A lintel contribution to fabric first

Catnic’s Richard Price offers a philosophy of designing and constructing homes where individual components and systems contribute to, rather than compromise, the overall building performance.

‘Fabric first’ remains the optimum way for housebuilders and developers to create homes that are liveable while meeting increasingly stringent energy performance legislation. Renewable technologies such as solar PV and ground source heat pumps are ‘nice to have’ but for many housebuilders and developers their cost, in both design and practical implementation, still precludes them from many specifications.

However, a fabric first approach, one where the building does the work without relying on more expensive ‘bolt-on’ technologies such as these, can ensure new homes meet the highest demands of the current Fabric Energy Efficiency Standard, Part L and SAP compliance.

Achieving this requires careful specification. It is an equation in which the sum of the parts equals the final outcome: a truly energy efficient home that is structurally sound and both cost effective and logical in its construction. Within this approach, no one component or system will compromise another.

Such an equation is a balancing act, between high performance components and systems and the potential thermal bridges they create. A key example is the steel lintel.

We cannot achieve structurally sound homes, cost effectively, without them. Indeed the steel lintel has become the byword for architectural achievement in the modern construction agenda. However the legislative landscape of increasingly demanding energy efficiency standards, combined with the sophisticated measurement of energy wastage through the building envelope, has highlighted a need for improvements in the psi values of lintels. It has prompted the introduction of a new type of lintel on the marketplace, one that is designed with a thermal break.

While steel lintels do provide a means to build homes quickly, safely and effectively, they also have the potential to directly convey heat from the inside to the outside of a building. In other words, without careful and knowledgeable specification, steel lintels have the potential to create thermal bridges that may compromise the energy performance and therefore the fabric first approach of a new building.

In fact, any material, component or system that forms a bridge linking two structural features – such as roofs and walls, or walls and floors, together – stands to risk the unwanted transfer and dissipation of heat.

The need for energy efficiency improvements has given rise to more sophisticated methods of measuring heat loss. Heat lost through the building fabric, for example through walls, roofs, floors and doors and windows, called U-values, is measured in W/m2K. Psi values measure heat lost at the junctions between these elements, for example at window heads, window jambs, window cills, corners, walls and floors as well as the walls and roof of a building at its eaves, and they are measured in W/mK. The total fabric heat loss of the structure is calculated as the sum of the U-value multiplied by the area plus the sum of the psi value multiplied by the length of the building.

For housebuilders and developers this requires close collaboration with manufacturers of systems and components. Working together they can fully understand individual performances and how building products might be designed together, to ensure the outcome embraces a fabric first philosophy efficiently and effectively.

A trusted lintel manufacturer can provide free lintel scheduling and technical support to housebuilders and developers to ensure the optimum lintel specifications are in place. Of course, all lintels from a reputable manufacturer can be used and will certainly comply with the Building Regulations. However, the psi value of all lintels specified must be taken into account when it comes to calculating heat loss and obviously, the lower the overall psi value, the better the resulting total fabric heat loss of the building will be.

Housebuilders and developers can derive lintel psi values from a number of sources. It is possible to work from the default values taken from BRE IP 1/06 or ACDs, these are the ones found in SAP calculators and really they present the worst case scenario, so are best avoided. Alternatively, it can be worthwhile looking to the lintel psi values that the insulation and block manufacturers include with their typical thermal details. However, by far the best option is always to use the psi values provided by the lintel manufacturer themselves.

Further to this it is possible to achieve improved lintel psi values by using lintels without a base plate (removing the potential for additional thermal bridging) and thermally broken lintels within cavity wall constructions. A lintel that incorporates a complete thermal break between the inner and outer leaf of a cavity wall will deliver the best possible thermal performance.

With a flat top providing a consistent junction with the cavity wall insulation, the reliance on site workmanship is removed and ensures the actual performance of the junction meets the design expectations. The specification of a lintel like this has added cost, time and logistical advantages because it can be installed in the same way as a traditional steel cavity wall lintel onsite.

The replacement of traditional lintels within typical housing designs can result in significant heat loss reductions for housing developments. By utilising thermally broken lintels, which offer the same benefits of a traditional cavity wall lintel, yet deliver industry leading psi values of 0.02 to 0.05 W/mK – in turn means that steel lintels can support a Fabric First approach to building design, with no compromise to energy performance.

Richard Price is technical director at Catnic