Envelope and building interfaces: HVAC

Last updated October 22, 2026

Values this chapter turns on

attic vent area as a share of insulated ceiling: 1/300; attic vent area as a share of insulated ceiling
minimum fireplace combustion air duct diameter: 100 mm; minimum fireplace combustion air duct diameter
minimum set temperature, electric storage heater: 60°C; minimum set temperature, electric storage heater
window glass area for living and dining rooms: 10%; window glass area for living and dining rooms

An HVAC designer's work touches the building envelope at many points: windows that provide natural ventilation and set heat loss, crawl spaces that may be heated, attics that must be vented, fireplaces that need outdoor combustion air, vapour barriers that control condensation, and water heaters that must reach a set temperature. The syllabus picks a targeted set of Articles from Sections 9.7., 9.18., 9.19., 9.21., 9.22., 9.25. and 9.31.

Windows, doors and skylights

Scope OBC Div. B 9.7.1.1.. Section 9.7. covers windows, doors and skylights separating conditioned space from unconditioned space or the outdoors, plus main entrance doors. "Skylight" includes roof windows and tubular daylighting devices; "doors" includes door glazing and sidelights.

Cross-references OBC Div. B 9.7.2.2.. Windows and skylights used for required non-heating-season ventilation must meet Article 9.32.2.2. Many other properties (egress, guards, fire spread) are sent to other Sections.

Minimum window areas for residential rooms come from the glass area table OBC Div. B 9.7.2.3. OBC Div. B Table 9.7.2.3.. Living and dining rooms need 10% of the area served whether or not there is electric lighting; bedrooms need 5%. Rooms like laundries and unfinished basements need no windows if electric lighting is provided. The glass area of a door or skylight counts as window area.

Performance and thermal rules OBC Div. B 9.7.3.1. OBC Div. B 9.7.3.3.:

Windows, doors and skylights resist precipitation, wind, air leakage, insects, forced entry (where required) and operate easily.
Metal frames and sashes need a thermal break, except vehicle doors, storm units and fire-rated units.
U-value or temperature index limits are in the window thermal table OBC Div. B Table 9.7.3.3.. For locations with a January 2.5% temperature between -15°C and -30°C, windows and doors are limited to U 2.0 W/m²K. Where SB-12 is stricter, the most restrictive value applies.
High-moisture spaces are designed under Part 5.

Installation OBC Div. B 9.7.6.1.. Install to CSA A440.4 and the manufacturer's instructions, and seal windows, doors and skylights to the air barrier.

Practice this topic

Crawl spaces and roof space venting

Heated or unheated? A crawl space is considered heated if it is used as a warm air plenum, contains heating ducts or pipes that are not sealed and insulated, or is not separated from heated space under Section 9.25. OBC Div. B 9.18.1.3.. A heated crawl space is heated to Section 9.33. and insulated with an air barrier and vapour barrier under Section 9.25.

Crawl space	Ventilation
Unheated	Natural or mechanical; natural vents at least 0.1 m² per 50 m² of floor area, uniformly on opposite sides OBC Div. B 9.18.3.1.
Heated	Ventilated in accordance with Section 9.32. OBC Div. B 9.18.3.2.

Crawl spaces as warm air plenums OBC Div. B 9.18.7.1.. Only crawl spaces under 1-storey portions of a dwelling unit may be used as plenums. Enclosing materials have flame spread not more than 150, and combustible ground cover is covered with noncombustible material extending at least 300 mm beyond each register opening, with turned-up edges.

Attic and roof space venting OBC Div. B 9.19.1.1. OBC Div. B 9.19.1.2.:

Roof	Minimum unobstructed vent area
General	1/300 of insulated ceiling area
Slope less than 1 in 6, or roof joists	1/150 of insulated ceiling area

Vents are spread on opposite sides with at least 25% of the required area at the top and 25% at the bottom. Keep at least 63 mm between insulation and sheathing in vented joist spaces OBC Div. B 9.19.1.3..

