Proposed Change 2267

Printable file (PDF 515 KB)
2267
Page: Last modified: 2026-09-18
Code Reference(s):
NBC25 Div.B 9.32.3.11. (first printing)
Subject:
Piping and Duct Insulation
Title:
Minimum Thermal Resistance Value for Duct Insulation
Description:
This proposed change increases the minimum thermal resistance value (RSI) for duct insulation.
This change could potentially affect the following topic areas:

Problem

Part 9 of the National Building Code of Canada (NBC) 2025 provisions mandate a minimum thermal resistance value of RSI 0.5 (m2×K)/W for the insulation of exhaust ducts passing through unheated spaces and of intake ducts carrying untempered outdoor air through heated spaces. As building envelope assemblies become tighter and indoor relative humidity levels are sustained at higher levels, RSI 0.5 is increasingly proving insufficient to prevent condensation in extreme temperature differentials. Furthermore, several provinces and territories have adopted more stringent thermal resistance requirements for these specific duct runs, creating a fragmented regulatory landscape.

Justification

This proposed change increases the minimum required thermal resistance value from RSI 0.5 (m2×K)/W to RSI 0.75 (m2×K)/W for the insulation of the following applications:

  • exhaust ducts passing through an unheated space or not separated from an unheated space by an insulated building assembly, and
  • intake ducts carrying untempered, unmixed outdoor air through a heated space.

The primary justification for this increase is to mitigate the elevated risk of condensation caused by the psychrometric realities of modern, airtight building envelope assemblies. Specifically, this enhanced thermal resistance provides a necessary buffer against extreme temperature differentials in the following:

Exhaust Ducts: For exhaust air travelling through unheated spaces, upgrading to
RSI 0.75 would ensure the moist exhaust air remains above its dew point until it terminates outside. This upgrade would prevent interior duct condensation, frost buildup, and subsequent water damage from backflow.

Intake Ducts: For cold outdoor air travelling through heated spaces, the higher RSI 0.75 insulation would act as a more robust thermal break. The increased insulation would prevent the outermost surface of the duct from cooling to the ambient indoor dew point, thereby mitigating the risk of concealed condensation, moisture damage, and mould growth within floor or wall cavities.

Additionally, this increase would align the National Model Codes with the more progressive standards for mechanical installation that have already been adopted by some provincial and territorial authorities, standardizing manufacturing outputs and simplifying cross-jurisdictional compliance for builders.

PROPOSED CHANGE

NBC25 Div.B 9.32.3.11. (first printing)

[9.32.3.11.] 9.32.3.11.Ducts

(See Note A-9.32.3.11.PROPOSED CHANGE A-9.32.3.11.)
[1] 1)Except as provided in Sentence (6), ventilation ducts and their fittings shall conform to the requirements of Article 9.33.6.2., except that exhaust ducts serving only a bathroom or water-closet room are permitted to be of combustible material, provided they are reasonably airtight and constructed of a material impervious to water.
[2] 2)Exhaust ducts shall not discharge into heated or unheated enclosed spaces.
[3] 3)Where an exhaust duct passes through an unheated space or is not separated from an unheated space by an insulated building assembly, the duct shall be insulated to not less than RSI 0.750.5.
[4] 4)Where a duct carrying outdoor air that is not tempered and not mixed with indoor air passes through heated space, it shall be
[a] a)insulated to not less than RSI 0.750.5, and
[b] b)provided with a vapour barrier.
[5] 5)All exhaust intakes located within 3 m horizontally of a cooktop shall be equipped with a grease filter at the intake end.
[6] 6)Ductwork for cooking appliance exhaust fans shall
[a] a)be of noncombustible, corrosion-resistant material,
[b] b)lead directly to the outdoors with no connections to other exhaust fans or ducts, and
[c] c)be equipped with a grease filter at the intake end.
[7] 7)All ductwork shall be installed to avoid crushing and shall be permanently supported to prevent sagging.
[8] 8)Joints in all ventilation system ducting shall be sealed with mastic, metal foil duct tape or the manufacturers' specified sealants.
[9] 9)Except where the size of a duct can be determined using Table 9.32.3.11.-A or Table 9.32.3.11.-B, duct sizes shall be determined according to Subsection 9.33.4.
Table [9.32.3.11.-A] 9.32.3.11.-A
Equivalent Duct Sizes
Forming Part of Sentence [9.32.3.11.] 9.32.3.11.([9] 9)
Fan's External Static Pressure, Pa Duct Diameter, mm Maximum Airflow in Duct, L/s
7 15 25 35 50 75 100
Maximum Length of Duct, m
25 75 6 1 0 0 0 0 0
100 25 6 2 1 0 0 0
125 60 17 6 3 1 0 0
150 60 42 16 8 4 2 1
175 60 60 34 18 9 4 2
200 60 60 60 35 18 8 5
50 75 16 4 1 0 0 0 0
100 60 16 6 3 1 0 0
125 60 47 18 9 5 2 1
150 60 60 44 23 12 5 3
175 60 60 60 49 25 12 7
200 60 60 60 60 48 22 13
62.5 75 22 5 2 1 0 0 0
100 60 21 8 4 2 1 0
125 60 60 24 12 6 3 1
150 60 60 58 31 15 7 4
175 60 60 60 60 33 15 9
200 60 60 60 60 60 29 17
100 75 38 9 3 1 0 0 0
100 60 36 14 7 3 1 1
125 60 60 41 22 11 5 3
150 60 60 60 53 27 12 7
175 60 60 60 60 57 27 15
200 60 60 60 60 60 51 30
150 75 59 14 5 2 1 0 0
100 60 57 22 11 6 2 1
125 60 60 60 34 17 8 4
150 60 60 60 60 42 20 11
175 60 60 60 60 60 42 24
200 60 60 60 60 60 60 46
[10] 10)In using Table 9.32.3.11.-A,
[a] a)when sizing branch ducts, “maximum length of duct” refers to the physical length of the duct from the interior grille served by that branch duct to the exterior hood,
[b] b)when sizing a trunk duct, “maximum length of duct” refers to the physical length of the duct from the interior grille of the longest branch served by that trunk to the exterior hood,
[c] c)outdoor air supply ducts shall be sized as trunk ducts,
[d] d)“maximum airflow in duct” refers to the maximum airflow rate that a given section of duct (branch or trunk) must provide to satisfy the ventilation system design, and
[e] e)“fan's external static pressure” refers to the external static pressure at which the fan is rated to achieve the maximum airflow rate that the fan is required or intended to provide.
[11] 11)Where flexible duct is used, it may be sized by choosing the next higher diameter in Table 9.32.3.11.-A or by choosing the diameter for a duct twice as long as the actual length.
[12] 12)Where rectangular duct is used in place of round duct, it shall be selected according to Table 9.32.3.11.-B.
Table [9.32.3.11.-B] 9.32.3.11.-B
Equivalent Duct Sizes
Forming Part of Sentences [9.32.3.11.] 9.32.3.11.([9] 9) and ([12] 12)
Required Round Duct Size, mm Permitted Equivalent Rectangular Duct Size, mm
Stack duct 100 mm depth 125 mm depth 150 mm depth
75 75 × 150 50 × 100 — —
100 75 × 250 75 × 100 75 × 125 75 × 150
125 75 × 250 125 × 100 100 × 125 100 × 150
150 75 × 300 200 × 100 150 × 125 125 × 150
175 75 × 350 275 × 100 200 × 125 175 × 150
> 175 Design to Subsection 9.33.4.

