One of the challenges for achieving low energy buildings is to significantly improve their air tightness. Ventilation provisions within the new Building Regulations Part F have been increased for commercial buildings and dwellings with a recommended design air permeability tighter or equal to 5 m3/(h.m2) @50Pa.
Focusing on construction, achieving an air tightness target of 5 m3/(h.m2) is not a difficult task. For many years specifiers have demanded significantly better standards of air tightness in quality buildings to ensure that the occupants enjoy a satisfactory state of comfort and well-being. For air-conditioned buildings, and buildings which aim to be low energy, a maximum air permeability standard of 3 m3/(h.m2) has been set by many building owners and operators.
The major benefits of tighter air tightness standard are far better control, fewer staff complaints and improved energy efficiency. Equally, many clients in the retail sector have adopted lower air tightness standards than required by the Building Regulations, such as 2 m3/(h.m2) for new build projects. Even extensions to existing buildings can routinely achieve an air permeability target of 3 m3/(h.m2).
In this regard, under normal practices for mix-mode and air-conditioned buildings, superstores, museums and storage, mechanically ventilated dwellings, factory and warehouses, the air-tightness should is expected to be better than 2010 amendments. Special consideration should be given to the design of naturally ventilated dwellings, schools, hospitals and naturally ventilated offices, when best practice for those type of buildings can achieve 40% - 60% better air tightness level than the new Part F standards.
The path to routinely achieving air tightness targets is as follows:
- Specify the air tightness target at a very early design stage;
- Specify the air seal line at a very early stage. The inside surface of the structure is usually the airtight surface. The airtight surface should be brought inside rooms which will be ventilated to outside, such as boiler rooms, plant rooms, electrical switch rooms and lift shafts;
- Require air sealing detail drawings from the architect or design and build contractor;
- Consider specifying an air tightness consultant to review drawings;
- Specify that air tightness testing be undertaken by an independent organisation which is a member of ATTMA, the testing organisation for the British Institute of Non-Destructive Testing;
- In liaison with the testing organisation, specify all aspects of the air tightness contract process. Where necessary, specify penalty charges for failures not rectified in a reasonable time-scale;
- Consider specifying an air tightness consultant to inspect the building during the construction process;
- Clearly communicate the requirements to all design and construction parties.
Specialists should have an early involvement and provide support to the contractors in the design and project management process. In some cases, advanced solutions may be required to meet the targets of the 2010 amendments, especially for construction solutions for which it was challenging to pass the 2006 amendments. It also vital that the project management fully understands and ensure co-ordinations of different trades with aspects of the external façade, and especially when structural supports, or building services, pass through a ‘perforated’ façade and external building elements.
With regard to ventilation systems, reference should be made to a new ‘Domestic Ventilation Compliance Guide’ for guidance on installing, inspecting, testing and commissioning ventilation systems in dwellings. For mechanical ventilation systems installed in new dwellings, air flow rates shall be measured on site and a notice given to the Building Control Body. This shall apply to intermittently-used extract fans and cooker hoods, as well as continuously running systems. In addition, the owner shall be given sufficient information about the ventilation system and its maintenance requirements so that the ventilation system can be operated to provide adequate air flow. All fixed mechanical ventilation systems, where they can be tested and adjusted, shall be commissioned and a commissioning notice given to Building Control Body.
What impact the changes will have on finance and profits?
Under Part F 2010, the contractor should have a greater focus on ensuring that the design is delivered according to correct specifications by specialists. Over the last four years, air-tightness levels were in many cases 50%-60% better than Part F 2010 at no additional cost. It is expected therefore that changes will not have an impact on profits when the design follows a proven assessment routine.
Although these changes will not have a fundamental effect on finance, as well as setting out physical performance requirements performance specification for building envelopes, procurement will need to ensure that contractors have the calculation competences and accredited details needed to secure the required air-tightness levels. Projects with many on-site design variations could be subject to greater misalignment with the expected air tightness results and a greater risk to the contractor team charged with delivery.
Dr Yianni Spanos is Associate Director at Capita Symonds.