Fire and Sound Control in Wood Frame Multi-Family Buildings

Designers strive to create living spaces that are safe and healthy–free from unwanted external noises and sounds and the rapid spread of fire. Buildings with many dwelling units demand more attention in this regard than single family buildings. In recent years, wood frame construction has been used much more frequently for multi-family buildings. These buildings are permitted to be up to four storeys high with units clustered both horizontally and vertically. Special steps are needed in their design and construction to prevent sound and fire from easily travelling between dwellings.

Statistics show that residential buildings account for 40 per cent of all fires and almost 80 per cent of all fire deaths1 , with one-third of these deaths occurring in multi-family buildings (MFBs). To reduce the loss of life from fire, Canadian building codes require designers and builders of multi-family buildings to consider the potential for fire ignition, fire growth and fire spread as well as ways of providing early warning and suppression.

Canada Mortgage and Housing Corporation’s (CMHC) new Best Practice Guide: Fire and Sound Control in Multi-Family Wood-Frame Buildings shows how to deal with the potential spread of fire and the transmission of sound between dwelling units. It focuses on control of fire and sound through the design and construction of the interior walls and floors separating dwelling units. Research performed by the National Research Council of Canada (NRCC) and for CMHC and other sponsors forms the basis for the best practices that are presented.

Wall and floor assemblies can be selected based on their ability to resist fire spread (rated as a fire-resistance rating, FRR) and to resist sound transmission (rated as sound transmission class, STC and impact insulation class, IIC). The FRR, STC and IIC are determined from laboratory performance tests. Field results can, of course, differ from those in the lab as many variables are introduced on-site. Nonetheless, FRR, STC and IIC can be used to select appropriate assemblies for specific applications.

Fire control by construction

To contain a fire to the compartment of its origin, wall and floor assemblies must be carefully constructed. Doors and windows (referred to as closures in building codes) must be properly selected. Most fires in multi- family buildings do not spread by the failure of compartment walls or structural assemblies, attesting to the effectiveness of such assemblies in controlling fire2. Fires most often spread through open doors or windows, or through concealed spaces that were not properly fire stopped. Fire separations are intended to contain a fire to a specific compartment, usually an apartment or another space, for a specific period of time, and to prevent the collapse of the building when exposed to fire.

Fire stopping is intended to prevent the movement of fire through concealed spaces within building assemblies and through openings in floors and walls, created by services penetrating the assembly. Fire stopping increases the reliability of a fire separation and therefore deserves special attention from designers and builders of multi-family buildings. The NBCC currently recognizes sheet steel and plywood or oriented strand board as acceptable fire stopping material. The Best Practice Guide identifies semi-rigid insulation boards as fire stopping material at the wall-floor joint with double-stud wall construction. While the material is not recognized by the NBCC as an acceptable generic fire stopping material, the research showed that both rock and glass fibre semi-rigid insulation boards will prevent fire spread in the cavity substantially exceeding the NBCC requirement of 15 minutes. Fire-stopping materials around penetrations can include mineral wool, gypsum plaster or Portland cement mortar, or listed proprietary materials.

Sound control by construction

While the physics of sound movement in buildings is quite complex, the steps that are involved in controlling sound are not particularly complicated. When airborne sound hits the surface of a building assembly, the energy contained in the sound wave is in part transmitted through the assembly, reflected off the surface and absorbed by the assembly materials. Sound not only moves through materials, it also passes around them, through cracks and voids, by air leakage. Airborne sound leakage needs very little air to transport it and can often be reduced through good detailing and planning. Good sound barriers must be impervious to air.

Another method of reducing sound transmission in a cavity wall or floor is to place absorptive material, such as fibrous insulation, within the cavity of the assembly. Absorptive materials are not sound barriers but interact with sound passing through them, converting the vibrations into heat. Each time sound passes through the sound absorbing material a little bit of the sound is absorbed. The fact that sound bounces repeatedly within the cavity from one surface to the other, through the sound absorbing material, results in a very significant decrease in the total sound transmitted. A single pass provides very little sound attenuation unless the material is very thick. It should be clear that adding a carpet or acoustic tiles directly onto a surface will not significantly improve the airborne sound insulation of the separation.

Sound transmission class, or STC, is a numerical rating assigned to a wall or floor assembly used to describe how well and how much it transmits sound. STC rates the average noise reduction in decibels for sounds that pass through an assembly. Table A shows generally how the STC ratings for walls relate to their ability to attenuate different sounds.