Impact analysis

The direct material cost increase associated with this proposed change would be marginal. Upgrading to an RSI 0.75 insulated flex duct or sleeve for heat/energy-recovery ventilator (HRV/ERV) systems adds a minor premium. As RSI 0.50 products cost $3.00 to $3.60 per linear foot and RSI 0.75 products cost $3.80 to $5.00 per linear foot, the premium would be only $0.80 to $1.40 per linear foot. Given the short lengths of these specific runs in residential occupancies, the impact on the total HVAC installation cost would be negligible. Labour expenditures would remain unchanged, as the installation and sealing methodology for RSI 0.75 products is identical to that required for current RSI 0.50 products.

Enforcement implications

No change to current enforcement procedures would be required. Building officials could verify compliance through visual inspection of the manufacturer’s printed RSI value on the outermost surface of the insulated duct or sleeve during the standard mechanical inspection.

Who is affected

Designers, engineers, architects, builders, building officials, and consultants.

OBJECTIVE-BASED ANALYSIS OF NEW OR CHANGED PROVISIONS

NBC25 Div.B 9.32.3.11. (first printing)

[9.32.3.11.] 9.32.3.11. ([1] 1) no attributions
[9.32.3.11.] 9.32.3.11. ([1] 1) [F81-OH1.1]
[9.32.3.11.] 9.32.3.11. ([2] 2) [F40,F52,F63-OH1.1]
[9.32.3.11.] 9.32.3.11. ([2] 2) [F52,F63-OS2.3]
[9.32.3.11.] 9.32.3.11. ([3] 3) [F63-OH1.1]
[9.32.3.11.] 9.32.3.11. ([3] 3) [F63,F80-OS2.3]
[9.32.3.11.] 9.32.3.11. ([4] 4) [F63-OH1.1]
[9.32.3.11.] 9.32.3.11. ([5] 5) [F41,F82-OH1.1]
[9.32.3.11.] 9.32.3.11. ([5] 5) [F01,F82-OS1.1]
[9.32.3.11.] 9.32.3.11. ([6] 6) ([a] a),([b] b),([c] c)[F02,F03-OS1.1]([a] a)[F80-OS1.1]([c] c)[F82-OS1.1]
[9.32.3.11.] 9.32.3.11. ([7] 7) [F41,F53,F80-OH1.1]
[9.32.3.11.] 9.32.3.11. ([8] 8) [F40,F41-OH1.1]
[9.32.3.11.] 9.32.3.11. ([8] 8) [F40,F63-OS2.3]
[9.32.3.11.] 9.32.3.11. ([9] 9) [F40,F50,F52,F53-OH1.1]
[9.32.3.11.] 9.32.3.11. ([9] 9) [F53-OS3.4]
[9.32.3.11.] 9.32.3.11. ([10] 10) no attributions
[9.32.3.11.] 9.32.3.11. ([11] 11) no attributions
[9.32.3.11.] 9.32.3.11. ([12] 12) [F40,F50,F52,F53-OH1.1]
[9.32.3.11.] 9.32.3.11. ([12] 12) [F53-OS3.4]
Date modified: