Page: Last modified: 2024-02-16
Code Reference(s):
NBC20 Div.B 10.9.36. (first printing)
Alteration of Existing Buildings
Airtightness of Altered Air Barrier Systems
This proposed change introduces requirements for the airtightness of air barriers in existing subjected to alteration.
This change could potentially affect the following topic areas:

General information

See the summary for subject Alteration of Existing Buildings.


See the "Problem" section of the summary for the subject Alteration of Existing Buildings.

If the continuity of the air barrier system is not maintained or repaired when significant repairs or alterations are made to the building envelope of an existing building, excessive heat loss or gain, drafts, high indoor relative humidity, or water accumulation could result. Failure to maintain the continuity of the air barrier system in an alteration may also lead to a decrease in the energy performance of the building and result in excessive use of energy.

However, if all the requirements for air barriers in Section 9.36. of Division B of the National Building Code of Canada (NBC) are applied to an alteration as if the building were newly constructed, the alteration could go far beyond its original scope.

If voluntary upgrades to assemblies that are required to include an air barrier are upgraded to at least a reasonable airtightness, an opportunity exists to reduce energy use in the existing building (and gain the related cost savings in energy billing).

This proposed change would also potentially save installation costs compared to later upgrades to a higher level of airtightness, which would involve reconstructing walls and ceilings.


When the air barrier system is added or upgraded to improve continuity or airtightness concurrently with other alteration work in an existing building, the energy performance of the building will improve, thereby reducing the incremental cost of the upgrade.

This proposed change not only clarifies the Code requirements for buildings subjected to alteration, but also safeguards against potential misinterpretation by various stakeholders, including authorities having jurisdiction, designers and other professionals. This enhanced clarity would ensure that the improvement of energy performance achieved through concurrent upgrading of the air barrier’s continuity or airtightness instead during alteration work is both effective and correctly implemented, reducing the risk of non-compliance and suboptimal results.


[10.9.36.] -- Energy Efficiency

[] ---Airtightness of an Existing Building Subjected to Alteration

(See Note A-
[1] --)Where the continuity of the air barrier system is adversely affected by an alteration, or where a continuous air barrier system does not exist throughout the extent of the alteration
[a] --)discontinuous areas of the air barrier system shall be constructed in conformance with Sentence, or
[b] --)the air barrier system shall be tested in accordance with Subsection 9.36.6. and achieve an Airtightness Level of at least AL-1A or AL-1B as specified in Article based on building type. (See Tables and

Note A- Airtightness of an Existing Building Subjected to Alteration.

Effect of Airtightness on the Building Envelope
The building envelope is required to effectively minimize heat transfer, air leakage, vapour diffusion, and precipitation ingress. The systems performing these functions are interdependent, and a material in one of these systems may have multiple functions. To ensure that alterations affecting airtightness do not adversely affect the overall performance of the building envelope, it is critical to understand that the air barrier system is one of several systems within the building envelope (see Note A-
For materials that are used to fulfill the air barrier function, changing the location of the materials or selecting materials with different performance characteristics may affect the performance of other systems within the building envelope. For example, where rigid foam board is used as thermal insulation, it may also act as a component of an air barrier system; replacing the foam board with a material that offers thermal resistance, but does not provide acceptable air leakage resistance would compromise the performance of the air barrier system. (See also Notes A-, A- and A- To avoid unintended consequences of alterations, it is important to consider the house or building as one system required to perform multiple functions (building-as-a-system concept).
The intent of Article is to improve the energy efficiency of buildings subjected to alteration by increasing their airtightness, which may be achieved in different ways depending on the broader scope of the alteration project and its impact on the building as a system. The follow examples demonstrate simple and complex cases where the air barrier system may be upgraded within the scope of an alteration to an existing building:
  • In a simple case of a single-room or -space renovation where the air barrier system is not the main focus, the goal might simply be to maintain, restore or improve the continuity of the existing air barrier system with minimal intervention.
  • Another example of a simple case, but on a larger scale and with significant intervention, is a deep energy retrofit. The improvement of the entire building envelope, including the air barrier system, is the main focus of the project, particularly where the entire air barrier system is exposed and accessible. This case is similar to new construction, and it may be relatively easy to adhere to the building-as-a-system concept.
  • More complex cases are renovation projects where as significant portion of the building is subjected to alteration, including cases where substantial upgrades are made to the building envelope, or where an extension is added to an existing building. In such cases, it is important to carefully apply the building-as-a-system concept to evaluate the risk of condensation in the parts of the building envelope that are unaltered. The risk of condensation may be lower where the thermal performance and airtightness of the new and existing parts of the building are relatively similar, while the risk of condensation may be higher where the new and existing parts of the building perform significantly differently.
Further information on airtightness and condensation control can be found in the Canadian Home Builders' Association (CHBA):2021, Renovators' Manual.
Effect of Airtightness on Other Building Systems
Improving the airtightness of a building not only improves the building's energy efficiency, but also affects the building's mechanical systems (e.g., ventilation, space heating, and cooling). Improved airtightness reduces the stack effect across the height of the building, which has the desirable effect of reducing unintended air infiltration through the building envelope assemblies.
However, indoor air quality may be adversely affected by the (unintentional) reduction of air flow, which reduces the dilution of contaminants. The performance of ventilation systems (and possibly forced-air heating systems) should be reviewed and adjusted, especially in partially renovated buildings where the unaltered part of the building may receive more of the unintended infiltration (and, with it, potentially moisture and other contaminants such as soil gases). One particular concern associated with increasing the airtightness of buildings is that the indoor radon concentration could also increase (see Note A-9.13.4.).

