Proposed Change 1954

Where an airtightness test is used to determine the airtightness of a proposed house, the measured airtightness of the building envelope must be reported in ACH₅₀ (air changes per hour at 50 Pa pressure differential) on the drawings and specifications for the proposed house as required by the National Building Code of Canada (NBC) 2020. A related proposed change (PCF 1819) uses NLR₅₀ (normalized leakage rate at 50 Pa pressure differential) as the regulated airtightness metric in the NBC.

To prevent confusion and enforcement issues associated with PCF 1819, the administrative provisions of the Code should also be revised to require the NLR₅₀ metric to be provided on the drawings and specifications for the proposed house.

NLR₅₀ (in L/(s×m²)) is an airtightness metric that is based on the airtightness of assemblies and surfaces, which is in turn directly aligned with the materials used and the assemblies built by the builders during construction. The prescriptive requirements in Subsection 9.25.3. address the airtightness of materials and assemblies as a means of achieving a continuous air barrier system that is a part of the building envelope. The required airtightness of this system should not vary with the size or geometry of the house being constructed, yet relying on a volumetric target such as ACH₅₀ effectively does just that. ACH₅₀ is a volumetric measurement that is calculated as an output of the measured effectiveness of the air barrier assemblies and varies by house size, even where assembly airtightness is consistent. Conversely, NLR₅₀ is a normalized metric, meaning that it is more consistently applicable to a wide variety of housing geometries.

PCF 1819 proposes maintaining the use of ACH₅₀ in Section 9.36. as a calculated value for input into energy models, but ACH₅₀ would be calculated as an output value based on an NLR₅₀ input value and building geometry rather than an assumed value. PCF 1819 would also remove ACH₅₀ from all tables in Section 9.36., which would simplify the requirements.

Using NLR₅₀ instead of ACH₅₀ as the metric to report the measured airtightness on the drawings and specifications for the proposed house would align the administrative requirements with the proposed revision to the technical requirements found in PCF 1819.

PCF 1819 revises Section 9.36. to use NLR₅₀ as the regulated airtightness metric, which provides a more representative assessment of the performance of the air barrier system. Making this change (PCF 1954) would align the requirements for drawings and specifications with airtightness testing practices. Since the Code requires builders to construct assemblies that are airtight, it makes sense to measure the airtightness of these assemblies, aligning technical requirements and enforcement requirements through the use of the NLR₅₀ metric.

Because the conversion between ACH₅₀ and NLR₅₀ requires only the exposed area and volume as inputs (both known quantities when performing a blower door test), and the NLR₅₀ metric is already specified in existing Code requirements, this proposed change would have no additional implementation costs for Code users. The authority having jurisdiction would need to be familiar with the airtightness requirements using the NLR₅₀ metric (i.e., the targeted NLR₅₀ values for different airtightness levels) when reviewing the drawings and specifications for proposed projects.

Using the NLR₅₀ metric instead of the ACH₅₀ metric as a measure of airtightness performance on the drawings and specifications for a proposed house would align the administrative requirements in Division C of the NBC with the revised technical requirements (as proposed in PCF 1819) in Division B.

This proposed change could be enforced using existing Code enforcement infrastructure.

Authorities having jurisdiction would need to become familiar with the NLR₅₀ metric if they are not already using it regularly.

Designers, engineers, architects, building officials, manufacturers, suppliers and energy advisors.