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Developers must carefully consider the technical impacts of any planning requirements which preclude the use of openable windows in residential developments.  The limited technical solutions currently available mean that any such requirements will severely constrain the ventilation strategy and have a direct impact on Part L1A CO2 compliance.

Where planning requirements for developments in areas with significant noise exposure preclude the use of openable windows, developers have limited options available to achieve Building Regulations compliant ventilation rates.  Under such circumstances, achieving Part F purge ventilation rates and the Part L1A requirement to mitigate the risk of summer overheating will have a major impact on the design of the dwelling and must be considered by developers at the earliest possible stage.

Whole house domestic ventilation systems can rarely achieve the required flow rates required for purge ventilation.  However, the use of non-domestic systems is not advised as this will make minimum Part L1A CO2 standards very difficult to achieve.

One technical solution for this issue is to use sound attenuating mechanical ventilators in each habitable room.  These can operate as background ventilators in conjunction with centralised mechanical extract ventilation systems, but also contain fans which can be used to achieve the higher air change rates required for purge ventilation and the avoidance of summer overheating.  This solution cannot generally be combined with heat recovery ventilation systems however, and will have significant internal space requirements as well as aesthetic impacts on external facades.

Noise exposure implications

Noise Exposure Categories were outlined in Planning Policy Guidance 24, with the following guidance for Noise Exposure Category C (typical for dense urban environments, or other locations subject to high levels of external noise):

‘Planning permission should not normally be granted. Where it is considered that permission should be given, for example because there are no alternative quieter sites available, conditions should be imposed to ensure a commensurate level of protection against noise.’

A typical Planning Condition for a development in a Category C area is to achieve Part F compliant background and purge ventilation rates without the use of openable windows. In some cases the Condition will also stipulate that the risk of summer overheating must be mitigated without the use of openable windows as well.

Case study

We modelled a typical south-facing 2-bedroom mid-floor flat with a floor area of 70m2 in SAP 2009 software.  In order to achieve the Part L1A minimum CO2 requirements, a centralised mechanical ventilation system was recommended to provide background ventilation to the dwelling.  For this case study, it was assumed that the flat is located in Noise Exposure Category C, and must not rely on openable windows for Part F or Part L ventilation requirements. 

Part F requires all habitable rooms to achieve a purge ventilation rate of 4 air changes per hour.  For the test dwelling this equates to a flow rate of 157l/s.  The air change rate for the avoidance of summer overheating as calculated in SAP 2009 software was 106l/s after other mitigation measures were applied (shading from brise soleil and solar control glazing).

Domestic MEV and MVHR units are not designed to achieve these levels of ventilation.  Typically a maximum flow rate for a SAP Appendix Q registered product will be approximately 100l/s.

There are two main options for achieving these flow rates:

  • Use of non-domestic ventilation systems.  These will dramatically reduce Part L1A CO2 performance as they are not SAP Appendix Q rated and therefore have higher specific fan powers than domestic units.  In our test dwelling, the use of a non-domestic system increased the dwelling emission rate by 60%, resulting in a fail for Part L CO2 requirements.  In addition, increased unit and ductwork sizes require more internal space, and external vents would impact on the appearance of facades.
  • Use of sound attenuating mechanical ventilators in each habitable room, in addition to the whole house background ventilation system.  This solution requires internal space in addition to the background ventilation system (internal units measure approximately 350mm x 450mm x 150mm).  They also require external vents which will impact on the façade design.  The units do not impact on Part L1A CO2 performance, but low energy systems should be installed to minimise running costs.  This solution is not without its challenges, but in some cases may be unavoidable in order to achieve the necessary ventilation rates required by Part F. 

 Part L1A Overheating Compliance

Criterion 3 of Part L1A 2010 requires designers to limit the effect of solar gains on internal temperatures.  This is often achieved by simply stating that windows in a dwelling can be fully opened during hot weather in order to cool the dwelling.

Part L only cites physical restrictions to window openings and security concerns (i.e. ground floor dwellings) as reasons to assume windows cannot be fully opened in hot weather.  Acoustic requirements are not currently covered in Part L1A overheating risk assessments. 

Mandatory requirements for mitigating the risk of summer overheating without the use of openable windows due to acoustic concerns are therefore currently only applied through local or regional authorities.  However, it is our opinion that any requirements should not be treated in isolation from Part L1A compliance.  Holistic strategies for adequate ventilation must always cover Building Regulations requirements as well as additional considerations required by local authority.

Posted on May 6th, 2013