Building the Zero Carbon Way

Our work with the Royal Institute of British Architects (RIBA) on embodied carbon has thrown up some fascinating facts – we could argue that’s its only common sense that historic buildings have a low carbon footprint but lets look at the figures to see if that’s the reality …

Historic buildings come in three basic types:

• Timber-framed which in the East of England is the dominant type
• Brick buildings
• Stone buildings

In a nutshell, timber-framed buildings have masses of carbon locked up in their component parts, and the longer they are with us the better this figure becomes. Brick buildings are generally the product of charcoal firing, again considered relatively low carbon although there were always emissions associated with this. Stone buildings are again zero carbon particularly as the stone was quarried by human graft.

Most modern building materials now come with a much higher carbon footprint, mainly because of mechanisation, but also because of high temperature kilns which burn high carbon fuels.

So, let's look at typical East Anglian buildings, largely timber-framed before the advent of plentiful cheap bricks, and usually made from materials very local to the building. A typical Yeoman's house of say 100 square metres on one and a half floors and built of large section oak returns figures which are eye wateringly good. The figures are expressed per meter squared of building for ease of translation:

• Foundations – usually of local stone or brick but minimal! PLUS 20 kg/m² (using current carbon figures but almost certainly much better than this)
• Timber frame – minus 80 kilograms per square metre
• Wattle and daub infill – 0 kilograms per square metre
• Thatch – minus 0.75 kilograms per square metre
• Windows (glass) – plus 1.75 kilograms per square metre

This gives us a total embodied carbon footprint of minus 59 kilograms per square metre of floor area. What this means is that the building is acting as a carbon sink. And if it stands for 500 years that figure needs to increase compared to modern buildings which have a declared lifespan of 60 – 80 years. So the final figure could be six or seven times better!

Of course, we are all aware that a simple timber-framed building no longer meets the zero carbon in use standards we all need to be aiming for so better insulation is a must both for the planet and for the comfort of building users. But again what we find is that with an emphasis on appropriate natural materials, our carbon footprint is substantially lower than with more conventional insulation materials, most of which are not vapour open, and therefore damaging to historic building fabric.

Materials such as wood fibre, wool, hemp/lime, recycled paper, are all appropriate and low carbon, and taking a typical (but perhaps controversial) refurbishment approach, we can improve on the figures because many of the deeper green materials have a negative carbon footprint.

For example, overcladding with wood fibre and lime render results in no net increase in embodied carbon because one cancels the other out, but a massive reduction in carbon in use down to Passivhaus levels of 15 kilowatts per square metre per year.

Now looking at appropriate modern construction, such as timber frame, natural insulation, airtightness to standards required for zero carbon, and mechanical ventilation, we find that we can achieve similar performance standards. The volume of timber is much less and the complexity of the building is greater, but the overall result is still better than zero embodied carbon. Here are a few figures to illustrate the point:

• Foundations (shallow lime foundations as opposed to concrete) – plus 20 kilograms per square metre
• Timber frame – minus 13.6 kilograms per square metre
• Insulation (taking hemp/lime) – minus 1 kilograms per square metre
• Roof coverings (clay tiles) – plus 0.2 kg/m²
• Glazing (triple) – plus 3.3 kilograms per square metre and likely to be double this for replacement at least once

None of the numbers for carbon emissions include excavation and disposal (with soft buildings a shallow foundation has minimal impact on this), mechanical services (mainly ventilation as heat is rarely required), off site works such as infrastructure connections (power, water, telephone, drainage etc) roads ...

But, overall the prospects for delivering zero carbon building now are good, provided we learn from the past and use materials which are friendly to the planet, and coincidentally also healthy and safe in fire.