Don’t forget your thermals

Karen Everitt of Ravatherm UK explains the importance of correct insulation specification in achieving high thermal performance.

Housebuilding is on the rise. Research suggests that by 2050 there could be a 23 per cent increase in UK households, and the Government has established a target of 300,000 new homes per year across the country to meet this demand.

Despite the requirement to increase housebuilding output, achieving carbon savings in buildings will be key to hitting the UK’s legally binding target from the 2008 Climate Change Act of an 80 per cent reduction in carbon emissions by 2050. The Committee on Climate Change (CCC) states that 18 per cent of UK carbon emissions come from buildings – most of them homes – with a further 15 per cent of emissions coming from electricity consumed in buildings.

It is therefore important for specifiers and housing developers to know the key considerations when specifying insulation, and how to achieve high thermal performance for low-energy homes.

Sustainable homes

At one time, ‘eco’ homes were often perceived as being against the norm, but with ever-tightening Building Regulations and mounting pressure to reduce carbon emissions, all newly-constructed dwellings must now consume much less energy in comparison with standard homes built as recently as five years ago.

In England alone, the change from Building Regulation Part L 2010 to Part L 2013 was intended to deliver a six per cent reduction in carbon dioxide emissions across all new dwellings through more thermally efficient building fabric. Building a sustainable home can include ‘bolt-on’ renewable technology – but first and foremost it is about achieving high quality fabric construction that will last for the life of the building, without ever losing its efficiency.

A low-energy home means careful and deliberate specification of building materials that will deliver the required performance – and thermal insulation has a significant role to play.

Extruded polystyrene

Meeting Building Regulations on paper can be very different from accurately achieving performance on site. Extruded polystyrene (XPS) is an increasingly popular insulation option for specifiers seeking confidence in a material that can be used in demanding conditions and applications, and still deliver good thermal performance. XPS insulation is not only thermally-efficient, easy to install and lightweight, but moisture tolerant too.

With moisture absorption as low as 0.6 per cent by volume, XPS products can be installed in conditions where moisture is present, such as below the damp proof course (DPC) level, or against the ground and to the external side of tanking membranes – all with negligible impact on the performance of the product.

Sustainable construction means more than the performance of the finished building; it also means the responsible manufacture of building materials. XPS solutions – manufactured by a reputable company – should be independently assessed under the Building Research Establishment (BRE) Green Guide to Specification, and should be able to demonstrate and achieve a certified Green Guide rating of at least A, as well as zero ozone depletion potential (ODP) and low global warming potential (GWP).

Housebuilders and developers should also look for XPS insulation products that have been manufactured in a factory with an ISO 14001 environmental manage- ment system, which accord with BES 6001 sustainable sourcing, offer Environmental Product Declarations (EPDs), and are 100 per cent recyclable at the end of the building’s lifespan.


Considerations for achieving sustainable, low-energy homes do not stop at the design stage. The position and use of insulation is vitally important to ensure correct thermal performance.

Properly insulated domestic floors are an integral part of high performance building fabric and help to increase the overall comfort for the end user signifi- cantly, including through using the heat storage capacity of concrete slabs and screeds where appropriate.

Ground-bearing floors can include insulation either below or above the concrete slab – the latter being ideal for intermittent heating, where a homeowner may only turn on the heating twice a day. The XPS insulation should be installed with a vapour control layer over it.
Ideally, the insulation should be positioned above the damp proof membrane, although it is not critical given XPS’ tolerance to moisture.

U-Value calculations

Heat loss through ground floors is unique to each property, being dependent on the size and shape of the floor, as well as the soil type. Thermal transmittance (U-value) calculations should follow the methodology in BS EN ISO 13370, which uses the ratio of the exposed floor perimeter to the floor area because more heat is lost at the edges of a floor than in the centre.

The measurement of the perimeter and area should be to the finished inside surfaces of the perimeter walls enclosing the heated space. Unheated spaces such as porches or garages should be excluded. Extensions to existing buildings can use the floor dimensions of the new build area only, or the complete building including the extension.

The same standard specifies how U- values should be calculated for basement walls and floors, taking into account the same criteria, while also factoring in the depth of the structure below ground. The calculation of heat loss through a basement wall also considers the insulation specifica- tion of the basement floor, so it makes sense to consider any basement design holistically.

Basement design

Basements are seeing a resurgence in residential developments. This is particularly true of refurbishment projects, as a result of limited available space to construct above-ground extensions. They can however cause problems if not properly designed and built. Insulation plays a key role in basement design, and there are a few factors to consider during specification. Some materials will only be suitable for use on the internal side of the tanking membrane, to avoid exposing them to the ground and moisture. XPS insulation with a compressive strength of 200 or 300 kPa (Kilopascals) can be installed outside of the basement structure and tanking, providing a continuous thermal envelope for the below-ground structure and resisting the loads imposed by the ground.

At the high end of the market, XPS is also ideal for use around swimming pool basins, resisting the loads imposed by the volume of water, and hitting the U-value targets specified in Building Regulations.


With tightening regulations and higher energy efficiency targets to meet in the coming years, it’s more important than ever to liaise with a trusted XPS insulation manufacturer which will offer a technical and specification advice, ensuring the building performs as per the specification.

Karen Everitt is marketing manager at Ravatherm UK