joint higher up the wall.  For this reason sealed facades should normally include provision for dealing with leakage.
 


Rainscreens
Ventilated rainscreen walls are used either for new-build or refurbishment/repair cladding solutions.  Their fundamental features are an outer rainscreen, a drained cavity and an inner air barrier.  The rainscreen outer layer serves to keep most rainwater from the cavity.  See Package 02.
 


Air leakage
Windows and glazed cladding systems such as curtain walls are always required to meet specified performance criteria for air leakage.  However, there are two different forms in which air leakage is reported, and this can cause confusion.  This Section explains the reasons behind, and the relationship between, different representations of air leakage.

Air leakage can be expressed as:

  • A leakage flowrate per unit area of facade (or product)
  • A leakage flowrate per unit length of joint

These different forms of expression can often be applied equally to the same component, but there are circumstances where one or the other is the more sensible:

a) Air leakage can occur through visible openings between parts of a component (even the smallest gaps may give an appreciable air leakage) or through invisible pores in the bulk of a material.  In the first instance the length of the opening can often be measured, and used to derive a length-dependent air leakage rate, but in the second case the area of the pores is indeterminate and only an area-based rating is possible;

b) A component or facade designer needs to know the leakage per unit length in order to calculate the total air leakage through an assembly.  The building services engineer or architect is concerned only with the overall leakage represented as leakage per unit area.  If the unit area air leakage rate is specified by the architect/building services engineer then the component/facade designer can select and arrange suitable details to achieve that level of leakage.
 

The need to assess air leakage is based on two issues: energy use and thermal comfort.  The energy use issue is obvious and usually dominates - air leakage through the facade implies an energy loss, which the services engineer must allow for when designing the heating and cooling systems.  From a comfort standpoint the problem is draughts - openings which permit gross air movement can generate strong air currents causing discomfort, and possibly generate noise as well.  It should be noted however that it is not always a requirement to eliminate air leakage;  air leakage can provide a valuable background service in dispersing pollutants and odours, and in reducing humidity levels.
 


Leakage flowrate per unit area
For a facade as a whole (or a large-scale test sample which incorporates all of the features of the facade) the air leakage is expressed as a leakage per unit projected area, in m3/hour/m2.  This image shows the allowable air leakage through a curtain wall, from Standard for curtain walling (CWCT, 1996).

Using an area-based leakage flowrate is the only option for large-scale facades and facade samples.  The diverse components in a facade often mean that there are several different types of joint seal, and it can be difficult to identify the total length of joint - for example, a glazing unit may be mounted in a vent frame, which is fitted into a fixed carrier frame, which is fitted into a stick-system curtain wall frame, entailing three distinct joints between the glazing unit and the primary frame.  In a large-scale test specimen there are also many types of joint which contribute to the overall volume of air leakage, and measuring the total length of ‘leaking’ joint may take longer than actually measuring the air leakage.  It should also be noted that the contribution of each different type of joint can be difficult to assess.
 


Leakage flowrate per unit length of joint
Where a product has defined joints the air leakage can be expressed as the leakage per unit length of the opening joint, in m3/hour/m.  This is of particular advantage to manufacturers who from one test can estimate the maximum or minimum size of a product which will meet specified performance requirements.  This image shows typical performance requirements from various standards.

The only requirement is that there is just one type of joint, which can be measured - usually based on the visible perimeter of the joint.  A typical performance requirement is given in BS 6375: Part 1 (1989), which states that for a window with only fixed lights the average air leakage rate should not exceed 1.0 m3/hour per metre length of the visible perimeter of the glass or glazing material, when tested at the same range of pressures as for opening lights.

For products which have opening lights, such as a door or a casement window with a side-hung opening light, the situation is slightly different.  Although the fixed joints contribute to the leakage through the window, the majority of the air leakage occurs through the opening joint; this is a necessary consequence of designing a jointing system that can be easily broken and resealed - a good seal requires a high contact pressure, but a high contact pressure would significantly increase the force required to open and close a window or door.  For such products the air leakage is expressed as a leakage flowrate per metre length of the opening joint, and the contribution of the fixed joints is assumed to be negligible.  Of course, as better sealing technologies are developed and different forms of hardware are introduced (geared and electrically-operated hardware types do not suffer from the same limitations on operating forces) it is probable that opening joints will be made more air-tight, and eventually all air-leakage characteristics could be expressed as a leakage flowrate per unit projected area.  However, at today’s level of technology, with (comparatively) poorly sealed opening joints it is still appropriate to use a different representation for air leakage at least for products with opening joints.
 


