Assessing airtightness metrics for new housing building code compliance: analysis of PCF 1819

Proposed Change Form (PCF) 1819 was submitted to public review during the 2020-2025 National Building Code (NBC) of Canada development cycle. The PCF proposed alterations to the way in which airtightness performance is defined and credited in the prescriptive and performance paths of Section 9.36 of the NBC. Under the Tiered Prescriptive Path of Subsection 9.36.8. energy conservation points may be awarded for testing and achieving airtightness “levels” defined in Subsection 9.36.6. Currently, a level is defined using airchanges per hour @ ΔP=50 Pa (ACH₅₀), normalized leakage rate @ ΔP=50 Pa (NLR₅₀), and normalized leakage area @ ΔP=10 Pa (NLA₁₀). PCF 1819 proposes to remove ACH₅₀ from the level definitions in Subsection 9.36.6., and only express airtightness performance in terms of envelope area-normalized metrics. In the performance path PCF 1819 proposed to define reference building airtightness performance in terms of NLR₅₀ instead of ACH₅₀. The PCF also proposes to use different reference airtightness performances between attached and detached homes where currently no such distinction exists; under 2020 NBC a lower airtightness performance (3.0 instead of 2.5 ACH₅₀) is only used if the proposed building is attached and its airtightness is tested using the unguarded approach. Finally, PCF 1819 proposes to allow reference and proposed building models to assume the same airtightness performance in the Tiered Performance Path if no airtightness testing is done and the air barrier is constructed per Subsection 9.25.3. and Articles 9.36.2.9. and 9.36.2.10. The 2020 NBC requires the proposed building assume a performance of 3.2 ACH₅₀ and reference 2.5 ACH₅₀ if no airtightness testing is done. Using the 240 new construction archetypes developed by Asaee & Ferguson (2019), the impacts PCF 1819 will have on prescriptive and performance compliance outcomes were evaluated. It was found that in the prescriptive path most attached and small detached buildings are unaffected by the proposed changes; they currently utilize NLR₅₀ to define their airtightness levels. Larger detached buildings, however, were found to mostly use ACH₅₀ to determine their airtightness levels, and with the removal of ACH₅₀ would need to achieve higher airtightness performance to meet a given level than what is currently required. The impact of PCF 1819 on the performance path was found to vary based on several factors: building geometry and attachment type, whether airtightness testing is being used for compliance, type of airtightness test (guarded or unguarded), and whether the base (Subsection 9.36.5.) or Tiered (Subsection 9.36.7.) performance paths were being utilized. The general observations were that if no airtightness testing is done as a measure for compliance there is minimal impact to compliance outcomes under Subsection 9.36.5., and a relaxation of requirements in Subsection 9.36.7. The latter is primarily due to PCF 1819 removing the airtightness performance gap if airtightness testing is not done. For cases where airtightness testing is used for compliance, the impacts of the PCF are shown to vary with attachment type and surface-to-volume ratio. Larger detached homes, which tended to have smaller surface-to-volume ratios in the archetype set, have an increase in reference building airtightness performance compared to current code. Subsequently, they must increase their airtightness and/or implement additional conservation measures to maintain compliance. Attached and smaller detached homes generally experience a reduction in reference building airtightness performance, and subsequently can use a reduced airtightness performance and/or implement fewer energy conservation measures while maintaining compliance. A case study using Tier 3 and Climate Zone 7A was used to further investigate the impacts of PCF 1819. Using the 240 archetypes, two sets of Tier 3 compliant solutions were developed: 2020 NBC and PCF 1819 compliant. In both sets, compliance was achieved using the performance path and measured airtightness. The results found that 63.7% of the archetypes have higher annual space heating loads under PCF 1819 compared to 2020 NBC. Generally, homes with conditioned volumes above 600 m³ see a savings in annual space heating load under PCF 1819, whereas those below 600 m³ experience an increase in annual space heating. It was noted that these general observations are applicable to all tiers and climates for cases where the performance path and airtightness testing are used for compliance.

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