Clad for a lifetime

This Issue This is a part of the Claddings feature

By - , Build 94

This article considers some of the things that should be taken into account when choosing claddings. Clearly aesthetics are important but so are the expected long-term costs of maintaining that cladding.

Te Papa has a cladding life of 150 years. (Photo courtesy of Te Papa.)
Te Papa has a cladding life of 150 years. (Photo courtesy of Te Papa.)
Some claddings constitute the entire building envelope.

Claddings are an important consideration in building design. Along with the basic structure they express the building’s shape and personality to the external viewer. They are an integral part of the building envelope and in some cases (e.g. unlined tilt-up concrete or concrete masonry and earth buildings) they constitute the entire building envelope.

Claddings keep the weather out and, along with the other components of the envelope (insulation, building wraps and linings), moderate the interior environment to make it suitable for human activities. The basic requirements for claddings are set out in the New Zealand Building Code, where the primary requirements are generally agreed to be E2 External moisture and B2 Durability. However, depending on the design and intended use of a building, other clauses, such as B1 Structure, C3 and C4 Fire safety, F2 Hazardous materials and H1 Energy efficiency, may also be relevant.

Life expectancy of claddings

This question is resolved by determining the client’s requirements along with a consideration of the minimum requirements set out in clause B2 Durability. The building code requires that buildings must meet all relevant clauses for their intended lives and sets default minimums of 50 years for the structure and 15 years for claddings (except those that are structural). A client can nominate a shorter or longer intended life for both the structure and a component such as a cladding. An example of this is the 150-year-life specified for the Museum of New Zealand Te Papa Tongarewa. In the case of a nominated intended life for a cladding of less than 15 years, this can only be achieved by nominating the intended life of the structure as less than 15 years. An example might be a 10-year-life specified for a temporary structure on a site that is to be redeveloped in the next 5–10 years.

Expected costs over that lifetime

Because building ownership is often relatively short term, many owners consider initial cost along with aesthetics as their prime consideration. However, whole-life costing (also called life-cycle costing or LCC) is becoming more widely used in decision-making, particularly where long-term ownership (e.g. public buildings) is intended. This concept considers the effective costs over the estimated service life of the building for various design options, to find which has the overall lowest cost while still meeting both client’s and building code requirements. For claddings, the information needed would include:

• estimated service life of the building, e.g. 25, 50, 100 years

• initial costs

• maintenance frequency, e.g. yearly or five yearly

• maintenance costs, e.g. scaffolding, building maintenance units (BMU), repainting, loss of revenue during maintenance cycles

• replacement cycle (if required)

• effect on ongoing operational costs (if any)

• disposal costs.

A simple example would be two alternative designs for a school building, one with higher initial cost but lower maintenance (Building A) and the other with lower initial cost but higher and more frequent maintenance and upgrading (Building B).

The life-cycle costing example in Table 1 shows that Building A is the better choice in terms of life costs even though its initial cost is higher.

Promoting sustainable design

As sustainability issues become increasingly important, life-cycle assessment or cradle-to-grave consider ations become increasingly important in building design. What this does is combine economic considerations of whole-life costing with sustainability issues associated with the production, transport, installation and maintenance of components, as well as their contribution to the overall ongoing impacts of the operation of the building and, finally, disposal at the end of its life. The widespread adoption of this technique is still limited by a shortage of reliable data on the sustainability aspects of New Zealand construction materials and easy-to-use tools that let designers and clients access information easily, in a form which allows sensible decision-making.

Some claddings constitute the entire building envelope.

Rating schemes for buildings incorporating various elements of sustainable design have been developed overseas to provide simple and standardised ways of assessing buildings. Schemes like this are being considered for New Zealand. The newly established New Zealand Green Building Council is currently evaluating schemes for office buildings. The BRANZ Green Home Scheme has been operating for 9 years and provides a simple way for designers to consider sustainability issues.

Another useful tool (commissioned by Building Research) is a spreadsheet which allows users to compare whole-life costs and some sustainability issues (embodied energy, CO2 emissions and recyclability) of some common wall claddings. The spreadsheet can be found online at the BRANZ website ( www.branz.co.nz), under Resources and Software; Life cycle – wall cladding.

The Building Act 2004 includes in its purposes the statement that ‘buildings are designed, constructed, and able to be used in ways that promote sustainable development’. This is further expanded in the principles to include whole-life costs, efficient and sustainable use of materials, the efficient use of water and water conservation in buildings, and the need to reduce waste during the construction process. This means these concepts will eventually be introduced into the building code. It is expected that the major review of the building code, currently under way, will see water efficiency and reduction of waste materials added to energy efficiency and conservation, which are already covered.

Table 1: Comparison of two alternative school designs.
 Building ABuilding B
Initial cost $1 million $0.9 million
Repaint $30,000 every 10 years $50,000 every 10 years
Internal fit-out $300,000 every 15 years $300,000 every 10 years
Total costs in today’s dollars $1.19 million $1.25 million

Note: Future costs have been discounted, assuming a 7% discount rate and a life of 60 years.

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More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Te Papa has a cladding life of 150 years. (Photo courtesy of Te Papa.)
Te Papa has a cladding life of 150 years. (Photo courtesy of Te Papa.)
Some claddings constitute the entire building envelope.

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