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Light Steel Framing Systems (LSFS)

• Introduction
• Technical issues - Durability
• Technical issues - Water, vapour, and air barriers
• Technical issues - Specifications
• Technical issues - Ancillary products
• Contractual issues
• Manufacturers
• Reference sources

Introduction

• In recent years there has been a significant increase in the use of light steel framing systems (LSFS) to provide the external wall structure of buildings. In general these systems have replaced concrete blockwork, as either the main structure of the wall or as the non-structural infill to a steel or concrete structural frame.
• Light steel framing systems are popular with contractors because of the cost advantages, and the speed of erection, compared with the use of concrete blockwork. The satisfactory use of LSFS systems requires that both specifier and contractor address a number of technical and contractual issues which may not be fully understood by those unfamiliar with these systems.
• For an in-depth review of the particular concerns and solutions when LSFS systems are used in conjunction with External Wall Insulation Systems, refer to the LSFS and EWIS page.

Technical issues - Durability

• The SCI publication "Durability of Light Steel Framing in Residential Construction", referred to below, is the most comprehensive, published, UK research into the durability of LSFS systems. It is significant primarily for:
• the confirmation that the G275 coating (275 gm/m²), commonly used as the protective coating for the metal studs in LSFS systems, is "satisfactory for most internal building applications" despite being thinner than the coatings specified in BS EN ISO 1461 (the standard for "post fabrication" galvanized coating on iron and steel products).
• the importance attached to the use of "warm frame" construction.
• The recommendation to use only "warm frame" construction is supported by:
• the SCI/INCA publication "Insulated Render Systems Used With Light Steel Framing", referred to in the reference sources on the LSFS and EWIS page.
• the Accredited Details for steel framing systems (which cover brick-clad systems only).

Technical issues - Water, vapour, and air barriers

• Whichever of the two forms of construction is selected, it is important to consider the design and detailing implications of achieving continuity of the water, vapour, and air barriers, remembering that the vapour control layer will be in different positions within the overall depth of the wall depending on which construction option is chosen.
• Where vapour is controlled by the use of a sheet material it is sensible to combine the vapour control layer and air seal along this line. The workmanship necessary to achieve adequate sealing for vapour control will also, generally, be adequate for the air seal.

"Warm frame" construction (all insulation external to LSFS)

This is the preferred form of construction, supported by the Steel Construction Institute (SCI), the Insulated Cladding and Render Association (INCA), and Accredited Construction Details (ACDs).

• It is generally desirable to include a sheathing board on the external face of the LSFS framing in all systems although, historically, it has been common practice to omit external sheathing boards when the LSFS system is clad with brick veneer or rainscreen cladding systems.
• The advantage of an external sheathing board is that it provides a firm base for the insulation and a rigid surface for air-sealing.

Vapour control layer/Air seal in "warm frame" construction

• The preferred location for the vapour control layer (VCL)/air seal is on the external face of the sheathing boards (internal face of the insulation).
• Air-sealing can be achieved by taping the joints between the boards, or by over-laying the boards with a polyethylene, or similar, sheet vapour control layer.
• Note:
• The VCL/air seal is protected from internal works, such as installing services within the stud depth.
• The VCL/air seal cannot be inspected, or easily replaced, in the future.

"Hybrid" construction (some insulation within LSFS)

This form of construction, although not specifically "banned", is not recommended by the Steel Construction Institute (SCI). Those intending to specify this form of construction should exercise extreme care and note, particularly, the statements highlighted at the end of this section.

• It is now difficult to meet the current thermal performance requirements by relying solely on insulation external to the frame ("warm frame") without a significant, and often unacceptable, increase in the overall wall thickness. For systems using polystyrene or cellular foam board insulation there will also be an increased cost as these materials are, in general, more expensive than mineral fibre slabs or batts. Specifiers will often be under pressure to reduce costs (and wall thickness) by minimising the external insulation and maximising the insulation within the stud depth.
• There may also be situations where it is necessary to include acoustic insulation within the stud depth to prevent flanking noise transmission where internal partition walls abut the external wall.

