The New Part L - The Impact on Air Tightness...

One of the most significant changes to Building Regulations was the updates to Part L. Rob Coxon of Stroma Technology Ltd outlines the targets in air-tightness testing in new homes.

Those of us that firmly believe in the "build tight - ventilate right" approach to reducing energy and carbon emissions within buildings can be reasonably pleased with the escalation of the requirements for air-tightness testing within Part L1A in particular.

Within both Part L1A and L2A, the "minimum standard" for air-permeability of 10 m3/(m2.hr) @ 50 Pa remains unchanged.  Many had argued for a ratcheting-down of this 'limit on design flexibility', myself included.  However, in reality, the ratcheting down of permissible carbon emissions rate, as calculated within SAP or SBEM (as required within Part L1A & L2A respectively) serves in effect to require significantly lower (i.e. better) air-permeability targets to be specified as part of the overall mix of carbon saving improvements over current requirements.  Indeed the air-permeability figure that is used in the 'notional dwelling/building' is now 5 m3/(m2.hr) @ 50 Pa.

Targets of 5 m3/(m2.hr) @ 50 Pa in the residential context are not likely to cause too much concern. Basic good practice on site will routinely yield results in this ball-park.  Good practice means simply mastic sealing service penetrations (in line with the plaster layer or floor boards), sealing the bottom edges of dry-lining, and so-on.

However, targets of around 5 m3/(m2.hr) @ 50 Pa are an entirely different proposition when it comes to complex commercial or public buildings such as schools or health-centres. Many contractors are still struggling to achieve 10 on projects such as this, not least as a result of the pressure to take cost out of their contracts. Achievement of compliance with the new Part L2A will not be achieved on present-day cost models.

Meanwhile, returning to the residential context, the new Part L1A: 2010 does include additional requirements for air-permeability testing, compared to the current 2006 version.  It is still the case that only a sample of each dwelling type (as is defined within the new Guidance) has to be tested - although the sample sizes required will increase substantially.  Nonetheless, while sample testing remains the requirement (as opposed to 100% testing), it is debatable whether the majority of plots on new-build developments that never get tested will ever be as air-tight as those that are earmarked for testing. While it may once have been envisaged as a 'random' test - it never actually is.

Recognising this, the authors Part L 2010 have added a significant 'penalty' on non-tested dwellings in the form of an increase to the result of +2 m3/(m2.hr) @ 50 Pa that must be added to the average test results from representative tested dwellings of the same type. Therefore, the results of sample tests can never be worse than 8 m3/(m2.hr) @ 50 Pa, so as to avoid the non-tested plots exceeding the limiting value of 10 m3/(m2.hr) @ 50 Pa.

As a result, some builders are seriously considering the option of testing all plots so as to avoid the need to compensate for this penalty in other potentially more costly ways; a prospect that air-tightness testing companies will be encouraged by, of course.

For further information on air-tightness testing and Part L compliance, please don’t hesitate to contact Stroma Technology on 0845 621 22 22 or email This e-mail address is being protected from spambots. You need JavaScript enabled to view it