UK Building Regulations
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  Ventilation Energy and Environmental Technology
                                     from VEETECH Ltd.              Updated 11th November  2009

Building Airtightness - Is Tight Right for British Homes?
Mechanical Ventilation Case Studies point to an 
Energy and Indoor Air Quality Disaster for Homes 

[View in conjunction with Pollutant Mitigation]

While sealing the building and reducing ventilation rate has a clear beneficial impact in preventing the ingress of outdoor pollutant, it is vital to understand how this approach impacts on metabolic indoor air quality. To investigate this, levels of occupancy of 1, 2 and 3 persons/10m² of floor area have been investigated (See Figures in the right hand column). 

In each case, ventilation rates of 0,1 and 8 L/s were applied (for each 10m² segment). A commonly established standard for the maximum metabolic carbon dioxide (CO₂) concentration in a space, for acceptable comfort, is 1000 parts per million (ppm). While a much higher CO₂ concentration  (e.g. up to 5000ppm) can be safely tolerated, the 1000ppm level represents the maximum threshold for acceptable comfort (and probably alertness and health) conditions. 

Using sedentary metabolic CO₂ generation rates as given in BS5925:1991, the rise in CO₂ concentration for each occupancy density and ventilation rate is illustrated in the Figures.

Figure (a) illustrates the impact of 1 person/10m² occupancy density. This presents an ideal scenario in which, under reduced ventilation conditions, the CO₂ concentration does not reach 1000ppm for at least an hour. The occupant therefore has an extended air quality reservoir in which he can protect himself from local outdoor pollution.

Figure (b) illustrates the situation for 2 occupants within the space. Under these more densely occupied conditions, the 1000 ppm threshold is reached after 30 minutes, therefore halving the effective capacity of the air quality reservoir. Also this Figure shows that an overall ventilation rate of 8 L/s is inadequate to meet the comfort needs of two people, since the CO₂ concentration for this rate of ventilation exceeds 1000 ppm after approximately 45 minutes.

Finally, the densely occupied conditions of Figure (c) (e.g. a school or meeting room) reveals a fresh air reservoir capacity of approximately 20 minutes at the reduced ventilation rate, and less than 30 minutes at the full ventilation rate.

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The above analysis shows that reduced ventilation (corresponding to shutting down the ventilation system and closing windows and vents etc.) can provide effective protection from short-term urban pollution, such as that caused by morning rush hour traffic. The period of protection ranges from between 20 minutes and an hour, depending on occupancy density. This analysis also highlights the need to provide ventilation on a per person basis rather than a floor area basis, in order to ensure that adequate ventilation is provided in instances when occupancy density is high. These calculations can be repeated for virtually any volume, occupancy density and outdoor pollution scenario.