Advantages and disadvantages
Joint-length related air leakage characteristics have the singular advantage that they allow the user to estimate directly the performance of a product of a different size.  For example, a manufacturer may have tendered a suitable design of window for a project as being a casement window, 900 mm wide by 1200 mm high, with a full-size opening light, and an air leakage of 4.0 m3/hour/m of opening joint, at a specified reference pressure of 75 Pa (this complies with class B of BS 6375: Part 1).  The approximate joint length is 4200 mm (900+1200+900+1200 - the real joint length will be slightly less than this), and so the total window air leakage is 16.8 m3/hour.  However, the project architect then decides to set a limit of 10.0 m3/hour air leakage per window.  The permissible length of opening joint is therefore 10.0/4.0 or 2.5 m (2500 mm).  This can be achieved if the window is re-styled as a top-hung opening light over a fixed pane, with the top hung light having a perimeter of less than 2500 mm (900 mm wide by 350 mm high).  This image demonstrates these two styles of window.
 


Origin of ‘Standard’ air leakage characteristics
A point worthy of note is that the curves described above, image, image, are based on an assessment of what was sensibly achievable at the time that the standards were written; much higher performance is achieved by some manufacturers.  Such curves are also generally of the form, image:

       q = constant x Dpn

where

q is the air flowrate in m3/hour per metre or per m2
Dp is the pressure difference in Pa

The exponent n depends upon the nature of the flow; for laminar (streamline) flow n is 1.0, and for fully-developed turbulent flow n is 0.5.  However, for a product with complex joints, of varying lengths and widths, the exponent n is often somewhere between 0.5 and 1.0, due to there being a combination of laminar and turbulent flows.  In the UK the exponent n is taken as 2/3 for windows and doors (BS 6375: Part 1), and Hutcheon and Handegord (1983) suggest a value of 0.65 as being suitable for many cases of window and wall leakage.

The variation of leakage flowrate with pressure differential for a typical glazed system is rarely a smooth curve, nor is it identical for positive and negative pressure differentials.  This image shows measured air leakage curves for different windows of the same size and style (900 mm wide by 1200 mm high, UK-style outward-opening top-hung light over fixed pane, opening joint perimeter 2760 mm) superimposed on the requirements of BS 6375.  The air leakage is clearly not a smooth curve, and this can be explained by the relative movement of the various parts of the window when under pressure.

These tests were all performed using a positive pressure differential, with the higher pressure on the external face of the window.  For a window which opens outwards this means that as the pressure increases the opening light is forced onto its weather-stripping, which usually increases the effectiveness of the seal.  If the glazing gaskets are well-designed and properly installed then the overall air leakage can decrease as the pressure is increased.  If the glazing gaskets are not properly installed or are poorly designed the air leakage through the fixed joints may then increase in a way that equals or outweighs the decrease in leakage through the opening joint.

It is usually the task of the specifier to identify the air leakage rate that is acceptable for the products used in a particular project.  However, the achievable leakage rate is often limited by the manufacturer’s choice of jointing method and the architects’ choice of pane or panel size.
 


Effect of facade design on air leakage
The window shown in the image above is very simple compared to a curtain wall which combines fixed lights (vision and non-vision panels), opening lights, smoke vents and doors, and which may also abut other wall constructions and the roof.  For such large scale constructions with a variety of joints the contribution of each to the overall air leakage is complex, and often hinders the comparison of facades which may contain the same components but in different proportions.  The following examples illustrate this point:

Case 1 - Influence of gaskets and their joins on air leakage
Gaskets may be produced by injection moulding, linear extrusion or a combination of both.  If moulded then the corners are an integral part of the gasket and can often be assumed to have the same air-leakage characteristics.  However, if the gasket is formed from extruded (or a combination of extruded and moulded) pieces then joins may need to be formed in the gasket using

  • heat-welding (thermoplastic rubbers)
  • adhesive bonding
  • butt- or mitre-joins buttered over with a wet-sealant
  • dry butt- or mitre-joins

These different methods of joining gaskets have different degrees of success, because some rely upon the skill of the installer for a successful join.  The air leakage rate of a made-up gasket, for example the gasket between a window frame and the glazing unit, can vary from the relatively air-tight frame gasket with moulded corners to the poorly installed ‘four lengths of rubber extrusion with visible gaps at the corners’ which offers little or no resistance to air leakage.

Beech and Saunders (1983) measured air leakage rates through linear pieces of a gasket and through jointed pieces of the same gasket.  A 295 mm long linear piece of gasket was placed between two mortar surfaces (with the ends embedded in a sealant) and the air leakage measured for different pressures and degrees of compression.  The same gasket was then formed into a square (inside measurement 145 mm per side, 580 mm total), with mitred corners sealed with a neoprene adhesive, and again tested at the same pressures and compression.

