Guidance on how to comply with this requirement is given in Approved Document L21. It explains (paragraph 1·20) that the requirement applies to naturally ventilated spaces as well as those that have mechanical ventilation or cooling. The idea behind this is to avoid the retrofit of cooling systems in naturally ventilated buildings that overheat.
The guidance in the Approved Document only applies to occupied spaces. Some buildings have stacks (figure 1) or atria to drive air movement and the guidance does not apply to them unless they are occupied. As a rough guide, spaces that are only occupied on a temporary basis, such as circulation spaces, do not count as occupied. But if an atrium, for example, contains a reception area or restaurant where people work for a substantial part of the day, then that would count as an occupied space and would need to comply.
The Approved Document then mentions (paragraph 1·21) three possible design strategies: appropriate glazing design; using solar shading; or using exposed thermal mass with night ventilation. It gives three specific ways to comply with the requirement in a space: limiting glazing area; limiting solar gain, with a simple calculation method; using a more detailed method to show the space will not overheat.
These are alternatives, so only one of them need be used to demonstrate compliance.
Glazing area
One way to comply is to show that each occupied space has a glazing area no more than a specific percentage of the internal area of the window wall (paragraph 1·22). This applies to spaces with glazing in one side only. The limiting percentage depends on orientation, so for a north facing wall 50% glazing could be allowed, while on an east, south east, south west or west facing wall the limit would be 32%. For a south, north-east or north-west facing wall the percentage is 40%. A space with rooflights and no side windows would comply if the rooflight area were less than 12% of the ceiling area.
In most cases these are quite large areas, which in some cases are greater than the base areas given in the guidance for avoiding excessive heat loss. However this guidance applies to individual spaces, rather than the building as a whole. So they could affect the freedom of the designer to trade off window area from other parts of the building to produce highly glazed, potentially overheating spaces.
For horizontal rooflights the area of glazing to limit overheating is less than the overall roof light area given in the guidance to reduce heat loss – because of the high summer solar gains. Here horizontal rooflights includes shed type rooflights inclined at up to 30° to the horizontal. If higher glazing areas are required, compliance needs to be shown in some other way. For example if shading devices or solar control glazing are included, the solar load method described below could be used to allow for their effect. For buoyancy ventilation using the height of the rooflit space, a more complex overheating calculation, perhaps involving cfd modelling, may be necessary. For vertical or near vertical sawtooth rooflights, the solar load method will normally allow higher glazing areas even without shading devices.
Where glazing areas are above the limiting values, or there are windows in more than one wall, or a combination of windows and rooflights, one of the other methods should be used.
Solar loads
The second way to comply is to show (paragraph 1·23) that the average solar load on peak summer days would not be greater than 25 W/m2 of floor area in each space. For rooms with standard low emissivity double glazing, lit from one side only, this gives equivalent results to method (a). However this method is more flexible. It can be applied to rooms with windows in more than one side, or with a combination of windows and rooflights. Also it can allow for the benefits of installing special solar shading devices3.
The method sounds complicated, but is actually straightforward. Appendix H in Approved Document L2, explains how to do the calculation. It involves:
- for each orientation of windows or rooflights in the space, taking a solar load factor from table H1 of the Approved Document;
- multiplying this by the area of glazing facing that orientation, and by another factor that depends on the type of shading installed;
- if there is more than one window wall, or a combination of windows and rooflights, adding together the solar loads from (2) above;
- dividing by the floor area of the space – areas more than 6 m from the window wall do not count unless they have rooflights;
- checking that the final answer is less than 25 W/m2.
There is scope here for large glazing areas in some spaces, provided suitable shading is specified. Appendix H gives a table of factors (taken from CIBSE Guide A2) for generic types of shading and glazing combinations. An alternative is to use the manufacturer's shading coefficient data. The shading coefficient is the ratio of the heat gain transmitted by the glazing/shading combination compared with that for single clear glazing. Equations H3, H4 and H5 in the appendix show how to use the shading coefficient data.
Equation H4 gives the correction factor for moveable shading. This equation assumes that moveable shading (for example internal blinds) would be in place only half the time, a reasonable assumption for manual control.
Approved Document L2 does not contain data for brise soleils, horizontal louvres and overhangs. A way of proceeding however is to follow the ASHRAE Fundamentals Guide 4 edition, which provides formulae to calculate the percentage of the window that will be shaded at each hour of the day. This information could then be combined with the hourly data for July from CIBSE Guide A2, tables 5.18-5.23, to predict the solar cooling load, taking the value from the relevant orientation for the unshaded part of the window and the north facing value for the shaded part (in both cases taking additional credits for any extra solar protection from blinds or special glass). These data can then be used to calculate the average solar load over the period 07:30 to 17:30 and to compare it with the target of 25 W/m2.
As an alternative, most computer programs used for design calculations have the facility for modelling the effects of external shading including overhangs and fins. Such a program could be used to demonstrate that the limiting solar load has not exceeded 25 W/m2.
Overheating
Finally, compliance is possible by showing (also paragraph 1·23) with detailed calculation procedures that the space will not overheat or require cooling when subjected to an internal gain of 10 W/m2. This is intended to provide a completely flexible method of demonstrating compliance. It could be used for example in spaces with night cooling and thermal mass, or where innovative natural ventilation techniques are used. An exact definition of what constitutes overheating is not given in the Approved Document, because different spaces will have different requirements, and different calculation tools use slightly different criteria. The Approved Document quotes chapter 5 of CIBSE Guide A2 as a source of suitable calculation procedures, but any reputable calculation technique could be used. The Nitecool program5 is an easy to use tool for calculating peak and average temperatures inside spaces with side windows and can be used to explore natural ventilation options. For more complex interiors, or the use of thermal mass, a more complex dynamic energy simulation program can be used. Where stack effects or air flows are important, cfd modelling can be used to simulate the way warm air rises and is dispersed through high level vents.
When using technical references and guidance not mentioned in the Approved Documents it is wise to agree in advance with Building Control that the proposed approach is acceptable.
Getting further information
The text of Approved Document L2 can be viewed on the DTLR web site http://www.safety.dtlr.gov.uk/ bregs/brads.htm.
Frequently asked questions on Part L are dealt with on the site http://projects.bre.co.uk/partlfaq/ where a hyperlink offers a list of all the material DTLR have published in connection with the Part L amendments. Further advice on the Approved Document can be obtained from the BRE helpline: environment@bre.co.uk.
Acknowledgments
Steve Irving of Faber Maunsell helped in the preparation of the original paper, which was produced as part of the research programme of the Building Regulations Division of the Department for Transport, Local Government and the Regions.
Source
Building Sustainable Design
Reference
1 Department for Transport, Local Government and the Regions, The Building Regulations 2000. Conservation of fuel and power. Approved Document L2, 2002 edition. TSO, London, 2001.
2 Chartered Institution of Building Services Engineers, 'Environmental design' CIBSE Guide A, 1999.
3 P J Littlefair, 'Solar shading of buildings' BRE Report BR 364. CRC, Garston, 1999.
4 American Society of Heating, Refrigerating and Air conditioning Engineers 'Fundamentals' ASHRAE Handbook, Atlanta, 2001, pages 30.45-30.46.
5 Nitecool is available on the web site http://projects.bre.co.uk/refurb/nitecool/
Postscript
Dr Paul Littlefair is associate director of BRE's Environmental Engineering Centre.
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