Verity Bird
Posted: Thursday, July 15, 2010 12:51 pm

The first two articles in this series were setting the scene; giving a perspective on sustainability.  The core ideas promoted are that

• with a sustainable approach everybody wins – even if anthropogenic climate change is not actually happening (climate change itself is proven, the debate is simply whether we are causing it), and;
• good design, taking a soundly based holistic approach, is at the heart of sustainable development.

Here we start to look at some of the issues that need considering in achieving this approach.  One of the ongoing debates is between the timber lobby led by the Timber Research and Development Association (TRADA) with the Timber Frame Association (TFA) and the heavyweight construction lobby including, for example, the Concrete Association (CA), both of whom appear to be lobbying for their approaches with the implication that the alternative is ‘less green’ and therefore ‘worse’.  Needless to say the reality is considerably more complex and as always, a whole building approach is needed to achieve the optimum solution for any given design brief and site.

The advantage of using mainly timber in relation to sustainability is that as a tree grows it locks CO2 into its structure.  This happens through the process of photosynthesis; the tree takes CO2 out of the atmosphere and water from the ground, and, using sunlight as its source of power, it combines them into carbohydrate in the form of wood, thereby locking up the CO2 (releasing oxygen back into the atmosphere).  This process is called sequestering.  The reason that burning fossil fuels releases CO2 into the atmosphere is that we are reversing the same process which happened many millions of years ago, releasing the CO2 sequestered by the plants that formed the oil, gas and coal reserves. When biofuels are burnt or vegetation rots down the CO2 is similarly released.  If we make use of natural materials like timber in our buildings the sequestered CO2 is locked into the structure, so we have a sound, positive carbon balance.  Ensuring that your timber is FSC certified will mean that CO2 released during harvesting, transportation, treatment and construction is addressed by the planting of more trees; win-win.

Concrete, on the other hand, is an energy intensive process to produce so a considerable amount of CO2 is released into the atmosphere during the production process.  A lot of research is going into finding alternative materials that will perform as well but with a lower carbon footprint.  Ancient Roman concrete, for instance, was made using lime and pozzolana (crushed volcanic material) as a binder.  Lime can still be used in some instances and, because of the chemical processes involved in its use, also locks up CO2 in its structure; it is, however, not as strong as concrete so it does not set as hard and it sets more slowly, thus limiting its usefulness.  Another alternative is the use of pulverised fly ash (PFA) – a by-product of burning coal to produce electricity – which can be used in place of a proportion of the cement in the concrete mix.

In terms solely of embodied carbon, i.e. the CO2 generated during construction, timber therefore wins hands down.  It is easy to insulate and, provided workmanship on site is good, achieving Building Regulations levels of airtightness need not be a problem issue.  However modern timber framed buildings are very lightweight.  Assuming they are well insulated, they will warm up quickly when the sun shines in or the heating comes on.  However, unless you can virtually eliminate unwanted heat loss by Passivhaus levels of insulation and airtightness, they will cool down equally as fast.  Typically a lightweight building with no mechanical heating/cooling will be only a degree or so cooler than the daily external high (possibly hotter if solar gain is not controlled), and a degree or so warmer than the external daily low temperature.

Dense, heavyweight materials like concrete have the capacity to, in effect, store heat within the bulk of the material; this effect is known as thermal mass.  High thermal mass materials can store more heat.  This has a number of beneficial effects that can be utilised in the design of low energy buildings.

Firstly, during the heating season once thermal mass elements (concrete floors, dense block walls etc) have warmed up they will hold their heat, only beginning to cool down again when the air temperature becomes lower than the temperature of the material itself.  The heat is then slowly released to the surrounding air, so the building cools down much more slowly.  If you position your thermal mass so that it is exposed to direct sunlight during the heating season this effect can be further increased to utilise free solar energy.  A heavyweight building will tend to have reduced extremes of temperature than a lightweight building of the same size and occupancy, and heats and cools more slowly, so it will, for example, remain cooler for longer in the heat of the summer, and retain its heat for longer on cold winter evenings as shown in this illustration³.

* Image reproduced from CIBSE AM10 'Natural ventilation in non-domestic buildings', by permission of the Chartered Institution of Building Services Engineers.
 

 For thermal mass to work effectively it needs to be positioned on the warm side of the insulation and directly exposed to the space it is heating/cooling, so a concrete soffit separated from the space below by a suspended ceiling will not be effective because the warmth/coolth will be trapped in the void above the ceiling.  Ideally it will have a hard finish (not carpet, which acts as an insulant).  If you are intending to use it to store solar heat it needs to be located where it will be exposed to low angled autumn/winter sun but protected from high angled summer sun to prevent internal overheating.  Darker colours are better at heat transfer, so ideally you will be looking for darker finishes, though this would need to be balanced against efficiency of lighting design because lighter colours reflect more light.

How much thermal mass is desirable?  Like everything else related to sustainability, a balanced approach to this is needed.  At a Green Register conference a couple of years ago it was suggested that 100mm is about optimum for the UK.  This is bulky enough to store sufficient warmth or give sufficient cooling, but is not so much that it would take an excessive length of time to heat up/cool down.  A floating screed needs to be 65 to 75mm thick to avoid cracking; this already gets you close to optimising thermal mass.  Adding a modest amount to its thickness might be worth while in the long run if the servicing strategy for the house warrants it.

This is a sketch design for a house on a north facing slope in a wood, a site far from the accepted ideal for a ‘typical’ eco-design.  There is as much glazing as possible facing south to maximise winter gains; solar shading would be provided by the surrounding trees in the summer.  In terms of thermal mass, the strategy here is to build a central thermal mass wall, warmed by the sun when possible, and also from the biomass fuelled Rayburn in the kitchen, and the wood-burning stove in the living room.  The design intention here was to provide a warm ‘heart’ running the full height of the house and connected to as many rooms as possible, though the structure would be predominantly an oak frame with lightweight, heavily insulated infill panels.

There are many benefits to using FSC certified timber in construction, not least its sustainable credentials, however the most effective is to couple it with some use of thermal mass to provide that heat/coolth storage for the building.  What is a complete nonsense, from a sustainability perspective, is to clad a timber framed building externally in a high embodied-energy, high thermal-mass material such as brick.

References/further reading:

1. www.esep.umces.edu/members/adam/images/carbon_cycle.jpg
2. www.tarmac.co.uk/termodeck/HowTermoDeckworks.aspx 
3. 2005; Natural Ventilation in Non-domestic Buildings; CIBSE