The STC rating ignores low frequency sound transmission below 125 Hz, which is often associated with mechanical systems and amplified music. Low frequency sounds can be a major cause for complaint in multi-family construction because they are much more difficult to control through construction techniques.

STC ratings are sometimes difficult to understand since decibels cannot be added and subtracted. A 10 dB increase actually means increasing the sound energy 10 times, which is perceived as being about twice as loud by the human ear. Therefore, an STC 45 wall will allow about 10 times as much sound to pass compared to a wall with STC 55, and the sounds that come through will be perceived as about twice as loud.

Impact sound is caused by a floor or wall being set into vibration by direct mechanical contact or impact. The sound is then radiated by the wall or floor surface into the cavity of the assembly. Floor vibrations can also be transmitted throughout the structure to walls and re-radiated as sound into adjoining spaces. For occupants, impact sound transmission can be a major issue and the source of considerable disturbance. Just as there is a standard test for airborne sound, there is a similar test for impact sound that results in a rating called impact insulation class (IIC). The IIC test method ignores sounds generated at the lower frequencies (below 100 Hz) such as those generated when people walk on lightweight joist floors. Thus, a high IIC rating does not necessarily guarantee there will be no problems from low-frequency sound through lightweight floors. Currently, the NBCC does not provide for protection from the transmission of impact sounds.

A significant amount of sound energy can bypass the wall or floor assembly cavity when front and back layers are solidly connected to each other. Sound can short-circuit the cavity by moving across its top, bottom or sides and is often referred to as flanking noise. A flanking path is a path for sound transmission that involves elements other than the common partition between two spaces although the latter may still be involved. Once sound has entered a structure and is propagating as vibration, it can travel over considerable distances. The vibrating structure continually re-radiates energy as sound from both sides. Flanking sound can propagate down walls to reach a room below. Even wh
en double studs are used for a party wall, there may be significant flanking transmission via the wall from dwelling units above, unless resilient channels are used (see figure A).

Proper detailing helps to reduce flanking noise, but avoiding it is nearly impossible. Flanking paths have much more serious consequences for impact sound transmission than for airborne sound transmission. Flanking noise can impair the performance of high STC walls and floors. Published sound transmission ratings for individual floor and wall assemblies typically do not account for this type of flanking noise problem. In practice, flanking transmission will reduce the specific airborne and impact transmission ratings of an assembly.

The ideal assembly to attenuate sound would include:

airtight construction, especially at penetrations;

two layers that are not connected at any point by solid materials;

the heaviest layers that are practical;

the deepest cavity that is practical, filled with sound-absorbing material.

Best Practice Construction and Details

The minimum STC required by the NBCC for walls and floors between dwelling units is shown in Table B. To provide a margin of safety to compensate for on-site conditions which could degrade lab performance, this Best Practice Guide recommends wall and floor systems be rated at least 5 points higher than the minimums specified in the NBCC.

A good assembly should prevent the passage of noise from all but the most inconsiderate neighbours. While it may not be possible to provide an entirely sound-tight assembly, many walls and floors in the STC 55 to 60 range are quite feasible to construct. The guide’s recommendations are also shown in Table B.

CMHC’s Best Practice Guide explores in detail important wall and floor design principles and provides 12 CAD-based construction details which illustrate best practices consistent with those principles. It also shows how services including pipes, electrical outlet boxes or other equipment that penetrate an assembly can be accommodated. An example of a typical detail in the guide is shown above.

Michael Lio, B.A. Sc., M.Eng. is a principal of Lio & Associates, consultants in building technology, and the chair of Ontario’s Part 9 Building Code Committee. He is an Assistant Professor at the University of Toronto in Building Technology and Building Science. Ken Ruest is a senior researcher with the Policy and Research Division of CMHC. He managed the development of the Best Practice Guide on Fire and Sound Control in Wood Frame Multi-Family Buildings.

Table A. The audibility of speech and music through walls of various STC ratings.

Table B. Minimum recommended STC and IIC for occupant satisfaction for various structural elements.

1. Fire Losses in Canada, Annual Report 1995, Council of Canadian Fire Marshals and Fire Commissioners, Human Resources Development Canada, Hull, QC.

2. Cot, A.E., Fire Protection Handbook, 18th ed., National Fire Protection Association, Quincy, MA, 1997.