Heat transfer considerations - why insulation specification is not just about the U-value

Mandatory minimum insulation levels were first introduced for new dwellings in the 1976 Building Regulations as a result of the need to conserve energy following the 1973 oil crisis.

The 1984 Building Regulations saw the streamlining of many local regulations into a single piece of legislation. The purpose of the regulations was to secure the health, safety, welfare and convenience of people in or around buildings and further the conservation of fuel and power. The latter led to the creation of Approved Document L, or Part L as it is often called. It had a simple elemental approach to compliance giving U-value targets for each element.

This elemental approach changed in 2006 in response to the Energy Performance of Buildings Directive (EPBD) from Europe. A whole building approach to calculate energy and carbon emission performance (SAP for dwellings and SBEM for non-domestic buildings) was introduced and EPCs (Energy Performance Certificates) were created from the software. The EPBD recognised that not just low U-values drive lower energy use and lower carbon emissions, factors such as the airtightness and efficiency of the heating system also had a part to play. This supported the EU’s aim to promote energy efficiency and decarbonise the building stock by 2050.

In 2006, the UK government introduced a policy that all new homes should be net zero carbon by 2016. Changes to Part L in 2010 and 2013 saw significant lowering of U-values to help achieve a 25% and 6% decrease in carbon emissions from dwellings. Eventually, in 2015, this policy was scrapped but the Part L changes remained in place.

In 2021 Part L changed again to facilitate a further reduction of 30% in CO₂emissions and to pave the way for the upcoming Future Homes Standard (FHS). Renewable technologies such as Solar PV and heat pumps were introduced to reduce carbon emissions. U-values were lowered slightly for the roof (0.11 from 0.13 W/m2.K) but the rest (walls 0.18 and floors 0.13 W/m2.K) remained unchanged.

This brings us up to the present day where a new Part L 2026 edition has been released with a change in its title from ‘The conservation of fuel and power’ to ‘Energy and greenhouse gas emissions.’ This new version of Part L is the update to the Building Regulations that is required to mandate the changes required by the FHS that will come into force in 2027. It creates buildings that only use electricity and renewable technology. This will allow them to become net zero carbon when the grid decarbonises. U-values are unchanged when using the interim version of SAP but notional U-values for upcoming Home Energy Model (HEM) calculation methodology will be tighter. Lower U-values, resulting in reduced heat loss through the building fabric, create the optimal conditions for the efficient operation of heat pumps, which deliver lower temperature heat compared with traditional gas boilers.

So, from that skip through history, U-values have proved a valuable mechanism to:

• Promote energy efficiency in an energy crisis in the 1970s
• Facilitate carbon emissions reductions in addition to energy efficiency in the 2010s due to the rising awareness of climate change
• Develop a building fabric with low heat transfer to support net zero carbon targets, enabling efficient use of renewable heat pump technology

The last point shows that we appear to have gone full circle as, just like in the 1970s, security of energy is a key issue for the government. Low U-values can contribute to reducing our reliance on fossil fuels and boost our national energy security.

So, the ‘U’ in U-values could be thought of as ‘Underlying’, reflecting their role as a fundamental measure that supports many aspects of building performance. However, the specification of insulation should not just be about U-values, as we will now explore.

There are several factors that insulation can influence other than only focusing its ability to deliver the U-value specification:

As discussed, when reviewing the history of U-values, 2006 saw the introduction of a calculation methodology for assessing a building’s energy efficiency and carbon emissions. This recognised that factors such as air permeability and thermal bridging also influence overall energy performance.

These elements can also have a significant impact on the internal environment of a home. Thermal bridging can create cold spots where condensation may form. In addition, buildings with high levels of airtightness require an effective ventilation strategy to remove stale and moisture-laden air, otherwise conditions can arise that encourage damp and mould growth.

Moisture management should therefore be a key specification consideration. As part of this, it is important to assess how the chosen insulation interacts with the building’s overall moisture management strategy.

Insulation can either be diffusion-open where it allows moisture to pass through or diffusion-closed when it doesn’t. Both types can be used as long as appropriate membranes are used to manage moisture to ensure that no water gets trapped within the structure of the building fabric. This is known as interstitial condensation, and it can cause long-term damage to the building fabric.

