Reducing heat transfer through the building envelope remains a key objective for both government policy and the construction industry. However, if factors such as moisture control, thermal bridging, airtightness and overheating risk are not considered alongside insulation performance, the quality of the indoor environment and the long-term durability of the building can be compromised.
A more balanced approach to building design is therefore essential. Materials such as wood fibre insulation can help deliver effective thermal performance while also supporting moisture management, occupant comfort, and the overall health of the building fabric. Rather than focusing solely on heat loss reduction, the industry must increasingly consider how different elements of building performance interact with one another.
What other factors need to be considered alongside U-values when specifying insulation?
There are several factors that insulation can influence other than only focusing its ability to deliver the U-value specification.
Moisture Management and insulation
Air permeability and thermal bridging can influence the overall energy performance 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.
In buildings that have been designed with materials that also allow moisture to pass through them, specifiers will often consider using a diffusion-open insulation, such as wood fibre, rather than focusing purely on U-value requirements. The wood fibre will work in harmony with the materials to allow moisture to flow freely through the element.
Diffusion-closed insulation can also be used, but both types of insulation need to use appropriate membranes to manage moisture.
Insulation choice and overheating
Approved Document O of the building regulations was introduced in 2022 to specifically reduce the risk of overheating in new build developments, caused by climate change.
Insulation can help to mitigate the effects of overheating. 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.
Insulation installation and thermal bypass
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.
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. STEICO supply a range of these products that can support improved heat transfer. 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.
Insulation for projects with a sustainability focus
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. 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.