David Russell, Director at Carbon Futures, outlines how offsite construction can deliver far-reaching thermal improvements for new UK housing.
A recent report by the Committee on Climate Change asked if the UK’s housing was fit for the future. The report listed five key priorities for government action: performance and compliance, skills gap, retrofitting existing homes, building new homes and financing and funding. Offsite construction has the potential to deliver on all these key priorities, however I would suggest that the most important of them all is performance. With plans for 1.5 million new homes by 2022, it’s vital that we learn from past mistakes, to ensure that we deliver quality as well as quantity.
If we fail to provide both, then any short-term success in terms of numbers has the potential to be blighted by long-term failures in performance. Good quality homes should be designed around five basic principles:
• Be suitably designed to reflect orientation whilst minimising overheating
• Have a well-designed high performance thermal envelope
• Be designed to reduce and ideally eliminate thermal bridges
• Be airtight in order to reduce unwanted infiltration
• Achieve good indoor air quality through the provision of well designed ventilation.
The key to successfully delivering these principles is to understand that they are all interconnected, in that a weakness in any one principle will have a detrimental impact on performance. To be as efficient as possible, designers must first endeavor to orient dwellings as efficiently as possible – ideally facing north/south if the site constraints permit. Window openings should be designed to suit orientation, with more glazing on the southern elevation to maximise solar gain and less glazing on the north elevation to limit heat loss.
It is also important not to size windows appropriately as a balance must be struck between solar gain and summer overheating. Overhangs should be designed into window openings to facilitate shading in the summer months, whilst permitting solar gain during the winter months, when the sun is lower in the sky, as this will reduce the heating and cooling demand, which is vital in order to moderate indoor temperatures for occupant comfort.
We must also endeavor to eliminate thermal bridging, which is an area of a building construction that has a significantly higher heat transfer than the surrounding materials. Thermal bridging can account for up to 20-30% of total building heat loss, which is significant. Any heated building should be designed and constructed to limit heat loss through thermal bridging.
Whilst repeating thermal bridges are accounted for in U-value calculations, a separate calculation is required to assess non-repeating thermal bridges at all the external junctions within a building. As homes become better insulated, the importance of thermal bridging increases exponentially. Failure to address thermal bridging can increasing the risk of surface condensation and mould growth, whilst also contributing towards occupant discomfort due to the presence of localised cold spots.
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