BUILDING DESIGN USING COLD FORMED
STEEL SECTIONS: ACOUSTIC INSULATION

External steel cladding (*general guidance only)

The tables accompanying this section provide a comprehensive overview of acoustic insulation test data relating to internal floors and walls, together with general guidance on the acoustic insulation provided by various types of steel intensive external walls.

As noted in Section 1, it is not considered practical, within this publication, to provide comprehensive guidance on the acoustic performance of proprietary external wall systems. Where such performance is an issue, advice should be sought directly from manufacturers.

The data presented in relation to external walls is intended to be su0icient to support basic scheme design calculations. Typical sound reduction figures are cited for each of the major generic types of profiled steel cladding system. It should he noted however that in many instances the dominant means of sound transfer will be through windows or other openings.

Similarly, sound reduction figures are provided for a typical brick clad external steel framed wall. In addition to the effects of windows etc., the overall performance of such a wall is influenced by a range of factors, including the density of the brickwork. Very high levels of sound reduction can be achieved by including additional layers of material within the construction, and where this is an issue advice should be sought from an acoustics specialist.

4.2 Materials used in steel framed and steel intensive construction

Cold formed sections are manufactured from cold reduced coil steel. The most common sections are the C or the Sigma shape, although other sections are used for special applications. Steel thicknesses range from 0.5 mm to 3.2 mm. Corrosion protection is normally provided by galvanising.

Cold formed sections can be used in various applications.

Stick built systems where the structural frame is assembled on site from individual members. Prefabricated panel systems where wall and floor panels are brought to the site ready assembled and bolted into position. Volumetric systems where buildings are designed and delivered to site as a series of prefabricated modules which are assembled in situ.

Built-up systems comprising an inner liner sheet and external profiled sheet, with insulation and spacer sections between. Built-up systems comprising an inner structural liner tray and external profiled sheet fixed to the returned edges of the tray, with insulation between. Composite panels comprising inner and outer steel sheets bonded to either side of an insulating foam core.

4.3 Reference guide: floor and wall types

4.3.1 How to use this guide

Tables 5 and 6 suggest particular floor and wall constructions for a range of building types. Of necessity this is general guidance since actual selection will depend upon the particular construction requirements of individual buildings, and will take account of structural and fire considerations.

• a) Within Activity, i.e. where rooms are not particularly sensitive to noise generated in adjacent spaces, such as within a single dwelling.
  
• b) Between Activities, i.e. where rooms are sensitive to noise generated in adjacent spaces, such as between hotel rooms or classrooms.
  
• c) Between Occupancies, i.e. where high levels of sound insulation are required between individual buildings, ownerships or tenancies.
  

The tables accompanying Figures 4.1 to 4.18 present typical sound insulation values for each of the generic types of construction covered by this publication. Sound insulation values are generally given in terms of Rw and Ln,w however, for completeness, certain American STC ratings have also been included. All of these values apply to the wall or floor without the influence of the surrounding structure.

It will be found that most regulations are expressed in the form of DnT,w and L'nT,w and STC equivalents, which are products of the particular room geometry and acoustic design of the building. For most domestic scale buildings with normal amounts of furnishing these requirements will approximate Rw, Ln,w and STC values, provided that there is not a significant alternative route for sound to travel via the rest of the structure i.e. no significant flanking transmission. A method of converting Rw to DnT,w is given in Appendix A.2.6.

Flanking transmission is unlikely to present a major problem in most framed structures (see Section 2.2). However, where the very highest levels of sound insulation are sought, such as when using generic wall types G2 and G3, special consideration may be necessary and advice should be sought from an acoustics specialist.

It is recommended that the information presented in the tables be used as guidance only, and that wherever possible actual sound insulation test evidence should be sought from nominated suppliers.

KEY

(A) Basic internal floors

(B) Internal floors / compartment floors with enhanced acoustic insulation

(C) Internal floors / compartment floors with composite steel and concrete construction

(D) Basic internal partitions

(E/F) Internal partitions / compartment walls with enhanced acoustic insulation

(G) Internal staggered stud partitions / compartment walls

(H) Internal partitions / compartment walls on double independent frameworks

(J) Brick clad external walls*

(K) External steel cladding*

*general guidance only.

