An Experimental Examination of Dead Air Space for Smoke Alarms

North American smoke-alarm installation standards and manufacturer’s recommendations require that smoke alarms be installed not less than 100 mm from any adjoining wall if mounted on a flat ceiling, and not closer than 100 mm and not farther than 300 mm from the adjoining ceiling surface if mounted on walls. The aim of this prescriptive rule is to avoid installation of smoke alarms in the “dead air space”, where it is assumed to be difficult for smoke to reach. However, there was little scientific basis and experimental evidence for this prescriptive rule. A series of experimental studies were conducted in a residential dwelling as well as in a laboratory room to examine the effect of “dead air space” on smoke-alarm response. Small and slow-growing smouldering and flaming fire sources were used to challenge the smoke alarms. The results show that smoke can reach the “dead air space” under the experimental conditions and the smoke alarms installed in the “dead air space” can respond to the fire at times comparable to, and in many cases even earlier than, the smoke alarms installed at conventional locations. There was no obvious evidence of the “dead air space” effect. The prescriptive “dead air space” as currently defined in the standards is questionable as far as early fire detection is concerned when fires are still small and smoke flows caused by the fires are still slow. Performance-based approaches should be used to identify those spaces where smoke may be difficult to reach.