Delivering Tomorrow's Quality Today

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.

Infiltration or air leakage can account for up to 50% of heat loss in modern homes significantly adding to the amount of energy required to keep a home warm and comfortable, resulting in higher carbon emissions. Air leakage allows expensive heated air to escape from a home, essentially wasting valuable energy, whilst also adding to occupant discomfort caused by drafts. Poor airtightness can also increase the amount of pollutants within a building such as particulates, pollen and volatile organic compounds (VOCs) that can all have a detrimental effect on our health and wellbeing, contributing towards the high instances of asthma within the UK. 

Airtightness and ventilation are inextricably linked. If you spend time and money designing a home to be very airtight in order to limit unwanted infiltration, it is vital that
you provided suitably designed ventilation to maintain good indoor air quality. Typical decentralised and centralised mechanical extract systems (without heat recovery) in homes suck warm moist air out of kitchens and bathrooms and rely on replacement air being drawn in via trickle vents and uncontrolled infiltration. This method results in a significant amount of lost energy as the extract fans draw heat out of the home and replace it with colder and potentially polluted air from outside. In energy efficient airtight homes with airtightness less than 3m³/hm² at 50 Pascals, mechanical ventilation with heat recovery (MVHR) is required in order to control the indoor air quality whilst also retaining heat within the home for longer. MVHR units are fitted with filters to remove pollutants, improving indoor air quality.

credentials, timber frame has many advantages over other forms of construction, when it comes to building homes. The material itself can be sustainably grown and procured from certified sources and has much lower embodied energy than masonry, or steel frame construction. It can also be easily prefabricated in factory controlled
conditions and be erected on site significantly faster than masonry construction. Erection is also less reliant on dry weather conditions. Timber frame can also typically achieve better thermal performance, with a thinner wall construction. The disadvantages are that it can be more susceptible to condensation, rot/infestation and the
risk of fire, however these issues can easily be overcome with good design and specification. Whilst the cost of a pre-designed timber frame can be higher than other forms of construction, there is generally greater cost certainty as it can be designed and manufactured offsite prior to starting on site. 

Offsite construction can help deliver good quality, low-carbon homes that meet the five key principles of good housing. Building in factory-controlled conditions helps to improve quality and performance, whilst significantly reducing waste, when compared against traditional building practices i.e. building outdoors on cold, wet, muddy building sites. This is particularly important when you consider that the UK construction and demolition industry is responsible for over 30% of landfill waste. It is also important to note that up to 15% of products delivered to construction sites are sent directly to landfill without being used.

Whether we choose to use modular construction or closed-panel construction, the key is to do as much as possible offsite in factory-controlled conditions, where stage-and-gate quality controls can be implemented. The quality control measures and working environment are much more likely to help close the ‘performance gap’ and make achieving a high performance thermal envelope, with high levels of airtightness, more readily deliverable.

The design and construction industries have an obligation to deliver on the governments key priorities. Whichever way we choose to construct homes, it is vital that we follow the basic principles in order to reduce energy demand, maintain occupant comfort and reduce waste. If we are to be truly sustainable, then we must also get out
of the habit of constructing poor quality homes, which are demolished 30-50 years later, to start the process all over again ad infinitum. This means that we must invest today for the long haul, by building good quality homes designed around the key performance principles. We know what is required, now is the time to deliver. 

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