A recent study from the University of Salford’s Energy House Labs has revealed crucial findings about the role of airtightness in improving energy efficiency for UK homes, particularly those with solid wall construction. Commissioned by the Department for Energy Security and Net Zero, the Demonstration of Energy Efficiency Potential (DEEP)[A1] report used advanced testing on a model Victorian terrace home to compare two retrofit approaches: the traditional piecemeal method and a holistic “whole house” approach. Researchers aimed to identify which retrofit strategies would best enhance airtightness, insulation, and resistance to moisture-related issues.
The DEEP study was motivated by the need to address heat loss in older homes, which often have high carbon emissions and draughts due to uninsulated walls and floors. With 20 million homes in the UK potentially needing energy efficiency improvements, this study’s findings highlight a clear path toward achieving energy savings, reduced emissions, and greater occupant comfort. For ATTMA members, who are actively engaged in airtightness and building performance, the DEEP report offers important insights into which elements of retrofit most effectively improve overall building airtightness.
Comparing Retrofit Strategies: Piecemeal vs. Whole House Approaches
Using the Salford Energy House, a controlled environment simulating a Victorian end-terrace, researchers tested common upgrades individually, such as loft insulation and window replacements, as well as a full-scale whole house retrofit. The piecemeal approach mimicked standard practice, focusing on one element at a time. In contrast, the whole house approach involved a coordinated sequence of upgrades to deliver a complete thermal barrier. Results from each method were assessed based on their impact on heat transfer coefficients (HTC) and air permeability (AP50). The study found the whole house approach significantly outperformed the piecemeal method in terms of cohesive energy savings and improved indoor air quality.
Ground Floor Airtightness: A Key Component of Thermal Performance
A critical finding was the specific impact of air sealing the ground floor compared to other parts of the house. In unsealed homes, floors with suspended timber structures often serve as major air entry points, letting in draughts that counteract other insulation measures. By sealing the floor, the researchers saw far greater efficiency gains than through traditional measures like loft or window sealing alone. This improvement in the building’s thermal envelope allowed heating systems to work more effectively, leading to lower energy consumption and greater comfort for occupants.
Paul Carling, ATTMA Technical Manager explains, “Sealing at a lower level reduces one of the two main causes of actual air leakage – stack effect. Stack effect is the movement of warm air upwards, which is replaced by cold air seeping in through gaps and cracks in the building fabric. If you cut the supply of cold air, you reduce the heat loss in other areas”.
Mr Carling goes on to say “with the sealing of windows, doors and other vertical components, you will see the benefit more when an external condition is applied, such as high winds, as wind against the side of a house creates a high pressure, in turn creating a low pressure and moving air through the vertical elements”.
Excerpts from The ATTMA Air Tightness Testing Level 1 Course delivered by The Building Performance Hub in High Wycombe
For ATTMA members, these findings suggest prioritising floor sealing in retrofit plans for older properties. Addressing these vulnerable areas provides a more robust barrier against air leakage, a factor that could significantly shift current retrofit practices toward a more whole-building approach.
Role of External Wall Insulation in Reducing Thermal Bridging
The study also highlighted the role of external wall insulation (EWI) in strengthening airtightness and reducing thermal bridging. EWI, especially effective in the whole house approach, accounted for a 51 percent reduction in HTC, particularly at thermal junctions where walls, floors, and roofs meet. In untreated homes, these junctions can become cold spots that lead to condensation and mould. The report found that the comprehensive whole house approach allowed for optimal treatment of these areas, reducing condensation and minimising health risks related to mould.
Practical Considerations and the Case for Whole House Retrofits
While the whole house approach delivered better results, the DEEP report acknowledged the cost challenges associated with it. The study found whole house retrofits were 61 percent more expensive than the piecemeal method, mainly due to the cost of materials and labour. This added expense, however, comes with significant benefits in long-term energy savings, maintenance, and occupant health. For properties where energy bill savings or occupant health benefits can offset this investment, whole house retrofits present a highly effective option for sustainable building performance.
Implications for the Industry and Next Steps
The DEEP report’s findings make a compelling case for the whole house approach as the new standard in energy-efficient retrofitting. The cohesive energy savings, increased airtightness, and improved indoor air quality this approach offers align closely with the UK’s 2050 net-zero targets. It is clear from this study that future building standards should evolve to prioritise airtightness and insulation across the entire building envelope.
For ATTMA members and other professionals in the building industry, the DEEP report underscores the need to address every component of a home’s thermal envelope to optimise energy efficiency. By treating each building element as part of an interconnected system, the construction industry can pave the way for a more energy-efficient, comfortable, and sustainable housing stock.
In summary, the DEEP study suggests that a well-planned, holistic approach to retrofitting will be essential for the UK’s journey towards net-zero housing. The insights from Salford’s Energy House Labs remind us that each element of a building contributes to its overall performance, and only by treating these elements as a unified system can we maximise energy efficiency and comfort in UK homes.
[A1]Link to https://www.gov.uk/government/publications/demonstration-of-energy-efficiency-potential-deep
