| TM23 understood to be within the margin of error of each testing method, it introduces another level of uncertainty into SAP calculations, in addition to the divide by 20 rule. It is hoped that both these aspects will be examined for SAP11 and the Future Homes Standard. As the industry will now be communicating air leakage values at two different reference pressures, it will be important when reporting results to be clear about the reference pressure to which they relate, and whether they were obtained directly from a test at that pressure, or by correlation from a test at the other pressure. For example, tested air permeability at 50Pa obtained by fan pressurisation test, or estimated air permeability at 50Pa obtained from a test result at 4Pa and the SAP conversion formula (the latter is not actually a test result at 50Pa). 18.00 16.00 14.00 50Pa, m3h-1m-2 GUIDANCE 12.00 10.00 8.00 Tested 50Pa 50Pa estimated by formula 6.00 4.00 2.00 0.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 Measured AP4 (m3h-1m-2, from LPP) Figure 3: Tested 50Pa (by blower door test) plotted against tested 4Pa (by LPP), obtained during the BTS field trials (BTS, 2018); the orange line is the estimated 50Pa obtained by conversion from tested 4Pa using the formula to be used in SAP (figure courtesy of Build Test Solutions) What next, and what can you do? An important source of uncertainty is not the techniques themselves, but what happens on the ground for instance, measurements, building preparation, departure from recommendations (whether intentional or not), and so on. The Competent Persons Scheme (CPS) can help address this. The TM provides a framework methodology for consistency in testing procedures and how test results are reported, but it does not go TM23 PROCEDURE RECOMMENDATIONS TM23 is based on ISO 9972:2015 and builds on best practice. As such, most recommendations will be familiar to testers. Points to note include: Record wind conditions during the test; take photographic evidence of building preparation and the buildings state of completeness Trickle vents can be closed, but should not be sealed With blower door tests, carrying out both pressurisation and depressurisation is not required for regulatory purposes, but is recommended whenever possible. The result is then the average of both. This provides a more complete picture of fabric behaviour, and can reduce the risk of errors because of outside wind, especially gusts. It is required when testing for Passivhaus. to the level of detail of industry resources such as the ATTMA suite of documents. It is expected that the CPS will revise its resources to align with the new TM, and CIBSE is open to collaborating with CPS. Martyn Reed, group managing director at Elmhurst Energy, says TM23 offers an opportunity for all airtightness testers to have a say in how tests should be performed. However, he believes the guide is just a starting point: In our view, the speed of publication has meant that some of the detail is lacking, and some of the previous assumptions needed to be tested, but we are delighted with the direction of travel. This is an evolving field. TM23 includes a literature review on the current state of evidence, including for method issues such as accuracy, repeatability, and influence of outside conditions. This has informed the recommendations and points to areas where project teams may wish to go beyond minimum requirements, or where knowledge and practice may still evolve. Testers and project teams are encouraged to contribute to the growing body of knowledge by sharing testing data with CIBSE, academia and the industry, especially in the following areas: feedback on TM procedures; test results obtained on the same building under both techniques; test results (under either technique) obtained on buildings where tracer-gas methods to assess infiltration rate are also used. CJ CIBSE TM23 is free to members of the Competent Person Schemes, ATTMA and Elmhurst Energy Airtightness. Contact your scheme to obtain the download code. Watch the CIBSE TM23 launch video at bit.ly/CJApr22JG1 References: 1 Smart meter-enabled thermal efficiency ratings (SMETER) technologies project: technical evaluation, 27 January 2022, BEIS bit.ly/CJApr22TM23 28 April 2022 www.cibsejournal.com CIBSE Apr 22 pp26-28 TM23.indd 28 25/03/2022 17:36