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LSFS and EWIS

• Introduction
• Water, vapour, and air barriers
• System options
• Reference sources

Introduction

• It is important to read the relevant pages on Light Steel Framing Systems (LSFS) and External Wall Insulation Systems (EWIS), to understand how these systems work in isolation, before reading this page about the specific issues to consider when the two systems are used together.
• Light Steel Framing Systems clad externally with brickwork, and External Wall Insulation Systems applied to a concrete or masonry background, are both well-understood forms of construction which have been used successfully in the UK for many years.
• In recent years there has been a significant increase in the use of External Wall Insulation Systems as the external finish to walls constructed using Light Steel Framing Systems. Towards the end of 2006 there were rumours of problems, in Canada and New Zealand, with this type of construction. These concerns led the CWCT to devote the entire technical content of its April 2008 Members' Meeting to the subject.
• The alleged problems relate primarily to conditions where the render has cracked badly enough to allow water to penetrate as far as the steel frame causing corrosion and, ultimately, structural failure. This has accentuated the existing concerns of some UK architects about the difficulty of ensuring continuity of the vapour control layer, and/or air barrier, when the construction process and the contractual responsibilities are so fragmented.
• The Steel Construction Institute (SCI) publication Insulated Render Systems Used With Light Steel Framing, referred to below, is essential reading for anyone specifying and detailing these systems when used in combination.
• As far as can be determined, there are no external wall insulation systems currently (November 2008) available in the UK with a BBA Agrément Certificate for use with steel framing systems. All the current Agrément Certificates refer to the use of the systems on a masonry or concrete background only.

Water, vapour, and air barriers

• Read the equivalent section on the Light Steel Framing Systems page, to understand the general issues, before reading this page about the specific issues to consider when LSFS systems and EWIS systems are used together.
• The Steel Construction Institute (SCI) publication referred to below, includes the following statement:

"The scope of the guidance given in this publication is limited to the use of light steel framing in "warm-frame" or similar designs, where the structural framework is not subject to condensation ... The recommended details and guidance assume sufficient insulation is present on the outside of the frame to ensure that a dew point does not occur behind the outer surface of the external insulation ... This ensures that condensation does not occur internally or on the light steel framework."

• The design of most external wall insulation systems assumes that the outer render coat provides a single barrier defence against water penetration. It is advisable to consider providing a continuous second-line-of-defence, within the system, to prevent water which has penetrated the outer render coat from reaching the inside of the building. This can be provided by the vapour control layer where this is external to the SFS system as in Options 1A, 2A, and 3A below, or by the breather membrane on the external face of the sheathing board in Options 1B, 2B, and 3B below.
• There are a variety of render types, see the website of the Insulated Cladding and Render Association (INCA) - www.inca-ltd.org.uk. Render systems for use with steel framing systems should have a higher vapour-permeability, and better crack-resistance, than might be the case when specifying render systems on a masonry background, i.e. silicone-based renders may be preferable to acrylic-based renders. It is also important to ensure that the detailing around doors, windows, and other penetrations does not allow water to penetrate into the framing zone.
• There is a general view, among those concerned about the potential for problems when LSFS and EWIS systems are combined, that incorporating a drained cavity within the system is beneficial. This is an NHBC requirement, and an SCI recommendation for severe and exposed locations.
• All the details in Section 5 of the SCI publication referred to below assume that the vapour control layer is on the inside face of the external insulation - as in the quotation at the very top of this section.

System options

The table below lists six system options, based on three different fixing systems, each with two insulation options. It is worth noting that only Option 3A complies with the requirements of HTM 05-02 (for healthcare buildings), NHBC (for residential buildings), SCI (representing the steel frame industry), and Zurich (a major insurer of school buildings). All other options require a compromise on one or more of the variables.

The links in the left-hand-column go to the Ancillary products section of the Light Steel Framing Systems page.