Vapour control layer/Air seal in "hybrid" construction

• The preferred location for the vapour control layer (VCL)/air seal is on the internal face of the LSFS framing system (stud-side face of the internal plasterboard lining).
• The standard industry response to providing a VCL/air seal in this location is to use plasterboard with an integral vapour control layer ("foil-backed" plasterboard) for the internal lining. These products are, unfortunately, notoriously unreliable for vapour control as the backing is (a) easily damaged and, (b) totally removed when holes are cut into the plasterboard (at recessed electrical sockets, for example).
• A proprietary sheet vapour control membrane is preferable to the use of foil-backed plasterboard.
• Note:
• The VCL/air seal is vulnerable to damage from internal works, such as installing services within the stud depth.
• The VCL/air seal can be inspected (by removing the plasterboard lining), and repaired or replaced as necessary, without disturbing the weather-sealing of the overall wall construction.
• In general, ensuring the continuity of the VCL at the interfaces with windows, doors, and other penetrating elements is more complicated than when the VCL is external to the framing (as for "warm frame" construction). All the details in Section 5 of the the SCI/INCA publication "Insulated Render Systems Used With Light Steel Framing", referred to in the reference sources on the LSFS and EWIS page, assume that the VCL is on the inside face of the external insulation.
• When considering the use of "hybrid" construction it is essential to ensure that condensation does not form on the metal studs. A simple rule is that at least two-thirds of the insulation should be placed externally to the frame, but condensation analysis of the specific wall construction is essential.
• When considering the use of "hybrid" construction it is essential that specifiers obtain a written statement from the manufacturer of the chosen Light Steel Framing System that their products are not damaged by being placed on the "cold" side of the vapour control layer.

Technical issues - Specifications

• The specification of these systems must be carefully considered as the complete wall construction is rarely carried-out by the same sub-contractor. A scan through the typical wall constructions below will give some idea of the potential complexity of specifying these systems.
• Who-does-what will need to be considered on a project-by-project basis as there is no accepted "industry standard" and different main contractors will wish to package the external wall construction in different ways.
• The light steel framing system itself should generally be specified using a bespoke, project-specific, performance-based specification. This applies even when all other elements of the wall, e.g. the cladding and linings, are specified prescriptively. There is currently no standard NBS work section specifically for light steel framing systems but Section G10 is the best starting-point.
• Irrespective of who-does-what, it is inadvisable to try and combine the elements into a reduced number of (NBS) specification sections. You should specify each element of the construction in the correct section to ensure that the correct workmanship clauses are included.

• The three examples below are all for "warm frame" construction.

Brick veneer

• Single skin external brickwork - Specify in Section F10.
• Cavity - Typically 50mm.
• Wall tie system - Specify in Section F30. Refer to Brickwork/Blockwork Fixing Accessories.
• Insulation - Specify in Section F30.
• Vapour control layer - See below for products and specification.
• External sheathing board - Optional but desirable. See below for products and specification.
• Steel framing system - Specify in Section G10, see above.
• Insulation, within stud depth - Undesirable, see above. Specify in Section P10 if essential.
• Internal lining board(s) - Specify in Section K10.

Rainscreen cladding (Metal panel,  terracotta tiles, and similar)

• Rainscreen cladding system - Specify in Section H92.
• Cavity - Typically 50mm.
• Insulation - Specify as part of the cladding system in Section H92.
• Vapour control layer - Specify as part of the cladding system in Section H92.
• External sheathing board - Optional but desirable. See below for products and specification.
• Steel framing system - Specify in Section G10, see above.
• Insulation, within stud depth - Undesirable, see above. Specify in Section P10 if essential.
• Internal lining board(s) - Specify in Section K10.

