The external façade must give expression to a building through both design and colour, and it must also protect the insulating layers in the wall from external influences. These functions can be fulfilled by almost all materials. If wood is to be competitive in this context, the wood material, the façade design and the surface treatment system must be chosen and interact in such a way that the façade is given a long life with little need for maintenance. A wooden façade will then in a broad sense be both economically and aesthetically attractive for the user.
This study illustrates the state of knowledge regarding the outdoor use of pine (Pinus sylvestris L.) and spruce (Picea abies L. Karst.) facings above ground. Specifically, it deals with the use of wooden facings with regard to the choice of material, façade design, surface treatment and recycling. The market demands wooden facing systems, and the requirements emphasized by the actors on the market, e.g. the builders, real estate administrators, architects, designers, frame suppliers, contractors and representatives for the single-family timber housing industry can be summed up as follows:
- There must be a specified life-time and given time intervals for maintenance of the wooden facings. (Shall be similar to those of competitive materials)
- The supplier of the facing system should shoulder the long-term responsibility for its maintenance.
- Flexibility, the supplier shall be able to replace or renovate the facings when necessary.
- Building requirements, the wooden facing materials must be able to interact with other, specially fire-classified, materials.
- The facing system shall have an attractive appearance.
The primary market for the new facing systems should be multi-family houses but not necessarily multi-family houses of wood. The focus shall lie in the flexibility of the facing system in architectural expression, and in relation to other materials and systems. New building is important, but the million program, renovation and additions (ROT) and greater energy efficiency are also important spheres.
The Swedish market is small (currently ca. 70 000 m3 wood for façades), but it should nevertheless be given priority before the Nordic countries, and thereafter Switzerland, Austria and Germany. The literature describes more or less well-founded recommendations for prolonging the life of wooden facing materials and extending their maintenance intervals, although some of the recommendations are directly conflicting.
Many details relating to materials choice, façade design and surface treatment are important for the durability of wooden facings. It is difficult to separate the most important factors, but without taking into consideration aspects such as costs, availability and other factors of a practical nature, the following key factors can be identified as important for an environmentally correct and durable façade of pinewood or spruce:
Choice of material
- The wood shall have a high proportion of heartwood, preferably exclusively heartwood
- The wood shall have vertical annual rings.
Handling from forest to the façade
- The wood shall be handled so that it does not suffer mechanical damage or microbial attack, or become wet or soiled, i.e. a rapid and correct handling with good packaging.
Design
- The façade shall start at least 30 cm above the ground level.
- The façade shall be ventilated so that moisture can rapidly dry out. Ventilation of the space behind the facing is an easy way of achieving this.
- Water run-off – no horizontal surfaces.
- Flexibility –both in the construction and in the architectural design. There is a demand for facing systems which can be simply “hung onto” existing buildings.
Surface treatment
- Sealed end-grain sections – sealing of the end-grain surface to prevent moisture absorption into the wood is decisive for the life-time of the wood material. Nailing can open up new end-grain surfaces and should thus be carried out carefully and only after due consideration.
- Rounded edges – increase the covering ability of paint and reduce the risk of mechanical damage to the facing boards.
- Choice of surface treatment – vital for the performance of the facings. The wooden facings shall be delivered as part of the complete maintenance package.
Nowadays there are many types of surface treatment where nano-technology is used to create an added value in a surface compared with what the more traditional products can offer. Nano-based surface-treatment products are already on the market, and they are said to make the surfaces dirt- and water-repellent, to prevent the growth of algae, fungi and moss, to improve UV- and temperature-resistance and colour permanence, to improve scratch- and abrasion-resistance, and to have anti-graffiti qualities etc. However, most of these products are new and for some of them there are still question marks with regard to their long-term performance and technical life-time, as well as their serviceability and thereto related economy seen from a life-cycle perspective for the product or system for which the surface treatment constitutes only a part.
A cost analysis carried out as a part of the study makes the assessment that the new nano-technology-based surface-treatment systems could lead at most to a reduction of 15 %. in maintenance costs. The assumption is then that the façade needs to be cleaned every fifth or seventh year when a traditional painting system is used.
According to the Swedish Standard, recovered wood from a wooden façade is defined as tree fuel and is generally designated recycled wood or, when the material is in a disintegrated form, as recycled chips,
There is a major problem in recovering energy from recycled wood when a part of the material has been treated in some way, e.g. impregnated with a wood-protection agent or surface-treated, or when it contains other design components of e.g. plastic or metal. Recycled chips are a very good fuel for energy recovery provided the plant has adequate flue-gas cleaning and the ash is handled in a correct manner. Contaminated ash constitutes a problem, since it is classified as dangerous waste and cannot therefore be returned to the forest. If the content of heavy metals is not too high, the ash can be used as a covering and filling material. Otherwise, the ash must be deposited as landfill. A better sorting of household waste and an overhaul of the regulations would mean that the cleaned recycled wood could be burned in conventional biofuel boilers and that the contaminated portion could be burned separately.