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4.3.2 Floor type A. Basic internal floors

Generic floor type A2

Timber flooring on steel deck on steel joists with plasterboard ceiling

Short span suspended floors typically comprise C or Sigma section joists at 400 to 500 mm centres, with 15 to 25 mm thick chipboard above and a single layer plasterboard ceiling of 9 to 20 mm beneath.

Where joists are at wide centres (900 to 2000 mm), a profiled steel deck, or equivalent, must be introduced beneath the chipboard in order to achieve the required spans. The plasterboard ceiling is then fixed to top hat sections beneath the joists and perpendicular to their direction of span.

Airborne sound insulation is typically in the range 40 - 49 Rw floor type A1, 46 - 55 Rw floor type A2; impact sound insulation is typically in the range 68 to 76 Ln,w

Dwellings are the only building type with regulatory requirements for sound insulation, however these are only applicable to walls and floors between dwellings. There are no standards for the acoustic performance of walls and floors within individual dwellings.

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4.3.3 Floor type B. Internal floors / compartment floors with enhanced acoustic insulation.

Generic floor type B2

Floating timber floor on plasterboard plank on isolating quilt on steel deck on steel joists with plasterboard ceiling

Short span suspended compartment floors are constructed using multiple layers of boards supported on steel joists. They typically comprise joists at 400 to 600 mm centres with plasterboard or chipboard laid beneath a resilient layer and the walking surface, with a multiple layer plasterboard ceiling. Various proprietary products are available which combine a resilient layer and walking surface. These are particularly useful for specialist applications such as refurbishment where room heights may be at a premium.

Where joists are required at wide centres (900 to 2000 mm), a profiled steel deck, or equivalent, must be introduced over the floor joists, and the plasterboard ceiling fixed to top hat sections as in Section 4.3.2 above.

Airborne sound insulation for this type of floor is typically in the range 47 - 52 Rw; impact sound insulation is typically in the range 60 to 65 Ln,w

There are regulatory requirements for the sound insulation (and fire resistance) of short span suspended compartment floors used between dwellings.

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4.3.4 Floor type C. Internal floors / compartment floors with composite steel and concrete construction.

These floors may he constructed using either conventional steel and concrete or ' slim floor' methods.

Conventional floor constructions comprise steel beams supporting a concrete slab, or a composite concrete/steel beam and slab construction. Slim floor construction is such that the beam is contained . within the depth of the floor with either precast concrete floor units or a deep steel deck spanning between the beams. These floors are sometimes heavier than conventional construction and therefore tend to have good acoustic properties.

In both cases the primary design requirement will be to support the required loads across the span and to have adequate protection against the effects of fire. These requirements will mean that the surface mass of the structure will be quite high. The airborne sound insulation will probably therefore be in the range 42 - 47 Rw floor type C1, 39 - 45 Rw floor type C2. If no special precautions are taken against impact noise, the impact insulation is likely to be in the range 75 - 89 Rw floor type C1, 82 - 89 Rw floor type C2 . If however the floor is carpeted or a floating layer is included then impact insulations of the order of 55 dB L'n,w can he achieved.

Long span and compartment floors are typically found in offices, retail buildings, hospitals and educational establishments. There are no regulatory requirements for the acoustic performance of floors in these building types.

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4.3.5 Wall type D. Basic internal partitions.

Basic internal partitions are typically constructed using layers of 9.5, 12.5 or 15 mm thick plasterboard either side of a steel stud framework. Dwellings are the only building type with regulatory requirements for sound insulation however, as already noted, there is no regulatory requirement for the acoustic performance of partitions within a dwelling.

Airborne sound insulation for this type of wall is typically in the range 37 - 46 Rw .

Recommendations have been made by the NHBC and Municipal Mutual Insurance for the sound insulation of partitions between WCs and bedrooms. Most new houses built in the UK will have partitions meeting this requirement (see Section 3.2). Since there has been no significant reaction against the current specification of such partition, it would appear that the standards are acceptable to most occupiers.

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4.3.6 Wall types E and F. Internal partitions / compartment walls with enhanced acoustic insulation.

