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Timber Cladding in Scotland

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Timber Cladding in Scotland

JOINT LEAKAGE PROCESSES

Five main processes force rainwater through joints or defects in the rainscreen (6):

1 Kinetic energy - drops of rainwater may have enough momentum to carry them through open joints either directly or by splashing;

2 Surface Tension - water can adhere to and run across the underside of horizontal surfaces;

3 Gravity - The force of gravity will pull rainwater downwards through any openings;

4 Capillary attraction - water is drawn into narrow gaps that are bounded by wettable surfaces;

5 Air-pressure differentials - the most common form of joint leakage is due to wind-induced differences in air pressure. When pressure on the outer face of a wall cladding is greater than that acting on its reverse face, rainwater will be forced inwards.

Over the 40 years since these mechanisms of joint leakage were first described, the rainscreen approach to preventing moisture penetration into a wall has developed into two distinct techniques:

THE DRAINED AND BACK-VENTILATED RAINSCREEN -

This is the more common and less technically demanding technique. It is based on the assumption that, although most rain is deflected at the outermost surface of the cladding, there will always be some wind-driven rainwater penetration through joints in the rainscreen and this is removed by cavity drainage and ventilation. In its simplest form, drained and back-vented timber rainscreens consist of a series of planks fixed to support battens which are designed to allow uninterrupted drainage and ventilation of the cavity. The joints between the boards are generally quite tight, and are designed to obstruct the passage of wind-driven rain droplets. Due to the combined effects of gravity and wind-induced pressure differentials, the joints do not prevent leakage. Thus, during periods of heavy driving rain, relatively large amounts of water may penetrate the joints and run down the reverse of the cladding boards. Successful design depends upon preventing this water from reaching the inner leaf of the cladding assembly. In other words, the claddings are allowed to leak to some extent and no deliberate attempt is made to minimise the effects of wind by pressure equalisation. Instead the cavity is drained and ventilated in order to remove moisture (6).

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THE PRESSURE-EQUALISED RAINSCREEN -

This more demanding technique seeks to eliminate rain penetration through the cladding, not by tightly sealing the joints, but by leaving some or all of them open to the passage of air but not water. This is achieved by controlling kinetic energy, surface tension, gravity and capillary attraction through careful joint design, using similar principals to the drained and vented cavity approach. Rain penetration caused by pressure differentials is controlled by pressure equalisation of the cavity. Pressure equalisation is enabled by incorporating sheltered openings of adequate size within the rainscreen combined with compartmentalising the cavity into separate vertical spaces. Splitting the cavity into separate spaces prevents lateral air movements which would reduce pressure equalisation. Full pressure equalisation is impossible to achieve in practice and minor leakage will still occur. This is drained and vented away using the techniques for the drained and back vented approach (6, 9).

Watermarks on the inside face of external timber cladding. Wind-driven rain will tend to penetrate through joints and run down the rear face of cladding boards. This is removed by cavity drainage and ventilation.

Both the drained and back-vented rainscreen and the pressure-equalised rainscreen incorporate a cavity behind the outer cladding. Confusion arises between the two because, whilst pressure-equalised rainscreens are designed in other materials they are difficult, if not impossible, to achieve in timber. In spite of this, various pressure-equalisation claims are regularly made for timber cladding that have little empirical basis. Anderson & Gill (6) caution that:

... very little is known about the wind response characteristics of rainscreens. When designing the cladding assembly, no account should be taken of the fortuitous effects of cavity pressurisation unless the results of adequate research are available.

RECOMMENDED BEST PRACTICE ON THE WEST COAST OF CANADA

In the exposed coastal climate of British Columbia, a number of building companies are currently the subject of litigation claims concerning the failure of various external cladding materials. In Canada these failures are popularly known as the 'leaking condo' problem. The failures have been appearing since 1985 and as a result the federal government's housing agency, the Canada Mortgage and Housing Corporation, instigated a comprehensive research programme to identify and develop solutions to the problems.

Many of the cladding failures discovered in the coastal climate of British Columbia were caused by building details designed for a sheltered climate being introduced to more exposed conditions (8). The use of non-rainscreen cladding (i.e. cladding without a drained and ventilated cavity) was a particular problem.

