Tabitha Binding of Timber Development UK answers readers’ questions on timber.
Wood fibre insulation provides excellent thermal performance, while also helping to mitigate heat loss and heat gain on a daily and seasonal basis. Photograph courtesy of Timber Development UK member Steico
Is there enough timber and is it sustainable?
The volumes of softwood and temperate hardwood biomass within plantation forests are increasing. This means we are currently growing more trees than we are felling. All plantation forests will have a management plan, often backed by an independent third-party certification scheme that ensures that forests are managed to meet environmental and social standards, and that trees felled are replanted. In the UK, look and ask for PEFC – the Programme for the Endorsement of Forest Certification, FSC – Forestry Stewardship Council, GiB – Grown in Britain, and FLEGT – Forest Law Enforcement, Governance and Trade, which covers tropical timber.
What is the difference between hardwood and softwood?
Hardwoods can be hard and heavy like greenheart, but they can also be soft and light like balsa. Softwood can be soft and light, like grand fir or hard and heavy, like yew. In general, hardwoods like oak are deciduous and lose their leaves in winter. However, Softwoods, which are mainly conifers, keep their leaves/needles all year round – unless they are larch, in which case, they shed them. So, it is the seeds that differentiate the two!
Hardwoods are angiosperms and their seeds are covered – think acorn as in oak. Softwoods are gymnosperms and their seeds have no coverings but are often held in cones from which the seeds are dispersed by the wind when ripe – think pinecone and Scots pine. A good place to start is TRADA’s timber species database.
How much carbon does a piece of timber store?
That depends on the density and moisture content of the timber. Trees absorb carbon as they grow through a process of photosynthesis. The biogenic carbon sequestered in wood (kg CO2/m ) is measured by a calculation. Take the dry mass of timber, deduct any moisture content, then multiply by a half (as only half the dry mass of timber is carbon) and multiply by the carbon factor. For example, the amount of carbon sequestered in 1m3 of construction softwood at 12 per cent moisture content is 655 kg CO2/m – (400/(400+12%) *0.5*(44/12).
How long can timber last?
The right timber species in the right conditions can last for thousands of years and possibly indefinitely. But get the species or conditions wrong and timber can degrade fast. Timber species have differing natural durability, which is divided into five classes from very durable at class 1 to nondurable at 5. Durability and longevity of species can be extended by coating, treating with a preservative, or modification by a thermal or chemical process. The desired service life of timber can be extended by understanding the environment that it will be placed in. For example, a non-durable timber that is kept below 20 per cent moisture content can last indefinitely in a use class 1 or 2 environment.
Why do some timber doors expand in the winter and shrink in the summer and others don’t?
Timber is hygroscopic which means its moisture content will change in response to the ambient temperature and humidity. Different species of timber have different movement rates. These are characterised as: small – one per cent for every five per cent change in moisture content (MC); medium – one per cent for every four per cent change in MC or large – one per cent for every three per cent change in MC. To reduce the risk of shrinking and swelling, specify a low movement timber that is appropriately coated and maintained. Some modified woods have very low movement characteristics. Timber has a good thermal performance, which makes it useful as a buffer between internal and external spaces.
Can timber be recycled?
Yes, timber can be both reused and recycled. Beams and floorboards in old buildings often had a previous life in another building. Timber from demolition or offcuts is turned into wood chip or wood fibre boards. At the end of its useful life timber can be chipped for animal bedding, which is then composted and returned to the earth or used in bioenergy plants.
Is timber good for us?
Yes, research is showing that wood in our built environment aids our health and wellbeing – a good thing when we spend 90% of our time indoors! Heart rates are lowered, we are more relaxed, we learn better, interact more, and are happier. Visit Wood for Good to find out more.
Can timber be used in retrofit?
Yes, there are lots of timber products that suit retrofit and renovation projects. Wood fibre insulation is one of them – it has excellent thermal properties. Increased thickness and density of natural fibre insulation give the building fabric greater thermal mass. As well as being energy efficient, this helps temper heat loss and gains on a daily and seasonal basis creating a substantially more comfortable living environment. The thicker layer of insulation also improves moisture buffering potential, acoustic performance and air quality.
TRADA and the Timber Trade Federation are merging to become Timber Development UK. For more information contact Tabitha Binding on 07815 948749 or tbinding@ttf.co.uk
Source: Architecture Today