Practice this topic

Chimneys and fireplaces: combustion air and heat circulating ducts

Section 9.21. applies to rectangular masonry or concrete chimneys not more than 12 m high serving fireplaces or appliances totalling 120 kW or less, and to flue pipes for solid-fuel appliances. Other chimneys, gas vents and flue pipes follow Section 6.3. OBC Div. B 9.21.1.1..

Fireplace combustion air OBC Div. B 9.22.1.4.. Every solid-fuel fireplace, including factory-built, gets outdoor combustion air through a noncombustible, corrosion-resistant duct at least 100 mm in diameter (or equivalent area) with an exterior intake. A tight-fitting damper near the interior outlet is operable from the fireplace room and shows its position. The outlet is close to the fireplace opening and blocks embers. Air delivered directly into the fire chamber follows CAN/CSA-A405-M.

Heat circulating duct outlets OBC Div. B 9.22.9.4.. Combustible material above the outlet is at least 300 mm away if it projects 38 mm or more from the face, or 150 mm if it projects less.

Practice this topic

Vapour barriers and envelope materials

The vapour barrier protects the warm side of walls, ceilings and floors, close enough to the warm side of the insulation to prevent condensation at design conditions OBC Div. B 9.25.4.3..
A sheet or panel material with low air leakage and a vapour permeance below 60 ng/(Pa x s x m²) must be placed per 9.25.5.2., unless an exemption applies (for example thin wood-based sheathing, or a more permeable insulating material in a location under 6000 degree-days) OBC Div. B 9.25.5.1..

Practice this topic

Hot water temperature

Where hot water is required, each dwelling unit is supplied with service hot water between 45°C and 60°C, and an electric storage-type water heater has a minimum set storage temperature of 60°C OBC Div. B 9.31.6.1A..

Practice this topic

Common traps

Venting a heated crawl space with 0.1 m² per 50 m² vents. That rule is for unheated crawl spaces.
Using a crawl space under a 2-storey portion as a plenum. Only under 1-storey portions.
Applying 1/300 to a low-slope roof. Below 1 in 6, use 1/150.

Chapter review

Chapter at a glance

One line per section: the idea to carry into the exam. Then the answers that most often catch candidates out.

Chapter at a glance: the key idea in each section
Section	Key idea
Windows, doors and skylights	Minimum window areas for residential rooms come from the glass area table.
Crawl spaces and roof space venting	Crawl spaces as warm air plenums.
Chimneys and fireplaces: combustion air and heat circulating ducts	Section 9.21. applies to rectangular masonry or concrete chimneys not more than 12 m high serving fireplaces or appliances totalling 120 kW or less, and to flue pipes for…
Hot water temperature	Where hot water is required, each dwelling unit is supplied with service hot water between 45°C and 60°C, and an electric storage-type water heater has a minimum set storage…

Windows, doors and skylights
Minimum window areas for residential rooms come from the glass area table.
Crawl spaces and roof space venting
Crawl spaces as warm air plenums.
Chimneys and fireplaces: combustion air and heat circulating ducts
Section 9.21. applies to rectangular masonry or concrete chimneys not more than 12 m high serving fireplaces or appliances totalling 120 kW or less, and to flue pipes for…
Hot water temperature
Where hot water is required, each dwelling unit is supplied with service hot water between 45°C and 60°C, and an electric storage-type water heater has a minimum set storage…

Usual exam traps

Venting a heated crawl space with 0.1 m² per 50 m² vents. That rule is for unheated crawl spaces.
Using a crawl space under a 2-storey portion as a plenum. Only under 1-storey portions.
Applying 1/300 to a low-slope roof. Below 1 in 6, use 1/150.

Practice this chapter

Questions on Envelope and building interfaces, in the exam format, every answer explained and cited to its article. Sign in to start; a free account gets the free questions.

Practice this chapter HVAC - House calculations

Code cited in this chapter

Based on the official 2024 Building Code Compendium on ontario.ca.

