Proposed Change 1758

From: National Research Council Canada

1758

Page: Last modified: 2022-10-13

Problem

All building envelopes (roofs and exterior walls) transfer heat and moisture.

The direction of vapour diffusion is always from a region of higher vapour pressure to one of lower vapour pressure. In certain situations, the direction of vapour difference is not from inside to outside: an example of this is cold storage warehouses, where the direction of vapour transmission can be from outside to inside over large portions of the year. Thus, building envelope systems are becoming more complex, therefore necessitating alternative forms of analysis beyond conventional steady-state system analysis. Non-linear methods of analysis, such as hygrothermal modeling, are now routinely used to validate or invalidate potential wall systems at the design stage and, as such, should be acknowledged in the NBC.

Justification

Building envelope systems are becoming more complex to accommodate increasing levels of insulation and address premature failures, particularly in regard to high humidity enclosures in harsh winter climates.

Dynamic analysis methods, such as hygrothermal modeling, have largely moved into “state of the practice” for building science, particularly due to the advent of commercially available hygrothermal modeling programs. In addition, international standards for hygrothermal modeling have been developed (ASTM E3054/ISO 18787) to help standardize procedures for a more accurate estimation of condensation potential from vapour diffusion. The NBC should therefore acknowledge that more advanced non-linear techniques are available for the assessment of condensation potential from vapour diffusion in building envelopes compared to traditional steady-state calculations.

As a caveat, the user must be adequately trained in the field of building science and must understand the input parameters and limitations associated with the results produced by hygrothermal modeling programs.

PROPOSED CHANGE

NBC20 Div.B 5.5.1.2. (first printing)

[5.5.1.2.] 5.5.1.2.Vapour Barrier Properties and Installation

Note A-5.5.1.2.(1) Vapour Barrier Materials and Installation.

In the summer, many buildings are subject to conditions where the interior temperature is lower than the exterior temperature. Vapour transfer during these periods is from the exterior to the interior. In general, in Canada, the duration of these periods is sufficiently short, the driving forces are sufficiently low, and assemblies are constructed such that any accumulated moisture will dissipate before deterioration will occur.Prescriptive designs and simple vapour diffusion calculations based on steady-state heat transfer and steady-state vapour flow (using fixed vapour pressure differences and material vapour permeances) have commonly been used to determine the appropriate location of the vapour barrier within an assembly prior to construction. By using these methods, designers can focus on preventing the accumulation of condensation on the surface of materials and components, although material and assembly failures may not necessarily be related to sustained condensation resulting from vapour diffusion. However, where the principal mechanism for moisture movement and ingress is vapour diffusion, the use of these methods may still result in an acceptable assembly design, provided that the driving forces of vapour diffusion are sufficiently low so as to avoid the surface accumulation or absorption of condensation or that, should condensation occur, it would dissipate before the onset of material or assembly degradation. It is important to note that these methods do not account for the moisture that can be stored by materials and ignore moisture transfer mechanisms other than vapour diffusion.

Buildings such as freezer plants, however, may operate for much of the year at temperatures that are below the ambient exterior temperature. In these cases, the “warm” side of the assembly would be the exterior and a detailed analysis on an annual basis is required.Prescriptive designs and simple vapour diffusion calculations have a limited application. Designers should determine if the properties of the materials within the environmental separator and the interior environmental conditions and climatic exposure are such that a detailed dynamic analysis using environmental data from one or more reference years is needed to ensure their design will prevent the development of harmful or damaging levels of condensation. Dynamic analysis (such as hygrothermal modeling) may be necessary to suitably establish the location of the vapour barrier within the assembly and its vapour diffusion resistance, as well as the location and properties of other components within the assembly that may offer resistance to vapour diffusion. In such cases, simulation parameters should be established with care by building science design professionals and with the use of applicable software. Further guidance on modeling may be found in CSA S478-2019, “Durability in buildings.”

Steady state heat transfer and vapour diffusion calculations may be used to determine acceptable permeance levels for the vapour barrier and to identify appropriate positions for the vapour barrier within the building assembly.

Impact analysis

This proposed change expands an existing explanatory note to provide clarifications to Code users and help them avoid the implementation of improper solutions to meet Code requirements. It is neither controversial nor complex, requires little effort to develop, does not involve any policy issues, scope issues, or enforcement issues, and has a minor impact on cost.

Therefore, this change to explanatory Note A-5.5.1.2.(1) warrants a very simple analysis.

The proposed change adds clarification for the Code user on existing tools or models that can be used to evaluate whether a design or construction conforms to the Code. It is understood that these tools and models have been used by the construction industry stakeholders and building professionals for many decades, but that the existing explanatory Note failed to describe the possibility and appropriateness of their use.

For the above-stated reasons, the following impacts are foreseen:

1) No quantifiable cost impact as the tools and models proposed in the explanatory Note are presently being used by all involved parties.

2) Beneficial impacts include:

a) reducing the risk associated with potential condensation within the wall assembly at the design temperature and humidity conditions that could cause deterioration or otherwise adversely affect

i. the health or safety of building users,

ii. the intended use of the building, or

iii. the operation of building services;

b) increasing the ability of the designer to improve the performance of buildings;

c) clarifying Code provisions that ease enforcement and save time; and

d) offering design flexibility and potentially providing less costly acceptable solutions to the industry.

Enforcement implications

The authority having jurisdiction relies on the professionals; the purpose of this change to the explanatory Note is to offer guidance to designers. Enforcement can be achieved by existing means and resources. However, if the AHJ decides to undertake a detailed review, then basic knowledge of the modeling software is required.

Who is affected

Designers, specification writers, contractors, and building officials.

Designers and specification writers: There should be minimal impact (if any) to designers and specification writers. Once the analysis is complete, it is a matter of verifying the location of the vapour barrier within the assembly.

Contractors: No impact on contractors as they will be following the plans that they have when it comes to selecting the vapour barrier and its location.

Building officials: They will review the plans submitted to assure that a vapour barrier is installed as per Code requirements. In some cases, building officials may also check what vapour barrier is specified. If the AHJ decides to undertake a detailed review, then basic knowledge of the modeling software is required.

OBJECTIVE-BASED ANALYSIS OF NEW OR CHANGED PROVISIONS

NBC20 Div.B 5.5.1.2. (first printing)

[5.5.1.2.] 5.5.1.2. ([1] 1) [F63-OH1.1,OH1.2]

[5.5.1.2.] 5.5.1.2. ([1] 1) [F63-OS2.3]

[5.5.1.2.] 5.5.1.2. ([2] 2) [F63-OH1.1,OH1.2]

[5.5.1.2.] 5.5.1.2. ([2] 2) [F63-OS2.3]

[5.5.1.2.] 5.5.1.2. ([3] 3) [F63-OH1.1,OH1.2]

[5.5.1.2.] 5.5.1.2. ([3] 3) [F63-OS2.3]

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Date modified:: 2022-10-13