Impact analysis

According to Statistics Canada, the greatest number of permits were issued for single-family houses in the late 1980s, peaking at around 130 000 permits annually [1]. For the purpose of providing a simplified calculation for estimating the cost-benefit of alterations, a demonstrative house (circa 1984–1995, two-storey, single detached, 2 000 ft.2 to 2 500 ft.2 of heated floor area and natural gas-fired furnace) in London, Ontario, (Zone 6) was used from a study conducted by CanmetEnergy [2].

Note that it is impossible to explore all permutations of alterations occurring in Canada. As such, this representative case has been selected to provide an illustrative example. The actual energy savings would greatly differ (i.e., may be understated or overstated), as they are based on the current airtightness of the building envelope being altered.

Where the building envelope is improved by this proposed change (and not have been improved otherwise), the amount of energy required to heat the building is typically expected to be nearly 30% less than that of the existing building with the original building envelope. The potential energy savings from improved enclosure airtightness could help meet energy-efficiency requirements and save utility costs [3]. Given this, up to 25% of energy savings could be contributed from alterations to improve airtightness. This implies that potential annual average energy savings would be around $75 per year (i.e., 25% of 30% of $995, which is the annual average natural gas bill for Canadian residences [4]).

Code users may comply with the requirements by applying certain sealing measures and conducting an airtightness test. A survey conducted in spring 2023 on the availability and cost of airtightness testing in Canada supports the rationale that airtightness testing is available at a fairly low cost ($150 to $3 250, see Table 1 below), and the increase related to travel costs for long distances is reasonable.

Table 1. Cost of an Airtightness Test by Geographic Region

  ON BC AB Prairies QC Maritimes Northern
Cost of blower door test(1)

$200–$3 000

$575 median

$150–$2 000

$475 median

$150–$2 000

$425 median

$250–$2 000

$500 median

$250–$1 200

$500 median

$250–$1 250

$750 median

$3 250(2)

Notes to Table 1:
(1) Costing data was collected from the survey on airtightness testing by Codes Canada.
(2) Costing includes estimated travel costs.


(1) Statistics Canada. Housing permit statistics.
(2) Clean Air Partnership. Archetyping-Guide-For-Energy-Efficiency-Programs-1.pdf,
(3) BC Housing. Illustrated Guide Achieving Airtight Buildings,
(4) Canadian Gas Association. Natural Gas Facts,

Enforcement implications

It is expected that a consistent set of provisions that apply to the alteration of existing buildings would help reduce the administrative and enforcement work of assessing the degree to which any particular requirement could be relaxed without affecting the level of performance of the building with respect to the Code objectives.

This proposed change would aid enforcement by identifying the work necessary to improve the energy performance of an alteration.

Who is affected

Designers, engineers, architects, manufacturers, builders, specification writers and building officials.


[] -- ([1] --) (a)
[] -- ([1] --) (b)
[] -- ([1] --) (b)[F90,F92,F95-OE1.1]
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