Lessons in ventilation

In the 1980s, the focus shifted to more tightly insulated school buildings, and on low energy technologies like passive solar design, trickle ventilation, and trombe walls. While these technologies were reported to improve energy efficiency, worrying signs began to emerge that indoor air quality was being sacrificed in the process.

In 1988, researchers reported that Looe School, a seminal passive solar development in Cornwall, was suffering carbon dioxide (CO2) concentrations in classrooms in excess of 1200 ppm, with peaks of 3000 ppm. This was attributed to low ventilation rates caused by over-emphasis on low heating energy1. Other monitoring work has also demonstrated that modern schools are increasingly under-ventilated, with values of 1000 ppm of CO2 often being reached2.

Research evidence suggests that the performance of occupants deteriorates at ventilation rates of less than 8-10 l/s/p. But the CO2 concentrations observed above imply that schoolchildren are suffering fresh air ventilation rates of just 1 l/s/p or even less. Clearly a wholesale revision of school ventilation design guidance is well overdue.

Where do we go from here?
The current ventilation requirements of schools are laid down by the Department of Education and Employment (DfEE), notably in Building Bulletin 87 (BB87). This is enforced by The Education (School Premises) Regulations 1999.

BB87 currently lays down two rates for ventilation: 3 l/s/p for background ventilation and 8 l/s/p for occupied areas. Despite the rate for occupied areas, the guidance expresses a preference for natural ventilation over mechanical ventilation.

This guidance is currently out of step with the ventilation rates laid down for other types of buildings. For example, CIBSE's Guide A recommends a fresh air supply rate of 12 l/s/person for offices, residences – and schools.

To its credit, the DfEE is well aware of these discrepancies and is seeking to amend BB87 to improve compliance not only with other engineering guidance, but also with the Building Regulations (a process that will be aided by removal of Crown exemption – see box).

The DfEE's Richard Daniels says that while every section of BB87 will be reconsidered, it is too early to say which clauses will be updated or removed. Speaking at a recent CIBSE ventilation seminar, Daniels hinted that the background rate of 3 l/s/p may be dropped in favour of a single ventilation rate.

Section A on acoustics will also be thoroughly overhauled to make it more suitable as a compliance document, says Daniels. The Building Research Establishment is currently working up Building Bulletin 93 on the acoustic design of schools, and a chapter of that document will be aimed at Building Control officers. Of course BB87 will also need to be in step with the forthcoming revision to Approved Document E: Acoustics in the Building Regulations, due out for consultation in the spring.

How will these changes affect planning and design? Possibly not very much. Plans will now be checked by Building Control whereas before they were not (unless the local education authority submitted them voluntarily). For the most part the construction standards already quote Building Regulations Approved Documents, but of course BB87 is still the primary source of environmental design data.

The other big choice DfEE faces, particularly in the light of revisions to Part L of the Building Regulations, is which parts of BB87 will remain enforceable and which will remain as design advice, such as the 4-5% daylight factor.

"The energy targets in BB87 may also have to be revised, but this will require further analysis of the energy consumption of existing schools" suggested the DfEE's Richard Daniels. "The energy calculation method also needs some attention, and there is also a plan to bring the Schools Environmental Assessment Method (SEAM) into BB87 as a separate section." The DfEE is planning to issue a draft revision of BB87 on the web. "This will help us to revise it further during the first year of Crown exemption in the light of feedback from building control officers and designers" said Daniels.

The argument for mechanical ventilation
So what about mechanical ventilation? Given BB87's preference for natural ventilation, only few mechanically ventilated schools have been built in recent years, despite the evidence that a mechanically ventilated system with high heat recovery can consume less energy than an equivalent naturally ventilated building3.

There is plenty of evidence to show that energy consumption of gas-fired heating (at, say, 100 kWh/m2/y) can easily be 10 times than of the transport energy for fans, which might typically be around 4-8 kWh/m2/y.

Given that air handling systems are available that can recover over 90% of waste heat from the extract air, the need for expensive heat raising systems can, given a highly insulated and airtight fabric, be almost totally avoided4.

Irrespective of the ventilation system and efficiency, there still remains a serious issue to be faced: airborne particulates. While it is quite possible to reduce CO2 concentration through dilution, the presence of unhealthy particulates – such as products of combustion, pollen and man-made fibres – can be harmful to health.

"There is a definite need for the Government to instigate standards relating to specific pollutants to get a truer picture of indoor air quality" says Plymouth University's air quality researcher, George Richardson.

"We have been concerned for some time about the ambiguities in BB87", says Mukund Patel, DfEE's head of the Architects and Building Branch. "We need to devise new standards that reflect the needs of a comfortable learning environment for both new and existing schools. With the ending of Crown exemption, we have an opportunity to put things right."