For a hollow section gasket, of a type commonly used to seal joints between cladding panels, the leakage flowrate past the linear section at 36% compression and 500 Pa pressure was measured at 22 ml/min per metre length.  For the same pressure and compression the leakage rate in the mitre-jointed gasket was 140 ml/min per metre length.  Each join therefore contributed a significant additional air leakage.

Based on these figures the actual leakage through the mitre-jointed gasket was 81.2 ml/min ( 140 x 580 / 1000 ), compared to an expected 12.8 ml/min ( 22 x 580 / 1000 ).  Each join therefore contributed 17.1 ml/min (( 81.2 - 12.8 ) / 4 );  each of this type of join, in this particular gasket, therefore contributes an air leakage equivalent to an additional 777 mm ( 1000 x 17.1 / 22.0 ) of linear gasket.

Further information on gasket design and performance is given in Section 01.06.

Case 2 - Influence of panel or pane size on air leakage
Many facades comprise a uniform grid of rectangular infill elements, each with a gasket around its perimeter.  However, as the grid spacing is reduced then the length of gasket per unit area increases, as does the number of corners per unit area.

Consider a curtain walling stick system which has been designed to be used in any type of facade, from a shop-front to a high-rise building, this may follow several different layouts, image.  Let the air leakage, at a specified reference pressure of 75 Pa, be 0.2 m3/hour per metre of frame.

If the curtain wall system is to be used as a shop front 3 m wide by 3 m high, image, then the total length of frame per glazing unit will be 12 m ( 3 + 3 + 3 + 3 ), giving a total air leakage of 2.4 m3/hour.  The overall area of a unit is 9.0 m2 ( 3.0 x 3.0 ) and so the area-based air leakage is 0.27 m3/hour/m2 ( 2.4 / 9.0 ), which complies with the 600 Pa rating of the Standard for curtain walling at the reference pressure.

If the curtain wall is to be used in a facade with a grid spacing of 1.5 m by 1.5m, image, the total length of frame per unit is 6.0 m ( 1.5 + 1.5 + 1.5 + 1.5 ), giving a total air leakage of 1.2 m3/hour.  However, the overall area of a unit is now 2.25 m2 (1.5 x 1.5) and so the area-based air leakage is 0.53 m3/hour/m2 (1.2 / 2.25), which only complies with the 300 Pa rating of the Standard for curtain walling at the reference pressure.

Now if the curtain wall is used in a facade with a grid spacing of 1.0 m by 1.0 m, image, the total length of frame per unit is 4.0 m ( 1.0 + 1.0 + 1.0 + 1.0 ), giving a total air leakage of 0.8 m3/hour.  However, the overall area of a unit is now 1.0 m2 ( 1.0 x 1.0 ) and so the area-based air leakage is 0.8 m3/hour/m2 ( 0.8 / 1.0 ), which is too high to comply with either rating class in the Standard for curtain walling at the reference pressure.

Note: the conversion of air-leakage characteristics from one grid spacing to another is not always as straightforward; the corners of a gasket may experience a greater rate of air leakage if not properly sealed.  For example, in the calculations above there are only 0.44 ( 4 / 9 ) corners per square metre for the 3m by 3m shop-front, 1.78 ( 4 / 2.25 ) corners per square metre for the 1.5m by 1.5m grid and 4.0 corners per square metre for the 1.0m by 1.0m grid.  As the grid spacing is reduced so the corners also become relatively more important.
 


Large-scale testing for air leakage
The issues raised above have the greatest significance when testing facades or facade specimens to determine their air leakage characteristics.  It is evident that project-specific testing on a sample which is representative of the particular application will give the most realistic assessment of the system performance, provided the same or better quality of fabrication and installation is then used on site.  However, the cost of such testing can be significant, particularly for small projects, and many facade suppliers would prefer to use existing test data to prove their system.  Clearly the above example demonstrates that it is not straightforward to compare two different arrangements even of the same components.  Care should be taken in assembling the test specimen that the numbers of joints and joins, and the total length of each type of joint is a sensible maximum for the test specimen; there would be some merit in assembling a specimen with two or more component spacings, and testing each area in turn.

It is also apparent that ‘standard’ values for air leakage often fail to recognise the range of constructions that are possible.  For example, stick-system curtain walling may vary from basic systems with a large number of opening lights to practically airtight structural sealant glazed systems with no opening lights.