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SOME OF THE JUSTIFICATION LOOKS LIKE SHOWING OFF

TECHNICAL & PRACTICE / PART F

In the usual helpful way of these regulations, the new Approved Document Part F: (ADF) Ventilation explains the main changes between it and the existing ADF 1995 by pointing out that in the new version 'more guidance has been given for domestic mechanical and natural ventilation systems'. Well, duh!

This document is split into: Section 1: Dwellings;

Section 2: Buildings Other Than Dwellings; and Section 3:

Existing Buildings. The ventilation rates are predicated on a higher level of airtightness than in Approved Document L.

Most dwelling ventilation rates remain as per, but there is a new requirement for the ventilation of basements. In order to assess the ventilation requirements, there is a reasonably straightforward diagrammatic representation of four options.

These are:

? background ventilation and intermittent extract fans;

? passive stack ventilation;

? continuous mechanical extract; and - continuous mechanical supply and extract with heat recovery.

'Purge ventilation' is the new name for 'rapid ventilation', which is something specific and intermittent in that it is ventilation required to clear air of paint fumes, vapour from extraneous heaters, burnt cooking, etc. Attention is drawn to the need for purge ventilation in sanitary accommodation for example, relating to the clear opening area of 1/20th of the floor area and for fully openable windows, separate and distinct to the mechanical-, passive- or trickle-ventilation requirements.

Each of these four 'types' relates to a separate page that describes a two- or three-step approach to reading off the clear equivalent ventilation area. In general, a ventilator area can be read off from a table by adding recommended allowances and reading off the cross sectional equivalent areas required.

The ventilation requirement for basements is new but straightforward. Where a basement is connected via an open stair to the main dwelling, ventilation must be as per the dwelling.

Where basements have one clear exposed wall, mechanical ventilation, with or without heat recovery, is the 'preferred' option.

Internal rooms can be vented through a secondary room, provided that background ventilation and purge ventilation (minimum 8m 2) requirements are met. If the outer room is a conservatory, the area of the closable partition between the two rooms - and from the secondary room to external air - must meet the clear ventilation opening equivalents relative to the floor area of both rooms combined, for purge-ventilation purposes.

In Section 2: Buildings Other Than Dwellings, the whole building ventilation rate for offices requires an air-supply rate increased from 8 to 10 l/s. In a sign of the times, this is predicated on a 'no-smoking' environment. This, we are told, is 'probably traced to an analysis of experimental studies of office buildings by Mendell [1993]'.

On reading this, it becomes noticeable that some of the extraneous 'scientific' justification looks like showing off and appears to be of little relevance to many domestic architects.

For instance, part of the ADF is premised on an assumed dwelling with a permeability of 3m 3/h/m 2 at 50Pa with continuous ventilation rates to remove 1kg of water at steady state to prevent 70 per cent and 90 per cent RH limits. This may please some engineers but is an indulgence many architects haven't time for.

A lot of helpful guidance is given on passive stack ventilation (PSV) systems. These can be used as an alternative to a mechanical extract fan for office sanitary areas, washrooms and food preparation areas. Recommendations are given on the location and extract rates but two pages are given over to referencing external compliance documents. The CIBSE Guide B2:2001 figures quite extensively. It is all too common for Approved Documents to keep their text short by obliging researchers to look at multiple external sources. The section on dwellings is 12 pages long; the section on offices just four - but you have to consult your Health Technical Memorandum for details.

Approved Documents are advisory guidance only and the directions contained within them are not mandatory (architects are free to comply as they see fit, the Approved Documents merely show one way of compliance). However, even though this new draft Part F (and draft Part L) still has the basic diagrams and read-off charts and tables, I get the sense that this round of Approved Documents has tended, if not to abrogate responsibility, then to introduce caution about its directives.

The duty of providing guidance is undermined by the background murmurs of a reliance on performance specification.

ADF prides itself on giving the designer suggestions 'on what level of ventilation should be sufficient, rather than how it should be achieved'. But that approach is simply a restatement of the Building Regulations position. In which case, what then are the Approved Documents for?

For example, Appendix A states that a proper ventilation system should mean that 'there should be no visible mould on external walls'. Fair enough, but it then goes on to stipulate that the principal performance criteria used for indoor pollutants should include confirmation that nitrogen dioxide levels not exceed 288µg/m 3 over a one-hour average and that carbon monoxide be below 10ppm averaged over eight hours, etc. It's all very well citing Department of Health guidelines and spelling out the 'assumptions used in applying performance criteria'; but such complex performance specifications seem somewhat cowardly.

Indeed, the ODPM has already expressed its own get-out clause in the introduction and passed its extreme level of compliance on to designers. Undoubtedly, risks and liability will be passed down the supply chain as 'designers' regurgitate the performance specification to fan manufacturers.

Superficially, this document is an improvement, but in reality it is not guidance. Its indecisiveness - stemming from a risk averse approach to specification - seems to hang around this document like a badly ventilated odour.

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