Rendered finish with external insulation

• This is Option 2A from the System options on the LSFS and EWIS page.
• Insulation with rendered finish - Specify in Section M21. Read External Wall Insulation Systems.
• Drained cavity - Typically 15mm.
• Vapour control layer - If required, see below for products and specification.
• External sheathing board - See below for products and specification.
• Steel framing system - Specify in Section G10, see above.
• Insulation, within stud depth - Undesirable, see above. Specify in Section P10 if essential.
• Internal lining board(s) - Specify in Section K10.

Technical issues - Ancillary products

• Breather membrane:
• Use : As the weatherproof external face of the insulation in "hybrid" construction.
• Product : Typically a proprietary sheet, often multi-layer polypropylene, which is watertight but with a very low vapour resistance. BS 5250 defines, in clause 3.2, a breather membrane as a "membrane with a vapour resistance of less than or equal to 0.6 MN s/g".
• Specification : Specify in Section P10 (NBS clause P10/320).
• Carrier board:
• Use : As the background for the render finish in "rainscreen" EWIS construction.
• Product : Typically cement-bonded particleboard to BS EN 633, BS EN 634-1 and BS EN 634-2, minimum 8/10mm thick, but often thicker for structural reasons.
• Specification : Specify in Section M21. (NBS Section M21 does not include a clause specifically for "rainscreen-type" construction. Specifiers may wish to consider writing a suitable clause, based on NBS clauses M21/210, 220, or 230, and include the carrier board in the new clause.)
• External sheathing board:
• Use : As the external sheathing to the LSFS in "warm frame" construction.
• Product : Typically cement-bonded particleboard to BS EN 633, BS EN 634-1 and BS EN 634-2, minimum 8/10mm thick, but often thicker for structural or acoustic reasons.
• Specification : Specify in Section K11 (NBS clause K11/485).
• External (breathable) sheathing board:
• Use : As the external sheathing to the LSFS in "hybrid" construction.
• Product : Typically an exterior quality timber-based board such as plywood or oriented strand board (OSB), generally 10mm to 15mm thick, although there are some proprietary board products available for this purpose.
• Specification : Specify in Section K11 (NBS clauses K11/415, 435, 465, or 485).
• Vapour control layer (VCL):
• Use : On the internal face of the insulation material, as a combined vapour control layer and air barrier.
• Product : Typically a proprietary sheet product which is watertight with a high vapour resistance (typically in excess of 250 MN s/g), e.g. 500 gauge (0.12mm) polyethylene sheet (200 to 350 MN s/g according to BS 5250, Table C.2).
• Specification : Specify in Section P10 (NBS clause P10/315).

Contractual issues

Design responsibility

• Issue : How to limit the designer's liability when most of the constituent elements are prescriptively specified (with the possible exception of the dead/live/wind load criteria for the steel framing system), the sub-contractors have little or no design responsibility for their own elements, and no one (except the architect) has any design responsibility for the complete wall construction. Steel framing systems are currently being "pushed" by many design-and-build, and PFI, contractors who appear oblivious of the additional design risk that these systems place on the architect.
• Solution : Single-point-responsibility for complete wall construction with one specialist sub-contractor. Although this might seem to be the obvious solution, very few companies offer this service and the few that do often exclude the internal linings. In addition, one of the reasons why these systems are favoured by main contractors is precisely because the systems do not tie them to a single sub-contractor for the complete wall (giving the main contractor greater cost and management flexibility).

Continuity of water, vapour, and air barriers

• Issue : How to achieve a satisfactory standard of workmanship, and hence performance, through the whole wall construction when so many different trades are involved.
• Solution (1) : Use "warm frame" construction and specify an external sheathing board for all forms of construction (including brick veneer). Include the board in the light steel framing system "package", and make the light steel framing system sub-contractor responsible for the continuity of the air and vapour barriers at the line of the sheathing board. The sub-contractor(s) responsible for installing the doors and windows can then be made responsible for sealing these components to the surrounding wall construction. This solution will not be appropriate with "hybrid" construction.
• Solution (2) : Clients and contractors should also be made aware that the increasingly onerous requirements of Part L of the Building Regulations, including the requirement for air pressure testing, mean that the responsibilities for ensuring the adequacy of air and vapour barriers should be defined in the contract documents. This alone might be sufficient to tip-the-balance in favour of the single-point-responsibility solution as described above.

