Home truths - low-carbon materials key to greener dwellings
1st June 2009
Forget complex, high-end technologies – basic truths and natural, low-carbon materials hold the key to sustainable and truly energy-efficient homes, argues James Hulme
In recent years there has been a great emphasis on the role of technology to improve the energy performance of residential and office buildings, whether newbuild or retrofitted. No shiny ‘eco house’ is perceived as complete without wind turbines, heat pumps, an array of photovoltaic panels and walls packed with foam-based insulation behind plastic membranes.
The truth, however, is that in our drive for energy efficiency we may be overlooking some basic truths about traditional building performance that are the key to real sustainability. In particular, relying on high-end technology may give short-term results at the evaluation stage, but the very specificity of these solutions mean they are going to be rendered obsolete by changes in the building’s use or configuration, or else fail in time due to their relative complexity and inability to be maintained simply. Membranes shrink or become damaged in routine alterations, while solar panels reach the end of their useful life after 20 years, even when specified for a building that should be around for 200 years or more. In the design of new buildings, clear distinction needs to be drawn between promoting the sort of smart technologies that sell new cars and those that will truly serve an activity that, above all others, is literally building our society out of its woeful carbon dependency.
Many experts now believe that true sustainability in the construction sector looks more like the solutions offered by natural, responsive materials, which are generally proven to have higher tolerances to climate variables. Instead of relying on technological fixes, the Prince’s Foundation for the Built Environment is promoting, in a live build project with the Building Research Establishment, an approach to residential design using natural materials grown or from the ground, achieving energy efficiency through simple wall construction of sound thermal performance. The emphasis is on vastly simplified wall and roof sections, capable of being produced from sustainable low-carbon materials wherever possible, and built and subsequently repaired by local labour without dependence on complex factory construction systems.
Further to this, there is now some circumspection about the targets that the Code for Sustainable Homes has set the construction industry, and the consequences for issues such as the airtightness of buildings. High levels of airtightness are essential to avoid unwanted heat-loss in highly insulated, low-energy buildings. This means the design and performance of ventilation systems becomes critical to human health, however, as we can no longer rely on unplanned draughts for fresh air.
The main problem with under-ventilated buildings is trapped moisture, which can lead to moulds, dust mites and illnesses such as asthma. While good ventilation is the primary means of dealing with this problem, natural materials designed into a breathable building shell can provide excellent support and back-up for a healthy building through vapour-open walls and roofs, and the hygroscopic buffering of internal moisture levels.
In both their production and life-cycle operation, a suite of natural materials is emerging that demonstrates excellent thermal performance when used appropriately, displaying good tolerances over a broad climatic range.
Natural roof-insulation materials
All experts agree that a well-insulated roof is critical for a building to achieve anything approaching acceptable performance targets in the future. What is less obvious is the capacity of natural building products to achieve this goal rather than highly processed synthetic materials. Supplied by Natural Building Technologies, Pavaroof is a solid, breathing roof system that has a very high thermal mass.
Pavatex boards are made of 95-99 per cent waste wood, bound together by its natural lignins under different pressures to provide a variety of boards for different applications (walls, floors and roofs). Pavatex boards lock up 1.2 tonnes of CO2 for every tonne of CO2 or product used.
The boards are applied over the structural elements of buildings to form a continuous, insulated, breathable shell. Airtightness is achieved on the inside of the structure with a large services void, thereby allowing internal fittings and services to be applied and later retrofitted without affecting airtightness. The boards also provide excellent acoustic insulation, typically resulting in an R-value of more than 50dB in roofs and walls. Finally the boards provide very good overheating protection because of their high thermal mass. A room in the roof insulated with wood fibre will typically have a peak summer temperature 4°C lower than a roof insulated with synthetic insulation to the same U-value.
The externally applied boards can be complemented in between studs or rafters by natural insulation such as Thermafleece sheep’s wool insulation. Launched in 2001, Thermafleece is a range of insulation made from the wool of British hill sheep. It is a high-density, wool-rich insulation that is the first choice when maximum thermal and acoustic performance and breathability are required or when space is limited. Manufactured to the highest performance standards, Thermafleece is the only wool-based insulation to hold BBA (British Board of Agrément) certification.
The Prince’s Foundation/BRE house uses Thermafleece insulation between 150mm rafters under 200mm of Pavatex wood fibre, achieving a U-value of 0.10W/m2K and excellent overheating and acoustic protection in a robust breathable structure. The roof will not need changing for 200 years.
