Controlling
Solar Overheating in Buildings
Designers
must now comply with regulations to limit solar overheating in buildings,
as Paul Littlefair of BRE explains.
A requirement to limit exposure to solar overheating in buildings, other
than homes, was included in the Building Regulations for the first time
in April 2002. It covers new non-domestic buildings, extensions and some
changes to existing buildings such as facade replacement.
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 retrofitting of cooling systems in naturally ventilated buildings
that overheat.
It only applies to occupied spaces. Some buildings have stacks or atria
to drive air movement, which are not covered 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 with the requirement.
Guidance on complying with the Building Regulations is given in Approved
Document (AD) L21. It suggests three design strategies for avoiding solar
overheating - appropriate glazing design, using solar shading or using
exposed thermal mass with night ventilation - and gives three specific
ways to comply with the requirement in a space. These are:
* limiting glazing area
* limiting solar load with a simple calculation method
* using a more detailed method to show the space will not overheat.
They are alternatives, so only one of them need be used to demonstrate
compliance for a particular space.
Limiting
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. This applies to spaces with glazing in one side only. The limiting
percentage depends on orientation. 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, sometimes greater than the
base areas given in the guidance for avoiding excessive heat loss. But
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 rooflight area given in the guidance to reduce heat
loss. This is because of the high summer solar gains through these rooflights.
Here 'horizontal' actually includes shed type rooflights inclined at up
to 30°.
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 Limiting 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 Limiting 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 to demonstrate compliance.
Limiting
solar load
The
second way to comply is to show that the average solar load on peak summer
days would not be greater than 25W per 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 the Limiting glazing area method
described above. But 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. And it can allow for the benefits of installing special
solar shading devices.
The method sounds complicated, but is actually fairly straightforward.
Appendix H in AD L2 explains how to do the calculation. It involves:
1. taking a solar load factor from table H1 of the AD for each orientation
of windows or rooflights in the space
2. multiplying this by the area of glazing facing that orientation, and
by another factor that depends on the type of shading installed
3. adding together the solar loads from (2) if there is more than one
window wall or a combination of windows and rooflights
4. dividing by the floor area of the space - areas more than 6 metres
from the window wall do not count unless they have rooflights
5. checking that the final answer is less than 25W/m2.
There is scope here for large glazing areas in some spaces, provided suitable
shading is specified. Appendix H of the AD gives a table of factors (taken
from CIBSE Guide A3) 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 AD Appendix show how to use the shading
coefficient data in this way.
Equation H4 gives the correction factor for moveable shading. This equation
(also based on CIBSE Guide A3) assumes that moveable shading (for example
internal blinds) would be in place only half the time, a reasonable assumption
for manual control. Building occupants prefer manually controlled shading
in spaces like offices.
AD L2 does not contain data for brise soleils, horizontal louvres and
overhangs. But a way of proceeding is to follow the ASHRAE Fundamentals
Guide which provides formulae to calculate the percentage of the window
that will be shaded at each hour of the day.
This information can then be combined with the hourly data for July (from
CIBSE Guide A3, tables A5.18-A5.23) 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. And 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 25W/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 therefore be used to demonstrate
that the limiting solar load has not exceeded the 25W/m2 target value.Demonstrating
the space will not overheat.
Finally, compliance is possible by using detailed calculation procedures
to show that the space will not overheat or require cooling when subjected
to an internal gain of 10W/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 (eg stack effects
in tall spaces) are used. An exact definition of what constitutes overheating
is not given in the AD, because different spaces will have different requirements
and different calculation tools use slightly different criteria. The AD
quotes chapter 5 of CIBSE Guide A3 as a source of suitable calculation
procedures, but any reputable calculation technique could be used.
The Nitecool program 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 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 the Building Control Body
that the proposed approach is acceptable.Further information:
The text of AD L2, including the legal technical requirements and Appendix
H, can be viewed on the ODPM website http://www.safety.odpm.gov.uk/bregs/brads.htm.
Those not familiar with the provisions in the Building Regulations applied
to building work (eg some building services engineers whose work has previously
been outside the bounds of building work covered) would be well advised
to look at the Building Regulations 2000 and the amendment.
Frequently asked questions on Part L are dealt with on the website http://projects.bre.co.uk/partlfaq/
where a hyperlink offers a list of all the material ODPM have published
in connection with the Part L amendments. Further advice on the requirements
and the guidance in the AD can be obtained by Emailing BRE at: mailto:environment@bre.co.uk.
Nitecool is available on the web site http://projects.bre.co.uk/refurb/nitecool/
Photographs:
(Top Right) Glazed stacks at the BRE Environmental Building.
(Top Left) Brise soleil at the Scottish Office, Edinburgh.