What is Through-Wall Design?

Assessing the suitability of a building system requires consideration of characteristics such as quality, versatility, sustainability and technical support, often meaning experience and judgement is needed to arrive at an informed decision.

Where performance is concerned, however, the specifier will be able to assess a structural system’s ability to meet requirements for loadbearing capacity, acoustics, thermal efficiency and, importantly, fire resistance, by using reliable published information on the system and materials used in its composition.

‘Through-wall’ performance

The Finishes and Interiors Sector (FIS) offers three approaches to the ‘through-wall’ concept in its Best Practice Guide, ‘Through-wall infill steel-framed systems (SFS)’. These are:-

• Option 1 – through-wall covers the zone from the inner surface of the plasterboard lining to the outer surface of the sheathing board.
• Option 2 – through-wall covers the zone from the inner surface of the plasterboard lining to the outer surface of the insulation on the face of the sheathing board.
• Option 3 – through-wall covers the zone from the inner surface of the plasterboard lining to the outer surface of the façade system.

Clearly, a specifier needs to be certain which approach has been adopted when referring to performance data for a particular building solution.

Where SFS is concerned, ‘through-wall’ is widely regarded as the part of the wall that provides the majority of the fire, thermal and acoustic properties, most commonly the zone from the inside face of the internal dry lining to the outside face of either the sheathing board or insulation, i.e. Option 1 or Option 2 from the FIS definitions.

The reason for this is that, with so many exterior façade materials available, it would be impossible to test every possible wall construction. This through-wall approach to performance allows SFS manufacturers to test a meaningful number of material combinations (SFS, internal and external boards plus insulation). The tested solutions will either be from the dry lining through to the sheathing board or from the dry lining to external insulation.

This approach provides the design team with the freedom to choose the desired external façade, provided, of course, that the necessary fire performance requirements are met.

Building Regulations guidance documents, such as Approved Document B, set out required fire resistance time periods, particularly with regard to integrity (passage of flame and hot gases), insulation (temperature on the unexposed side of the wall) and loadbearing capacity where the wall is being used in a structural capacity.

Through-wall performance data

In order to achieve through-wall fire performance data, SFS manufacturers construct different complete wall build-ups, commonly measuring 3m x 3m, comprising light gauge steel frames fitted with various combinations of boards and insulation materials for testing at UKAS-accredited laboratories.

Each build-up is tested using a large scale, high heat test (reaching up to 600 degrees Centigrade in 5 minutes), which measures the performance of the whole wall by simulating fire trying to break out through a solid wall. Tests are conducted to British Standards BS EN 1364-1 to achieve the 120-minute, 60-minute and 30-minute fire ratings typically required by building regulations.

In most cases, fire performance is an inside to out requirement, aiming to prevent the spread of fire from its source to adjacent compartments. However, where buildings are in close proximity to each other or if the wall performs a loadbearing function, fire resistance will also be required from outside to in.

Through-Wall Design: The Metsec approach

Metsec’s approach to through-wall fire testing is to invest in independent laboratory tests on complete wall build-ups using our own SFS and Metframe systems in conjunction with dry lining, insulation and sheathing from the industry’s leading building materials manufacturers.

The aim is to provide the design team with a comprehensive database of performance data which will cover the vast majority of requirements. The approach also affords the design team a choice of solutions for cost, materials and availability as well as thermal and acoustic performance. As mentioned, by not relying on the external finishes for performance achievement we are providing the design team total freedom of choice in external façade materials.

Our database of more than 400 different test wall build-up configurations include internal boards from British Gypsum, Knauf and Siniat, with external boards from British Gypsum, Euroform, Klasse, Knauf, Obex, RCM and Siniat. The data can be easily accessed in a number of ways, including via our new online specification manual (which can be viewed here) or the NBS platform, where the wall build-ups are grouped by sheathing board and dry lining products.

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