In addition to careful design that promotes drainage and ventilation of the cladding, Canadian researchers also recommend that all inaccessible structural and non-structural timber outside the breather membrane should be pressure-treated with a suitable preservative unless it is of a high natural durability. Canadian researchers do not believe that a surface coating will provide adequate weather protection to timber species with low natural durability (7). While spruce and lodgepole pine claddings are treated with preservative, Douglas fir is used without pressure treatment but is given a surface coating. Western red cedar cladding is not treated when used for cladding, but pressure treatment is recommended for roof shingles (9).

In British Columbia the current recommendations for timber cladding generally involve drained and back vented designs. However, on very exposed coastal sites it is argued that these types of design may not give sufficient protection against driving rain penetration. In such situations, the use of pressure-equalised timber cladding has been proposed though these proposals are hedged with caution:

Very few of these assemblies have been constructed, so construction detailing has not yet been fully developed (7).

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RECOMMENDED PRACTICE ON THE WEST COAST OF NORWAY

In Norway, as in the rest of Scandinavia, virtually all external timber cladding is made from either European redwood or, more commonly, European whitewood (Norway spruce). No attempt is made to remove the sapwood and the use of preservative treatments are only recommended on coastal sites exposed to the full force of wind driven rain (10). Less than 10% of Norwegian cladding is preservative treated and the detailing of most timber cladding there is generally very plain and follows established convention. Considerable emphasis is placed on cladding being well drained and ventilated and thus able to rapidly dry out after periods of wetting. Throughout Scandinavia this type of timber cladding is given a service life of 50 years in most conditions although poor detailing can reduce this figure considerably

Norwegian timber cladding is almost always coated, generally with a fully opaque moisture vapour permeable coating. It is also well maintained through, for example, repainting the coating every 5-10 years, and replacing any boards that are damaged or starting to decay. The moisture-shedding properties of coatings are not, it is believed, as important in defending against against moisture problems as adequate drainage and ventilation.

Both the heartwood and the sapwood of Norway spruce are relatively resistant to the uptake of moisture and for this reason whitewood cladding is preferred to the more permeable redwood. Norway spruce is also less resinous than redwood and so less prone to resin staining on a warm, south facing wall.

No attempt has been made to remove the (lighter coloured) sapwood of newly installed redwood cladding in Stavanger, western Norway. This cladding would subsequently have been painted with an opaque moisture vapour permeable coating.
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Technical standards for timber durability and preservation are harmonised throughout Scandinavia. Thus, although Norway is not a member state of the European Union, its technical standards could be expected to be broadly in compliance with the European standards quoted in this study.

In contrast to the UK, Norwegian recommendations stress the importance of relatively tight cladding designs that seek to minimise the gaps where wind-driven rain can penetrate the outer rainscreen. It is believed open-jointed designs may put an undue reliance on the water shedding qualities of the breather membrane. This is because - even allowing for the best of modern breather membranes being very reliable - such designs are vulnerable to:

  • Poor installation practice;
  • Damage caused by vandals pushing sharp objects through the cladding joints;
  • In extreme cases, particularly with open jointed vertical cladding, sunlight may penetrate the joint and cause UV degradation of the breather membrane.

Vertical open-jointed designs are not accepted at all because these can allow sunlight to reach the breather membrane leading to UV breakdown of the membrane. Current Norwegian guidance only includes one type of horizontal open-jointed cladding and this is only recommended for sheltered conditions (10).

Brightly coloured opaque coatings add vibrancy to buildings on the west coast of Norway.
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Keith HUnter
An open-jointed cladding design recommended for sheltered sites in Norway. As with all open-jointed designs, it is not suitable for use as vertical cladding. This type of open-jointed design can be adapted for use with invisible clip systems, such a system is currently being marketed in the Netherlands.
Open-jointed, drained and vented cladding can take many forms. This visitor centre at Mount Stuart on the Isle of Bute by Munkenbeck and Marshall Architects uses horizontal timber louvers which, although they allow a considerable amount of rain to penetrate, will protect the breather membrane behind from UV damage. Open-jointed cladding appears to be suitable for sheltered sites in Scotland providing that it is understood that the breather membrane is required to do more work than would be the case with tighter cladding designs.