In buildings that have been designed with materials that also allow moisture to pass through them, often referred to as a ‘breathable’ construction, such as solid walls, specifiers will often consider using a diffusion-open insulation rather than focusing purely on U-value requirements. A diffusion-open insulation, such as wood fibre, will work in harmony with the materials to allow moisture to flow freely through the element. Any moisture vapour that enters the wall can diffuse safely through the materials, including the diffusion-open insulation, to the outside.  

The effects of climate change are increasing the risks of overheating. In the Committee for Climate Change report^[1], it states that under current UK weather conditions, 50% of existing UK homes will suffer from overheating risk. If global temperatures were to rise by 2°C, this number would rise to 90%.

Approved Document O^[2] of the building regulations was introduced in 2022 to specifically reduce the risk of overheating in new build developments. Whilst there are a number of ways to address overheating, passive measures are preferred to active ones such as air conditioning as the latter use energy and require maintenance and repair.

Insulation can help to mitigate the effects of overheating and certain types of insulation are more effective than others. For example, wood fibre insulation combines high density, high heat storage capacity, and low thermal conductivity to help reduce the risk of summer overheating. These properties enable the insulation to absorb and retain heat during the day, slowing its transfer into internal spaces of the home, so peak solar heat does not immediately affect indoor temperatures. The insulation acts as a passive way to help to mitigate the effects of overheating as well as helping to retain heat inside the house when it is required during the colder months.

The expected U-value performance of an element can be undermined by gaps in the insulation. These gaps create what is known as thermal bypass – where air is allowed to move around or behind insulation or between gaps left between insulation boards, or between insulation and the element into which it has been fitted such as rafters or timber studs. A report by the Passivhaus Trust^[3] looks at the effects of an air gap on the warm side of the insulation:

• 2mm gap             =             12% increase in heat loss
• 7.5mm gap         =             203% increase in heat loss
• 15mm gap          =             520% increase in heat loss

Selecting insulation that is best suited to its intended application can help minimise gaps and improve overall performance. Wood fibre insulation is available in both flexible batts and rigid boards. The flexible batts can more easily fill spaces between timbers, reducing the risk of gaps and voids compared with cutting rigid boards to fit precisely.

Where insulation is installed over or under rafters or timber framed walls, the tongue-and-groove design of wood fibre boards helps create a continuous layer, reducing the potential for thermal bypass and improving the effectiveness of the insulation.

In the same report it also poses an interesting question about the effect that specifying lower U-values can have with respect to the effects of thermal bypass. As the U-value of an element gets lower, the proportion of heat loss attributed to thermal bypass increases ‘dramatically’.

Although the report states this is pending further research, the magnitude of the heat loss caused by gaps shows the importance of specifying insulation that is in the best physical form to provide continuity of insulation.

Where a construction project has a focus on sustainability the contribution of products to embodied and operational carbon can come under scrutiny. The wood fibre constituent of wood fibre insulation binds a carbon dioxide equivalent within the finished insulation board or batt^[4]. This is reflected within their EPDs and can benefit projects looking for ways to reduce their carbon footprint. In some sustainability projects, the source of the materials can also feature as a potential selection criterion and those materials such as wood fibre insulation using wood from PEFC sources can help specifiers to meet that requirement.

STEICO has a comprehensive range of products suitable for use in walls, floors and roofs in new and existing buildings. These include rigid wood fibre insulation boards and flexible wood fibre batts as well as a range of render, sarking and sheathing boards.

They are all made from at least 80% wood sourced from PEFC (PEFC/04-31-2550) certified forestry and have characteristics that can help specifiers deliver more than just U-value targets as Martin Twamley, Technical Director at STEICO, explains, “The focus on achieving low U-values is understandable given that improving thermal performance of the building fabric is the primary reason for installing insulation. However, the positive impact of choosing the right insulation type, on other aspects of the building should not be overlooked.  Wood fibre insulation not only helps to keep occupants warmer in winter, but it also assists with the mitigation of overheating in the summer. Its diffusion-open structure gives specifiers the ability to match the moisture management profile of buildings designed with a ‘breathable’ structure and being derived from a natural material it can also appeal to projects where material sourcing is a specification criterion.  Wood fibre insulation is able to tick many more boxes than just ‘delivers low U-value’ and our customers appreciate its contribution to other key performance aspects of their projects.”

To discuss how STEICO products can support with improving heat transfer within your next building project and a whole lot more besides, get in contact with our expert team who will be happy to help.