What it costs: Thermal insulation

  • Date: 18/07/2008

Building Magazine: Insulation is an effective way to help meet Building Regulations targets on carbon emission reduction. Peter Mayer of Building LifePlans examines the effectiveness of different materials

Since October 2006 all new housing in England and Wales has needed to meet the energy requirements in Part L1A of the Building Regulations. Certain components and elements must achieve a minimum thermal or energy efficiency but it is the complete building that is assessed as being compliant, based its carbon dioxide emission rate.

Housing compliance is determined by the Standard Assessment Procedure (SAP). This defines the size and location of the planned building and then determines the “target CO2 emission rate” (TER), based on the 2002 Building Regulations. The new building then has to be designed to reduce this by 20% – this is known as the “dwelling CO2 emission rate” (DER).

The government predicts that the extra cost of achieving the DER value would vary from £450 for a 65m2 two-bedroom mid-floor flat to just under £1,000 for a 140m2 four-bedroom detached house.

Designers have some flexibility in achieving the DER – increasing thermal insulation levels is one the ways it can be done.

Thermal insulation levels

The limiting area-weighted average U-values are 0.35W/m2K for walls, 0.25W/m2K for roofs, 0.25W/m2K for floors and 2.2W/m2K for windows. These can be lowered to achieve compliance, provide a more energy-efficient dwelling and reduce carbon emissions and heating bills.

There are many materials with different thermal conductivities that can be used for insulation. The lower the thermal conductivity the better the U-value and each insulant can be designed to achieve a specific value – it is just a matter of specifying adequate thickness. The service lives for insulants are expected to be more than 60 years, when correctly specified, designed and installed.


  • Plant and animal-based insulants such as sheep’s wool and cellulose fibre These have thermal conductivity in the range of 0.037–0.040W/m2K. The specification issues are similar to those for mineral fibre. 
  • Cellular plastics These are the most thermally efficient of the common insulants. Phenolic and polyisocyanurate foams have the lowest thermal conductivity: 0.023W/m2K. Rigid polyurethane, extruded and expanded polystyrene have higher values, in the range of 0.032–0.040W/m2K. The thermal conductivity of cellular plastics reduces over time, owing to gas exchange. This can be limited by gas impermeable facing and should be allowed for in the thermal rating. Joints should be close fitting or taped and filled to minimise air movement and gaps.
  • Mineral fibre, stone or glass wool These have a thermal conductivity of about 0.033–0.040W/m2K. When using these materials it is is important to maintain a dry environment during construction to protect them from rain and minimise risk of condensation. Thermal performance is reduced if moisture comes in contact with this type of insulation. Air movement can be limited by specifying mineral fibre insulation with a facing. Mineral fibre used for flooring or flat roofs should have adequate compressive resistance for the expected traffic and loads to avoid compression.

Designs should minimise thermal bridging. Accredited details are available from www.planningportal.gov.uk. If accredited details are not used, thermal bridges should be assessed to BRE information Paper IP1/06: Assessing the Effects of Thermal Bridging. Account should be taken of thermal bridges formed by fixings and key construction details such as corners, lintels and junctions.


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