Reflections on Energy Efficiency
This article originally appeared in the August 2018 edition of RCI magazine
The governments recently announced “Clean Growth Grand Challenge” aims to halve the energy use of new buildings by 2030. New building construction needs to explore cost-effective ways of meeting this target and the standards laid out in Building Regulations Approved Document L. Is sustainable timber frame construction, using advanced wall membranes and energy-efficient cladding, the future?
In July, Business Secretary Greg Clark presented “Reimagining the buildings of the future” at the Northern Powerhouse Business Summit. “Our next challenge is to make the buildings we live and work in more efficient. Buildings account for around 30% of total emissions and around 40% of final energy consumption in the UK. We want to lead the world in designing and building safe, smart, energy efficient, affordable homes and commercial buildings.”
The stated goals are to make sure that every new building in Britain is safe, high quality, much more efficient and uses clean heating; using innovation to make low energy, low carbon buildings cheaper to build; driving lower carbon, lower cost and higher quality construction through innovative techniques. This raises the bar for the construction industry, especially in the domestic sector: deliver better, cheaper, more energy-efficient homes – while maintaining commercial and economic viability.
Changes in 2016 to Buildings Regulations Approved Document L (Conservation of Fuel and Power) included revised recommended U values for both new dwellings and conversions. U values (measured in W/m2K) express the thermal conductivity of building materials and installations; the lower the value, the more energy efficient the building. Approved Document L states that new buildings should have a U value of 0.28 W/m2K for walls, 0.16 W/m2K for pitched roofs with insulation at ceiling level and 0.18 W/m2K for flat roofs and for pitched roofs with insulation at rafter level.
Timber frame construction is growing in popularity in the UK, particularly for dwellings: it offers a reduced carbon footprint, is faster to construct and energy efficient. Since the 1990s, the market share of timber frame construction has grown from 6 per cent to 28 per cent in 2018. It’s anticipated that there will be a further five per cent growth by 2020. Timber frame construction also lends itself to the use of efficient, cost-effective pre-built modules, completed offsite. Timber frame homes can be clad with traditional materials such as brick, stone, render and tiles, or more unusually (for the UK) timber, fibre cement, composite weatherboards or even metal.
Although current timber frame construction offers sustainable, energy efficient results, the drive to improve the thermal performance of new dwellings means exploring the use of new and alternative materials and techniques. Performance can be improved by the use of deeper solid timber studs (with the industry moving from 90mm studs for external walls to 140mm) and additional layers of insulation (in roofs and walls). The passive house movement for ultra-efficient, low energy buildings with a reduced ecological footprint, outlines the principle insulating the entire building envelope to reduce the typical 70 per cent loss of heat through roofs and walls. Air tight construction, without any weak spots, results in “thermal bridge free design”: it is effective thermal insulation, not heat storage, that makes for energy efficiency.
The ideal solution for improving the air tightness of a building is an air and vapour control layer, applied to the warm side of the insulation, fully taped and sealed, to help achieve the thermal bridge free goal. Increasing the use of AVCLs in new dwellings would have a significant impact on achieving the goals of the Clean Growth Challenge. Tiny gaps or cracks in plasterboard, gaps between insulating panels, unsealed overlaps in a vapour control layer - all will create a thermal bridge, allowing heat to transfer. This can reduce the overall U value of the installation and the building substantially. As well as improving the air tightness and reducing convective heat loss, vapour control layers also perform the vital function of protecting the insulation from moisture from inside the building and prevent water vapour from passing through to cause interstitial condensation.
Protecting insulation and the building envelope from external elements is equally critical. To achieve this, a breather membrane is typically applied to the external, cold side of the insulation. These wall breather membranes protect the building envelope from external moisture while allowing water vapour from the interior to pass through. On the Continent, it is not uncommon to tape wall breathers to seal joins, further improving their efficiency. Most wall breathers are single layer, non-woven fabrics that perform their function – water resistance and vapour transmission – but their inherent U values do not contribute significantly to the overall thermal efficiency of the building.
One innovation, which is a relatively cost-effective way of lowering a building’s overall U value, is to improve the U values of these two key membranes: vapour control layers and wall breathers. This can be achieved by incorporating a thermo-reflective foil layer in both. These heat-reflecting materials are becoming more common, and offer improved thermal efficiency over traditional, unfaced membranes.
A thermo-reflective wall breather membrane is in fact a thermal-resistant material, its foil layer reducing the transmission of heat both from the exterior and interior. Similarly, a VCL with an integrated foil, thermal-resistant layer facing inwards, installed on the warm side of the insulation reduces the amount of heat transferring from the inside of the building to the wall. When both traditional unfaced wall breathers and VCLs are replaced by thermal resistant versions in a wall installation, there can be a significant overall reduction in the U value. Many factors, including the type of insulation used (mineral roll, PIR, mineral batt), can affect the resulting U value for the installation, but using both a thermo-reflective wall breather and a foil-faced vapour control layer can reduce the U value by up to 25% compared to a similar installation with standard (non-foil-faced) membranes.
All dwellings sold, and some new conversions or extensions, must pass the Standard Assessment Procedure (SAP) - the methodology used by the government to assess and compare the energy and environmental performance of dwellings. The calculations determine, among other criteria, how well the fabric of the dwelling contains heat and the predicted CO2 emissions. In many cases, an installation must exceed the standards set out in Approved Document L to achieve a “pass” in the SAP calculations. Upgrading building membranes to those with thermal resistant foil layers is a cost-effective, efficient way to improve the U rating of new-build timber frame dwellings.
By Paul Harrison, Building Materials Technical Specialist at Industrial Textiles & Plastics
Industrial Textiles & Plastics manufacture an extensive range of Powerlon building membranes, including UltraBlock thermo-reflective vapour control layers and ThermaPerm, a thermal resistant wall breather.