Acoustically enhanced internal partitions/compartment walls usually include two layers of plasterboard either side of a supporting framework of steel studs. Studs may be of normal width i.e. 40 to 100 mm (defined as type E in the tables) or, where large vertical spans are required, may be up to 150 mm (defined as type F in the tables). Where necessary, studs can be nested together in pairs to provide increased stiffness, and mineral wool mats incorporated into the cavity between studs to improve sound insulation.

Airborne sound insulation for this type of wall is typically in the range 46 - 54 Rw wall type E1, 44 - 49 Rw wall type E2, 52 - 58 Rw wall type F1 and 50 - 55 Rw wall type F2. In general, the higher levels of insulation are achieved using wider studs.

Typical applications include executive offices, interview and consulting rooms, and rooms containing plant or reprographic equipment.

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4.3.7 Wall type G. Internal staggered stud partitions / compartment walls.

Generic wall type G1

Partitions can be designed using staggered or alternating studs, each set of which support one wall face independently of the other line of studs. Wall surfaces may comprise one or more layers of plasterboard, and mineral wool mats can be included between the wall skins (and routed around the studs) to increase sound insulation.

Airborne sound insulation for this type of wall is typically in the range 49 - 68 Rw . In general the higher levels of insulation are achieved using wider studs.

This form of construction combines the dimensional benefits of single stud walls with a higher sound insulation approaching that of equivalent walls with completely separate stud frames.

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4.3.8 Wall type H. Internal partitions / compartment walls on double independent frameworks.

Double layer internal partitions comprise two independent stud frames each of which carry one wall face. The extra isolation means that high levels of sound insulation can be achieved without significantly increasing the weight of the construction. Sound insulation can be increased by using multiple layers of plasterboard on the wall linings, or by introducing a mineral wool mat between the frames. This type of partition will be used where the highest level of sound insulation is required, primarily between dwellings. Even higher levels of sound insulation may be necessary for specialist applications such as recording studios, but this is beyond the scope of this document.

Airborne sound insulation for this type of wall is typically in the range 60 - 70 Rw .

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4.3.9 Wall type J. Brick clad external walls.

Generic wall type J1

Brick outer leaf, cavity, insulation, cold formed steel framed inner leaf, plasterboard internal lining

The external walls of cold formed steel framed buildings often comprise a structural frame clad internally with plasterboard and externally with insulation, with an external brick skin separated from the frame by a cavity. This form of construction is known as 'warm frame' since the position of the insulation means that the frame is generally kept at the internal temperature of the buildings and is not exposed to interstitial condensation. The approach has proven to be extremely durable. This type of construction combines the sound reduction due to the mass of the brickwork with that of the framed structure and therefore has good overall performance.

Airborne sound insulation for this type of wall is typically in the range 48 - 56 Rw .

There is however considerable potential to upgrade the sound insulation of this type of wall. Additional quilt can be incorporated between the steel studs, or additional layers of board type material (such as plasterboard or GRC) can be applied to one face of the frame (normally the external face). The appropriate strategy will depend both upon the level of performance that is required, and the type and frequencies of the sound that is to be attenuated. Advice may be sought from an acoustics specialist.

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4.3.10 Wall type K. External Steel Cladding.

Generic wall type K3

Composite panels comprising inner and outer steel sheets bonded to either side of an insulating steel core

Various types of steel cladding are available as outlined in Section 4,2. There are no mandatory regulations requiring specific levels of sound insulation from external cladding, however it may be used either to reduce the egress of sound through the building fabric from a noise generating operation, or to reduce the ingress of sound generated outside the building envelope.

The inclusion of an air gap will usually increase sound insulation, whilst the introduction of perforated liner panels will increase sound absorption, therefore reducing the amount of reverberant sound within the building. Perforated liner panels may either be substituted for the normal unperforated liner, in which case the sound insulation of the cladding will be slightly reduced; or may be fixed infront of the normal liner panel with additional quilt inbetween, in which case both sound absorption and insulation will be increased.

Airborne sound insulation for this type of wall is typically in the range 28 - 37 Rw wall type K1, 40 - 48 Rw wall type K2 and 23 - 35 Rw wall type K3.