Home Features FACADES: A thermal bridge too far

Market pressures and increasingly onerous regulatory burdens are driving the demand for carefully detailed solutions to ensure facades deliver an improved thermal and fire performance. Andy Pearson reports.

Plastering Contractors Stanmore - 21 Wapping Lane 2In the new build facade market the pressure to deliver cost-effective thermal insulation solutions has increased the focus on design details and, in particular, solutions that help prevent the occurrence of thermal bridges. “Careful consideration needs to be given to preventing thermal bridges when drawing up a system specification to ensure that poor detailing does not result in the need for additional energy efficiency measures, such as renewables,” explained Andrew Orriss, head of business development at SIG Distribution.

This increased focus on thermal bridging can be directly attributed to buildings becoming better insulated. In the past a thermal bridge may have been considered insignificant in the context of a relatively poorly insulated building. Now, with increased levels of insulation, that same thermal bridge will be the cause of a significant proportion of a building’s total heat loss.

A thermal bridge, or cold bridge, occurs where the insulating layer in a building’s fabric is breached creating a thermal link between the inside and outside of the building envelope. Breaches are usually caused by gaps in the insulation or at the joints between different facade elements, or even around window and door openings. The heat loss associated with a thermal bridge has the rather grandiose term of linear thermal transmittance or psi-value. “The regulations require psi-values to be dealt with properly,” added Mr Orriss.

The government publishes Accredited Construction Details (ACDs) to assist the construction industry to minimise cold bridging and achieve the performance standards required to demonstrate compliance with Part L (Conservation of Fuel and Power) of the Building Regulations. However, because ACDs are standardised solutions they are not always the most cost effective for a particular application.

Mr Orriss says that design services offered by insulation providers like SIG Insulation can help by advising customers on the best options to ensure they select the most cost-effective Building Regulations compliant system for a particular application. “The SIG360 technical advice service is like a value engineering service – they get into the detailing so that customers don’t have to.”

Thermal bridges are also an issue in the Steel Framed System (SFS) sector. Knauf has developed its ThermaFrame facade system to help reduce thermal bridging through the system’s lightweight, cold-rolled galvanised steel sections. The system features a clever stud design which greatly reduces thermal bridging through the steel studs by offsetting rows of slots formed within the web; this simple off-set design helps increase the length of the heat path, thereby reducing the thermal bridge.

In addition to reduced heat transfer through the studs, the system also has the advantage of providing improved levels of thermal insulation without sacrificing usable space. This is because in conventional SFS systems insulation is often applied externally, which can add to the thickness of walls and decrease space internally. Knauf’s ThermaFrame facade system avoids this problem by placing insulation within the thickness of the steel frame, effectively sandwiching it between the gypsum-based sheathing boards and the internal plasterboard lining to provide a U-value of between 0.32 and 0.15W/m2K depending on the depth of system used.

In addition to an impressive thermal performance, the system also has health and safety benefits because it reduces the amount of time operatives have to work outside at height. Once the sheathing boards are attached to the steel frame so that the building is weathertight, operatives can then install the insulation from inside the building.

Another detailing issue currently occupying facade contractors and specifiers is that of ensuring a cladding system’s compliance with the fire regulations. The focus on cladding is the consequence of a fire in a 14-storey block of flats in Scotland in 1999, where cladding on the outside of the block was suspected of contributing to the fire’s severity. Following the incident, the Building Regulations were reviewed and as a consequence attention has been increasingly focused on the external spread of fire, particularly on housing projects where facades extend above 18m in height.

“Although this is not a new standard or regulation of late we have found increasing levels of scrutiny and discussion concerning compliance with BR 135 and BS 8414 parts 1&2 legislation,” said John Taylor, technical manager at Euroform Products. It is an issue with which Kate Hawkins, head of design at contractor Stanmore, agrees. “Although document B2 of the Building Regulations has not been revised, the interpretation and insistence of compliance has now been enforced,” she added.

BR 135 sets out the principles and design methodologies to minimise the spread of fire through non-loadbearing cladding systems, particularly those over 18m in height. “The documents and subsequent test standards of BS 8414 are fairly complex but are clearly essential in ensuring that buildings are protected from the risk of fire spread through the external facade system where external wall insulation is used,” explained Mr Taylor.

Where a BS 8414 fire test is commissioned it is the system’s performance and not simply the performance of the individual components that will prove a cladding system’s compliance. “As a supplier of sheathing boards that are used in external wall insulation systems, we have had significant involvement in test work and specifications in collaboration with external wall insulation manufacturers,” he added. The work has been vital in ensuring that various EWI systems applied to buildings over 18m in height comply with the regulations and, where necessary, tests. “Our contribution to test design and general development work has been to specify high-performing sheathing boards to work in collaboration with various thermal insulants in the EWI package,” Mr Taylor concluded.

The increased scrutiny on a cladding system’s fire performance is being felt by contractors, who also have to ensure a cladding system has sufficient thermal insulation to comply with Part L of the Building Regulations.

One option is to have BRE carry out a desk top assessment of a system’s compliance with the BR 135 fire test. “Although we are now utilising this method it is not without its limitations  because the testing conducted by insulation manufacturers is often based on specific wall constructions and insulation thicknesses,” commented Stanmore’s Ms Hawkins. “At present we are working closely with all interested parties to review current and upcoming projects to ensure we can provide the most cost  effective and compliant external wall solution.”

It seems that market pressures and increasingly onerous regulatory burdens will continue to drive up the demand for carefully detailed facade solutions for some time yet.