Date of Publication: November 26, 2025 [2025.09.V-01.C]
Subject: Smoke-Tight Barriers in Houses with Secondary Suites
2024 Ontario Building Code Reference(s):
Subsection 9.10.9.
Subsection 9.10.10.
Incoming Request:
A smoke-tight barrier consisting of 15.9mm type X drywall has been proposed between the two suites in a new house with a secondary suite. What (if any) protection is required for piping penetrations, electrical outlet boxes, HVAC inlets/outlets or where the layer of type X drywall is interrupted by framing members? Are door and window openings required to be protected by closures?
Executive Summary:
MMAH has advised that a smoke-tight barrier is not a fire separation, and is intended to resist the passage of smoke only. At this time, the Code does not contain prescriptive requirements for maintaining the continuity of the smoke-tight barrier. Therefore penetrations, joints and other potential discontinuities should be sealed with industry-accepted sealants, tapes, etc that will function as a barrier to smoke movement. Current BCAS recommendations are based on information from MMAH advisors and typical trades practices. These recommendations may not apply to all situations.
Discussion & Considerations:
The 2024 OBC has introduced the concept of a “smoke-tight barrier”; permitted to be used to separate corridors, service rooms, common spaces and dwelling units from one another in a house with a secondary suite. [Sentences 9.10.9.16.(4), 9.10.9.17.(4). and 9.10.10.4.(2).] These sentences were introduced as part of the harmonization with the 2020 National Building Code (NBC), with the main difference being that the NBC permits the smoke tight layer to consist of ½” drywall, and the OBC requires 15.9 mm (5/8”) thick type X drywall. As this is a new concept and an undefined term, many questions have been asked. MMAH has been helpful in clarifying some aspects of the smoke-tight barrier, chiefly, that it is not a fire separation, but is intended to function as a “barrier against the spread of smoke”.
Continuity:
The 2020 NBC and the 2024 OBC contain the same sentences [9.10.9.2.(2) and (3)], and Appendix Notes [A-9.10.9.2.(2) and (3)] regarding continuity of smoke-tight barriers. MMAH has advised that where the smoke-tight barrier meets another assembly or penetration, it must be sealed to maintain the continuity and resist the passage of smoke, i.e. mud and tape or compatible sealant. The smoke-tight barrier would not necessarily be required to extend across framing members to maintain its continuity.
Piping Penetrations:
Since a smoke-tight barrier is not a fire separation, firestopping requirements of Articles 9.10.9.6. and 9.10.9.7.would logically not apply. However the smoke-tight barrier must be sealed to remain continuous. MMAH has advised that this could be done in a similar manner as sealing an air barrier by using compatible tapes, gaskets and/or sealant.
Outlet Box Penetrations:
Articles 9.10.9.8. (in addition to 9.10.9.6., 9.10.9.7., and 9.10.9.9.) of both the 2020 NBC and the 2024 OBC also do not appear to be applicable to a smoke tight barrier, as they refer to fire separations or assemblies required to have a fire resistance rating. However many have expressed concern that large amounts of electrical outlet boxes and similar penetrations (i.e. recessed light fixtures) could provide an avenue for smoke to move from one space to another. Where these concerns arise, the provisions of 9.10.9.8.(2),(3),(4), and (5) might serve as a guideline, but would not be strictly enforceable.
HVAC Inlets/Outlets
The most common penetration/discontinuity of a smoke tight barrier that causes concern is the typical ceiling HVAC outlet or exhaust fan inlet. Unfortunately neither the 2020 NBC nor the 2024 OBC provide much direction in this area. As mentioned above, MMAH has indicated that since the smoke-tight barrier is not a fire separation, a fire damper is not required. In some cases, lining the interior of the joist cavity with 15.9mm Type X drywall (sometimes known as a reverse drywall box) has been accepted as a means of maintaining the continuity of the smoke-tight barrier. Note that Sentences 9.33.1.1.(3), and 9.33.6.13.(7.1) speak to the circulation of air between suites and other spaces in a house with a secondary suite.
Large Openings
2020 NBC Sentence 9.10.9.3.(2) and note A-9.10.9.3.(2), speak to the protection of openings:
9.10.9.3. Openings to be Protected with Closures
9.10.9.3.(2) Doors in smoke-tight barriers shall
a) be solid-core, wood doors at least 45 mm thick, and
b) have a self-closing device.
(See Note A-9.10.9.3.(2).)
A-9.10.9.3.(2) Openings in Smoke-Tight Barriers to be Protected with Closures. Doors described in Sentence 9.10.9.3.(2) are deemed to provide a minimum 20 min fire-protection rating, which is considered an acceptable level of protection against the spread of fire in a house with a secondary suite. They are not required to be marked to identify conformance to CAN/ULC-S113, “Standard Specification for Wood Core Doors Meeting the Performance Required by CAN/ULC-S104 for Twenty Minute Fire Rated Closure Assemblies,” as is the case for solid-core doors installed in fire separations.
However the 2024 OBC does not include Sentence 9.10.9.3.(2)., or the Appendix Note. MMAH is aware of this omission and has indicated that it is being proposed for a subsequent amendment package. It remains unclear if a door is the only opening in a smoke-tight barrier that is required to be protected by a closure.
Risk/Benefit Analysis:
A smoke-tight barrier is a new and undefined term within the Code, and users are still becoming comfortable with its application. MMAH has indicated in a number of forums (correspondence with LMCBO committees, BCAS, and MMAH presentations) that the smoke-tight barrier is not a fire separation, and is only intended as a barrier against the spread of smoke. MMAH has also indicated that its continuity may be maintained in a manner similar to sealing an air barrier. Although BCAS hopes for more clarity in future amendments to the Code, at this time establishment of typical details and proactive policies on how to deal with unusual openings or penetrations (such as access covers and recessed lighting) are recommended to ensure consistent Code enforcement and reduce municipal risk.
Final Recommendation:
Although the term is undefined, a smoke tight barrier is not a fire separation. It is intended to be continuous and resist the passage of smoke only. Its continuity may be maintained by the use of compatible sealants. A solid core 45 mm thick door with a closer would be acceptable as a closure in a smoke-tight barrier.
Recommendation to Ministry of Municipal Affairs and Housing:
Please provide further Appendix Notes or other clarification.
Referenced Documents:
National Building Code of Canada 2020
2024 Ontario Building Code O.Reg 163/24 Amended to 5/25
Disclaimer:
This guidance document is intended to assist building officials by gathering relevant information to interpret the OBC Act and the prescriptive requirements of the Ontario Building Code, and is intended to be a best practice aid for building officials.
The views expressed within this guidance document should not be considered as the official interpretation of legislated requirements based on the Ontario Building Code, as the final responsibility for interpretation rests with the local Authority Having Jurisdiction.
The views of this advisory committee should not be construed as legal advice.