Exploiting another lesson from traditional design to enhance long-term performance, the eaves of the house have been made generously deep to give adequate protection to the rendered walls below, as well as to withstand heavier downpours as a result of climate change.
Lime plasters and renders
Modern lime-based plasters and renders are being supplied for the Prince’s Foundation house by Baumit, through its partner Natural Building Technologies. These maintain the traditional breathable and environmental qualities of lime, but are designed for modern site and cost conditions.
On a house with a 200-year lifespan, it is vital that the renders are robust and attractive. The main failings of renders on UK buildings occur due to the incompatibility of render and substrate, and poor application. Baumit limebased renders are specifically designed for compatibility with the aerated clay blocks and come pre-bagged to avoid mixing errors. Furthermore they are spray-applied, ensuring correct pressure of application and a considerable reduction in application time (and cost). A silicate-based, self-cleaning topcoat called Nanopor will be applied as a finish coat.
Internally a modern lime plaster will be used as part of the airtight breathing wall construction. Resistant to mould, lime also helps provide a healthy internal atmosphere.
James Hulme is director of policy and research at the Prince’s Foundation for the Built Environment, and manages the Natural House project, due to complete this summer, at the BRE Innovation Park, Garston, Watford. Additional information provided by Neil May of Natural Building Technologies
Hemcrete is an innovative building material that uses the inner, woody core of the industrial hemp plant combined with a lime-based binder. Being cellulose, the hemp particles are a carbon sink and provide the insulation qualities; the formulated lime binder is a breathable (vapour-porous) matrix that renders the hemp fire- and vermin-proof, provides the strength and has a low-carbon footprint.
Depending on the relative mix ratio of these two constituents, the resulting mixture can be cast or sprayed to form highly insulating walls and roofs (using more hemp), and is also available as a 3N structural block (440 x 215 x 100mm) by using a greater proportion of binder – the blocks are carbon-neutral.
Most newbuild applications use a lightweight timber frame resulting in a fast build process forming a monolithic wall with the unique characteristics of insulation, airtightness and thermal inertia. Consequently, carbon-negative Hemcrete saves carbon emissions from the building ‘in use’ as well in construction. Hemcrete is also suitable for renovation of older buildings, and is entirely recyclable.
Aereated clay block
Updating the traditional clay brick, aerated clay block is now available from several manufacturers and is widely used in the European construction industry, despite being hardly known in the UK. Rendered externally and plastered internally, the blocks provide a method of building without cold bridging, and with internal insulation and high thermal mass. The blockwork is easy to construct and can be readily mastered without specialist skills, making it a suitable system for more localised building operation.
The system provides good airtightness in a vapouropen construction with no artificial membranes. The pure clay is highly durable and easy to adapt and extend. When manufactured in the UK the thermal blocks will have approximately 50 per cent less embodied energy and embodied carbon than a conventional block cavity wall meeting the same thermal performance.
Resilient and long-lasting, production of ThermoPlan Zeigel Block will commence in the UK this summer through a partnership between Natural Building Technologies, Ibstock and ZWK.
Other natural choices
While the average homeowner may not be in the position to commission structural work in block or lime cement, there are many other areas of home improvement and decoration where it is possible to effect positive environmental outcomes that will improve your carbon footprint and ensure a healthy environment. Appropriate floor coverings include Marmoleum and cork linoleum, both available from Scottish manufacture Forbo and made from natural materials and naturally occurring resins, reducing the likelihood of VOC (volatile organic compound) offgassing. They are also a deterrent against asthma, as they reduce exposure to dust mites and have natural bactericidal properties.
Replacement windows can have serious deleterious environmental impacts if poorly chosen. UPVC frames represent a cheap option for the unwary, but have high embodied energy in production and a short lifespan. Wood frames are more expensive but can be locally and responsibly sourced, allow for the expression of traditional carpentry techniques and, because they expand and contract in response to seasonal climatic changes, often outlive double-glazed units. English Heritage and the National Trust offer advice on sourcing suitable windows for historic properties.
When choosing paint for your home, manufacturers such as Farrow & Ball are now actively trying to reduce the impact of oil, with an emphasis on natural ingredients such as linseed oil and China clay, and by rejecting harmful ingredients such as ammonia and formaldehyde. VOC for most Farrow & Ball paints are so close to zero that the leading finishes are classified ‘zero-VOC ’ when tested to US Environmental Protection Agency standards. A new generation of eco paints reduces solvents and oils without compromising quality of colour or finish.
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