RECOMMENDED BEST PRACTICE IN SCOTLAND

In the UK, TRADA currently recommends that where cladding timbers are classified as being less than moderately durable, or where they contain sapwood, the timber should receive preservative treatment by impregnation (11). This view is essentially the same as Canadian guidance but contrasts strongly with Norwegian recommendations.

The requirements of the Building Regulations in England and Wales correspond to TRADA's recommendations. However Part B 2.2 of the Scottish Building Regulations corresponds more closely to Scandinavian practice in that the regulation permits timber of low natural durability to be used without preservative treatment providing the boarding can be readily replaced. The regulation states: Materials, fittings, components, and other manufactured products or parts thereof whose suitability depends upon proper maintenance or periodic renewal must be readily accessible, or positioned so that replacement is reasonably practical (12).

Under BS EN 460:1994, both approaches appear to be consistent with European standards. This standard reconciles the two approaches by putting the onus on the designer to decide what level of decay risk is acceptable for a particular cladding job. Where only a minimal risk is acceptable, or the site is very exposed, the specifier has little alternative but to specify an appropriate preservative treatment or a relatively durable timber. But where safety and economic considerations indicate that minor remedial action is acceptable, preservative treatment could be avoided, and even timber classed as being not durable might be used in many situations. It should be emphasised that:

  • the low durability option requires that the owner of the building is aware of and accepts the possibility of greater ongoing maintenance;
  • the cladding is designed so that the most vulnerable components are readily replaceable should the need arise;
  • this approach presupposes a much higher standard of detailing, construction and maintenance than is common in the UK construction industry.
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In contrast to Norway, current UK recommendations for timber cladding include designs for relatively open-jointed drained and back-vented claddings. These designs have a long history in sheltered continental climates (10, 13) and were first used in the UK on the Henley Rowing Museum (1998). Open-jointed designs such as Henley assume the cladding itself is not fully resistant to water penetration, and so the use of a high performance breather membrane behind the cladding cavity is essential to resist water penetration into the wall assembly. These types of cladding appear to be acceptable on most sites providing it is recognised that rain penetration through open joints places increased demands on the breather membrane. Indeed, they offer the advantage that, because they can accommodate a degree of timber shrinkage, green (i.e. unseasoned) oak can potentially be used. Green oak can be considerably cheaper than seasoned oak and may permit the use of homegrown oak in situations where it might otherwise be too expensive to use for cladding.
'Behind the Wall' in Falkirk by Zoo Architects uses green oak sourced from forests in the Trossachs for its exterior cladding.

References

1 HMSO ,1989, The Climate of Scotland
2 Quine, C.P., Humphrey, J.W. and Ferris, R., 1999, Should the wind disturbance patterns observed in natural forests be mimicked in planted forests in the British uplands?, Forestry, Vol 72, No. 4.
3 Hulme,M., Crossley, J., and Lu, X., 2000, An Exploration of Regional Climate Change Scenarios for Scotland, Scottish Executive Central Research Unit
4 Birkeland, O., 1962, Curtain Walls, Norwegian Building Research Institute;.
5 Garden, G.K., 1963, Rain Penetration and its Control, National Research Council of Canada
6 Anderson, J.M., and Gill, J.R., 1988, Rainscreen Cladding - A Guide to Design Principals and Practice, CIRIA and Butterworths.
7 Canada Mortgage and Housing Association, 1999, Wood-Frame Envelopes in the Coastal Climate of British Columbia.
8 Canada Mortgage and Housing Association , 1996, Survey of Building Envelope Failures in the Coastal Climate of British Columbia
9 Canadian Wood Council, 2000, Wood Reference Handbook
10 Byggforsk, 1998, Byggforsk kunnskapssystemer ver 1.0, Norwegian Building Research Institute
11 TRADA Technology, 2001, Timber frame construction
12 Hamilton, W.N. et al., 1994, The Scottish Building regulations explained and Illustrated, Blackwell Scientific Publications
13 Detail, 2000, Vol. 40, No. 1, Timber construction
British Standards
BS EN 460: 1994. Durability of wood based products. Natural durability of solid wood. Guide to the durability requirements for wood to be used in hazard classes.