Specifying insulation to meet a low-energy agenda is increasingly complex. Barbour Index and Scott Brownrigg look at calculations, tests and safeguards to help save the planet

Insulation is a key part of meeting the increasingly demanding low-energy agenda, but it is becoming progressively tougher to specify. The familiar measure of overall thermal performance, the U-value, is becoming outdated as construction requirements become more demanding, and is being replaced by other parameters. How well insulation performs over its life is crucial if buildings are to maintain good thermal performance. In addition, using materials with good green credentials in the manufacture of insulation products is also increasingly important. To successfully specify low energy buildings, or even buildings that can be claim to be zero-carbon entities, a much more comprehensive and detailed level of requirement is needed. As performance is so critical these days, it may be worth specifying thermal imaging tests and vapour control layer tests when the building is completed to ensure that workmanship is up to the standard required.

Units of thermal measurement

When specifying materials, it is important to consider the thermal properties of the materials in question. The ubiquitous U-value is a measure of thermal conductance of all materials in a construction element, such as a wall. An alternative is to work out the thermal performance of individual materials in a construction element, and add them together to determine overall thermal performance. The thermal resistance, or “r” value, should be identified and specified as the performance value. The r value is calculated by taking the thermal conductivity of a material, or its l (lambda) value, and its thickness, then dividing the thickness by the l value. The results for individual materials can be added together to arrive at a performance value for the overall construction element. l values are readily available as manufacturers are required to declare them when the material is new.

Look for the “90/90 value” to ensure consistent performance: this value indicates that 90% of the tested samples have a value that falls within 90% of the quoted value. These values are only approximations of actual thermal performance as they are all based on “steady state” criteria, and buildings are rarely in this condition. The specifier should think about how the building will actually be used and how this relates to the calculated values. FENSA, the Fenestration Self-Assessment Scheme, is trying to cater for this in its window rating system, which measures the energy flow across the unit, and provides a rating system based on relative performance.

Lifecycle performance

Determining lifecycle performance can be surprisingly complex. The energy required to produce insulation materials must be considered along with its likely life and the energy saved by a product during its life. Determining the life of materials is difficult as the available data is rather vague. Materials will also degrade over time, so the specifier should ask for l and r values several years after the production date. These values are a useful specification parameter, but in practice difficult to obtain. There is a fall-off, particularly with foamed materials, and what may seem very good when new may not be good several years after completion. Materials can be offered with a guarantee, but this may not reflect the materials’ actual life. Good quality data can be found from some of the more established manufacturers.

Vapour control

Substantial quantities of insulation will inevitably be used in any low-energy situation. It is therefore important to ensure adequate vapour control is established to prevent the formation of condensation. Review the energy ebb and flow through the fabric, as this can help or hinder the development of condensation depending on the amount of water vapour present. Use a good quality continuous and well-sealed vapour control layer – these are specified by their vapour resistance. Separate vapour control layers are always better than attempting to seal components together, such as in roof liner trays.


Good quality insulation materials may be the cornerstone of sustainable construction, but don’t overlook the details of the materials used to manufacture them in the first place. Insulating materials may claim to be free of ozone-depleting CFC and HCFC agents, but question what has replaced them. The specifier may need to check the chemical make-up of the material, its production process, the route to site, any cleaning or maintenance regime and the ability to recycle the material at the end of its life. Each should be tabulated with values and comments to draw up a schedule for the overall sustainability factor of a specific project. The good news is that clients are increasingly asking for proof of the sustainable nature of materials used in construction. There is some evidence that manufacturers are also waking up to this demand, particularly in the insulation industry.


Recycling is currently difficult for foam insulants, but less problematic for mineral fibre materials. Recycled plastics are now used to make some types of insulation and are becoming increasingly common, particularly in bubble form. Typically, 5-9% of the contents of foam products on the market are manufactured from recycled resin. Fibreglass insulation is made from 25-30% recycled glass, and mineral wool can consist of up to 70% recycled material. Cellulose alternatives are also available in in the form of reused paper or wool, although each comes with drawbacks. Cellulose suffers from condensation and can attract mould. Moreover, it needs to be used with fire retardants. Wool has also been found to suffer from infestations, and so needs to be treated and sealed. Manufacturers’ information on these products is useful and worth reading prior to formulating the specification. But read these with care, as they only portray one point of view.

Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification Expert services. For further information, contact Barbour Index on 01344-899280 or visit www.barbour-index.co.uk