1 : Insulation adhesive-, or mechanically-, fixed without external cavity
OPTION 1A - "warm frame" construction Render finish (5mm). Insulation (typically 125mm to 200mm). Vapour control layer (VCL). External sheathing board. SFS stud system (typically 100mm to 200mm). Two layers of 12.5mm plasterboard (25mm). Typical thickness: 250mm to 450+mm.	Compliance BBA : NO : No Agrement Certificate. HTM/Zurich : YES : Insulation choice. NHBC : NO : No drained cavity. SCI : YES : 'Warm Frame'. Construction comments All insulation is external to the LSFS frame. VCL is "buried" within construction. Maximises overall wall thickness. Poor acoustic separation (room-to-room).
OPTION 1B - "hybrid" construction Render finish (5mm). External insulation (typically 50mm). Breather membrane. External (breathable) sheathing board. SFS stud system (typically 100mm to 200mm). Internal insulation (within SFS studs). Vapour control layer (VCL). Two layers of 12.5mm plasterboard (25mm). Typical thickness: 200mm to 300+mm.	Compliance BBA : NO : No Agrement Certificate. HTM/Zurich : YES : Insulation choice. NHBC : NO : No drained cavity. SCI : PARTIAL : Condensation analysis required. Construction comments Some insulation is within the LSFS frame. VCL is accessible, although with difficulty, for future inspection and maintenance. Minimises overall wall thickness. Improved acoustic separation (room-to-room).
2 : Insulation mechanically-fixed with drained, unventilated, cavity
OPTION 2A - "warm frame" construction Render finish (5mm). Insulation (typically 125mm to 200mm). Drained cavity (typically 15mm). Vapour control layer (VCL). External sheathing board. SFS stud system (typically 100mm to 200mm). Two layers of 12.5mm plasterboard (25mm). Typical thickness: 250mm to 450+mm.	Compliance BBA : NO : No Agrement Certificate. HTM/Zurich : NO : Foam insulation only. NHBC : YES : Drained cavity. SCI : YES : 'Warm Frame'. Construction comments All insulation is external to the LSFS frame. VCL is "buried" within construction. Maximises overall wall thickness. Poor acoustic separation (room-to-room).
OPTION 2B - "hybrid" construction Render finish (5mm). External insulation (typically 50mm). Drained cavity (typically 15mm). Breather membrane. External (breathable) sheathing board. SFS stud system (typically 100mm to 200mm). Internal insulation (within SFS studs). Vapour control layer (VCL). Two layers of 12.5mm plasterboard (25mm). Typical thickness: 200mm to 300+mm.	Compliance BBA : NO : No Agrement Certificate. HTM/Zurich : NO : Foam insulation (ext). NHBC : YES : Drained cavity. SCI : PARTIAL : Condensation analysis required. Construction comments Some insulation is within the LSFS frame. VCL is accessible, although with difficulty, for future inspection and maintenance. Minimises overall wall thickness. Improved acoustic separation (room-to-room).
3 : Rainscreen system
OPTION 3A - "warm frame" construction Render finish (5mm). Carrier board. Ventilated cavity (typically 40mm). Insulation (typically 125mm to 200mm). Vapour control layer (VCL). External sheathing board. SFS stud system (typically 100mm to 200mm). Two layers of 12.5mm plasterboard (25mm). Typical thickness: 300mm to 500+mm.	Compliance BBA : NO : No Agrement Certificate. HTM/Zurich : YES : Insulation choice. NHBC : YES : Drained cavity. SCI : YES : 'Warm Frame'. Construction comments All insulation is external to the LSFS frame. VCL is "buried" within construction. Maximises overall wall thickness. Poor acoustic separation (room-to-room).
OPTION 3B - "hybrid" construction Render finish (5mm). Carrier board. Ventilated cavity (typically 40mm). External insulation (typically 50mm). Breather membrane. External (breathable) sheathing board. SFS stud system (typically 100mm to 200mm). Internal insulation (within SFS studs). Vapour control layer (VCL). Two layers of 12.5mm plasterboard (25mm). Typical thickness: 250mm to 350+mm.	Compliance BBA : NO : No Agrement Certificate. HTM/Zurich : YES : Insulation choice. NHBC : YES : Drained cavity. SCI : PARTIAL : Condensation analysis required. Construction comments Some insulation is within the LSFS frame. VCL is accessible, although with difficulty, for future inspection and maintenance. Minimises overall wall thickness. Improved acoustic separation (room-to-room).

HTM/Zurich compliance required

• Preferred option : 3A.
  Why? The only option which includes non-combustible insulation, a drained cavity, and a 'warm' frame.
• Acceptable options (in descending order) : 1A, 1B, 3B.
  Why? Option 1A maintains a 'warm frame'; Options 1B and 3B require condensation analysis.
• Unacceptable options : 2A, 2B.
  Why not? All currently-available systems utilise a combustible foam board which is unacceptable to HTM/Zurich.

NHBC compliance required

• Preferred options : 2A, 3A.
  Why? Both systems incorporate a drained cavity and maintain a 'warm frame'.
• Acceptable options : 2B, 3B.
  Why? Both options require condensation analysis.
• Unacceptable options : 1A, 1B.
  Why not? NHBC require a drained cavity within the system.

Reference sources

Centre for Window and Cladding Technology

TN 59 - Introduction to externally rendered cladding systems. 2008.
TN 60 - Performance of externally rendered cladding systems. 2008.

• These two Technical Notes, intended to be read together, provide some useful general information which is not covered by the SCI/INCA publication referred to below. They are both available, to CWCT members, as free electronic downloads from the Cladding Forum section of the CWCT website at www.cwct.org.uk. They are also available to Non-Members at £20 per copy.

The Steel Construction Institute

Insulated Render Systems Used With Light Steel Framing. 2006.

• By far the best publication on the subject and essential reading for anyone detailing and specifying these systems. It costs £35 and is available direct from the Steel Construction Institute at http://shop.steelbiz.org/ or INCA at www.inca-ltd.org.uk.

© 2008. ArchiMentor.