Manufacturers

• Kingspan Off-Site : www.kingspanoffsite.com : Kingspan supply a variety of systems, from fully-panellised walls installed by approved contractors down to steel frame components intended for onsite erection. Their website is now less-informative than it used to be, maybe reflecting the complexity of the different systems on offer, but Kingspan are one of the few companies offering the "complete wall" based on SFS systems.
• Light Steel Frame Solutions : www.lsfs.co.uk : LSFS provide a complete design-and-installation service for the steel framing system and some of the associated elements, e.g. insulation. The Technical Manual, which requires a (free) online registration prior to downloading, provides excellent generic details which are clearly annotated to show which elements are provided by LSFS, which are optionally provided by LSFS, and which are not. These are an excellent pointer to what can be considered as "normal" industry practice and illustrate the complexity of these systems.
• Metsec SFS : www.metsec.co.uk : The Technical Handbook/Specification Guide (downloadable from the website) is probably the best simple introduction to SFS-type systems. There are some simple details (Page 15), and examples of typical constructions (Page 44), together with clear diagrams about the assembly of the steel framing system. Metsec only supply their system components for others to install, they do not provide a design-and-installation service.

Reference sources

In addition to the reference sources on the LSFS and EWIS page:

The Steel Construction Institute

Durability of Light Steel Framing in Residential Construction. 2000.

• SCI Publication P262. A summary of in-depth research on the durability of galvanized (zinc coated) cold formed steel sections used in housing. It costs £35, is available direct from the Steel Construction Institute at http://shop.steelbiz.org/, and includes the following:

(Summary) The performance of galvanized (zinc coated) steel components within warm frame applications is very good. This research shows that the predicted design life of the standard G275 coating, based on the measured loss of zinc from the strip steel, is over 200 years, provided that the building envelope is properly maintained.

(1.1 Light steel framing in housing) Most light steel framing systems have been assessed by the British Board of Agrément (BBA), based on a rigorous testing regime. In these BBA approvals, the frames are required to remain dry and reasonably airtight in the so-called warm frame construction envelope in all reasonable circumstances during the life of the building.

(1.1.1 Warm frame construction) ... In other cases, insulation may be placed between the wall studs, provided that there is sufficient insulation outside the studs to avoid cold bridging and therefore to avoid condensation on the studs. However, insulation to external walls positioned solely within the depth of the studs will not prevent interstitial condensation from forming on the stud members themselves.

(2.1 The hot dip galvanizing process) In the UK, the working standard has been 275 gm/m² (i.e. a surface thickness of about 20 microns). ... The coatings are thinner than those formerly specified in BS 729 [now BS EN ISO 1461] because research shows that the corrosion resistance is satisfactory for most internal building applications.

(3.8.1 Conclusions from case studies - Warm frame applications) The monitoring studies have shown that the environmental conditions present in warm frame construction are such that moisture levels are very low and that the galvanized steel components are not subject to a risk of significant corrosion within the expected life of well maintained modern buildings.

(4.2 Measures to control water penetration) Provide a water resistant barrier on the exterior of the wall studs.

(4.3 Control of condensation) In light steel frame construction, a warm frame is achieved by positioning the insulation on the external face of the studs. Additional insulation may be located between the studs, but care must be taken to ensure that the calculated position of the dew point lies outside the zone of the studs. ... A simple rule is that at least two-thirds of the insulation should be placed externally to the frame (i.e. to preserve the warm frame).

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