Back to basics: Thermal insulation in buildings
Polyfoam XPS’s factory in Hartlepool has been the site of extruded polystyrene (XPS) manufacturing for more than forty years. Over that time, the business has built up a wealth of knowledge about insulation production (much of which informed the commissioning of our new production line) and building construction.
Among Polyfoam’s workforce are people who have been there since day one, and who have forgotten more about extruding polystyrene than many of us will ever know. We pride ourselves on using that knowledge to talk about the product we manufacture and how it can be used.
Sometimes, however, it’s possible to be too close to a subject. There are readers happy to learn about the historic use of HFCs in UK-made XPS, or the compressive creep performance of floor insulation boards. But there are also readers who want a more general introduction to thermal insulation and its use in buildings. That’s where this blog post comes in.
What are building regulations?
National building regulations set a minimum standard that must be achieved to safeguard the health and welfare of people in and around buildings. They also set out requirements for conserving water and, most relevant to this blog post, energy.
The way in which the conservation of energy is demonstrated varies slightly by the regulations of each country of the UK. Broadly, however, a proposed building must demonstrate that it meets or betters defined limits for carbon dioxide emissions and energy consumption. The following regulations and supporting guidance apply.
- Part L of Schedule 1 in England, supported by Approved Document L Volumes 1 and 2.
- Part L of Schedule 1 in Wales, also supported by Approved Document L Volumes 1 and 2.
- Section 6 of the Domestic and Non-domestic Technical Handbooks 2022 in Scotland.
- Part F 2022 in Northern Ireland, supported by Technical Booklets F1 and F2.
The calculation software used to establish whole-building performance takes into account many and various aspects of building design, including heat loss through elements of the building fabric (floors, walls, and pitched roofs and flat roofs, as well as glazed elements such as windows and rooflights).
How is building fabric heat loss measured?
Thermal transmittance is the rate of movement of heat energy through building fabric elements. It is expressed in terms of the transfer of heat energy in Watts (W) per square metre (m2), per degree of temperature difference between inside and outside (K, for degrees Kelvin).
This gives the units W/m2K, and is more usually called a U-value.
U-values can be established by laboratory testing (which is expensive and time consuming) or on-site measurement (which requires the building to have been built). Mathematical calculation methods are therefore most commonly used for design and specification purposes.
Thermal modelling can be used for all forms of construction, including thermal bridging details, and is best suited to complex construction forms.
A simplified form of calculation, known as the combined method, can be used for building elements that feature defined layers of consistent thickness and thermal properties. This describes most common construction types. The combined method is defined in the international standard BS EN ISO 6946.
How are U-values calculated using the combined method?
Within a combined method U-value calculation, material layers are built up to reflect the composition of the building element as designed and/or constructed.
A typical material layer comprises the thickness and the thermal conductivity (or lambda value) of the construction product. Thickness is divided by thermal conductivity to give a thermal resistance for each layer. The thermal resistances of the complete construction are then totalled, and a reciprocal taken to work out the U-value.
Additional calculations are made where a layer is bridged by a material of different conductivity.
BS EN ISO 6946 details a number of corrections that can be applied to a U-value to reflect how the element will be constructed. These corrections make the end result more accurate and a better reflection of how the building element will perform in service, over the life of the building.
In many construction elements thermal insulation makes up the bulk of the thermal resistance, and therefore makes the greatest contribution to the final U-value. Thermal insulation materials are essential to meeting the energy efficiency requirements of national building regulations while retaining relatively thin, buildable constructions.
In the UK, BR 443 supports construction professionals working with U-values by describing conventions that should be followed when calculating them. These conventions give more detail about material data that should be used, and how to apply the calculation method as described in BS EN ISO 6946 to different construction types.
What U-value target do I need to meet for my construction project?
As touched on above, compliance with energy efficiency/conservation of power regulations is assessed via a whole-building calculation. This is true for both domestic and non-domestic new-build projects.
Alongside U-values, the calculations take into account other aspects of building fabric performance including airtightness and thermal bridging heat losses. They also factor in heating and hot water systems, as well as building form and orientation.
Altering any of these factors individually impacts on the overall calculation and can result in a change to the building fabric specification. In recognition of this, current building regulations provide example specifications which, if followed, are assured to be compliant.
The U-values that form part of these ‘notional’ specifications can be used as a rough guide to understanding the likely insulation requirements for an individual project. However, project-specific U-values should always be established as part of the overall building specification, and U-value calculations carried out accordingly.
What to do next
Energy efficiency regulations in the UK are in something of a state of flux. For example, at the time of writing, Part L 2021 in England has only been in force for a few years but is expected to be replaced soon by the Future Homes Standard. To ensure you have the most up to date information, we recommend referring to current Approved Documents (or equivalent).
You can also contact our technical services team for advice about your project, and take advantage of the technical support resources on our website. We provide specification advice and free U-value calculations, carried out using the combined method and following the conventions of BR 443.
Did you find this blog post useful? What other topics could we go ‘back to basics’ on? Let us know using our contact page, or by following our page on LinkedIn.
April 2026 note: this blog post is based on content written for the first edition of The Polyfoam Orange Book of XPS Insulation, our comprehensive guide to extruded polystyrene (XPS) published in March 2026. Written for designers, specifiers and contractors, you can learn more about the book, including how to request a copy, here.