The specific dwellings to be tested should be selected by the BCB in consultation with the pressure tester. The dwellings should be selected so that approximately half of the scheduled tests for each dwelling type are to be carried out during construction of the first 25% of units in that type. The aim of this is to enable any required remedial work, following any failed tests, to be learned and to be adjusted in the design or procedure prior to the majority of the units being completed.

Sample Testing – Pro’s and Con’s
Whilst it might seem like a simple decision to make (less tests = less cost!) unfortunately things are never that simple. Please see below for a list of the pros and cons to sample testing:

Pros
• Providing that all tested plots pass, by achieving a result better than their design air permeability targets, then the fewer tests will mean less cost.
• Having less tests to schedule means that there will be less time delay for the build team to allow for testing.

Cons
• All sample testing results should be notified to the BCB, including details of any failures.
• When your SAP assessor enters your air test results into the SAP software, they must declare whether the result is from testing that specific plot or not. If not, the convention for entering the data is to take the average of all tested results for that dwelling type and add a 2 point ‘penalty’. This means that if the average of results were a score of 3.5, all untested plots would be assumed to score 5.5.
• If a plot fails an air test during sample testing, not only must that plot be retested until it passes, but another plot of the same dwelling type must be tested as well. This can drastically increase both time and financial costs.
• The 2 point ‘penalty’ applied in SAP can also have adverse effects on ventilation strategies. This is because it means that in some cases the figure that needs to be achieved by the tested plots can make a standard System 1 ventilation strategy (intermittent extractors and background ventilation) to air tight. For example, with a design target of 4, the tested plots would need to achieve 2 or under to allow the untested plots to also pass. This would mean that those buildings were too airtight for standard ventilation.