OBC Div. B 9.7.1.1. Application
(1)This Section applies to
(a)windows, doors and skylights separating conditioned space from unconditioned space or the exterior, and
(b)main entrance doors.
(2)For the purpose of this Section, the term "skylight" refers to unit skylights, roof windows and tubular daylighting devices.
(3)For the purpose of this Section, the term "doors" includes glazing in doors and sidelights for doors.
Open OBC Div. B 9.7.1.1. in the code viewer
OBC Div. B 9.7.2.2. Other Requirements for Windows, Doors and Skylights
(1)Minimum sizes of doorways and doors within a barrier-free path of travel shall conform to Section 9.5.
(2)The protection of window and door openings to protect persons from falling through them shall conform to Article 9.8.8.1.
(3)Properties of windows and doors within exits shall conform to Section 9.9.
(4)Windows and doors installed to provide the required means of egress from bedrooms shall conform to Subsection 9.9.10.
(5)The location and protection of windows, doors and skylights in order to control the spread of fire shall conform to Subsection 9.10.12.
(6)Doors between dwelling units and attached garages shall conform to Article 9.10.13.15.
(7)The surface flame-spread rating for doors and skylights shall conform to Article 9.10.17.1.
(8)Windows and doors installed to provide the required access to a building for firefighting purposes shall conform to Subsection 9.10.20.
(9)Windows and skylights installed to provide required non-heating season ventilation shall conform to Article 9.32.2.2.
Open OBC Div. B 9.7.2.2. in the code viewer
OBC Div. B 9.7.2.3. Minimum Window Areas
(1)Except as required in Article 9.9.10.1. and Sentence (3), the minimum window glass areas for rooms in buildings of residential occupancy or rooms that are used for sleeping shall conform to Table 9.7.2.3.
(2)The unobstructed glass area of a door or skylight is considered equivalent to that of a window.
(3)Work areas in live/work units shall conform to Clause 3.7.2.1.(2)(a).
(4)Where rooms with different requirements for window glass area are combined as described in Sentence 9.5.1.2.(1), the more restrictive requirement shall govern.
Open OBC Div. B 9.7.2.3. in the code viewer
OBC Div. B Table 9.7.2.3. Glass Areas for Rooms of Residential Occupancy — Forming Part of Sentence 9.7.2.3.(1)
Table 9.7.2.3 — Glass Areas for Rooms of Residential Occupancy — Forming Part of Sentence 9.7.2.3.(1) Location | Minimum Unobstructed Glass Area — With No Electric Lighting | Minimum Unobstructed Glass Area — With Electric Lighting Laundry, basement recreation room, unfinished basement | 4% of area served | Windows not required Water-closet room | 0.37 m² | Windows not required Kitchen space | 10% of area served | Windows not required Kitchen alcove | 10% of area served | Windows not required Living rooms and dining rooms | 10% of area served | 10% of area served Bedrooms and other finished rooms not mentioned above | 5% of area served[1] | 5% of area served[1] Notes to Table 9.7.2.3.: (1) See Subsection 9.9.10.
Open OBC Div. B Table 9.7.2.3. in the code viewer
OBC Div. B 9.7.3.1. General Performance Criteria
(1)Except as provided in Sentences (2) to (4), windows, doors and skylights and their components separating conditioned space from unconditioned space or the exterior shall be designed, constructed and installed so that, when in the closed position, they
(a)resist the ingress of precipitation into interior space, (See Note A-9.7.4.2.(1))
(b)resist wind loads,
(c)control air leakage,
(d)resist the ingress of insects and vermin,
(e)where required, resist forced entry, and
(f)are easily operable when not intended to be fixed.
(2)Skylights and their components shall be designed, constructed and installed so that, when in the closed position, they resist snow loads.
(3)Main entrance doors and their components shall be designed, constructed and installed so that, when in the closed position, they
(a)control air leakage,
(b)resist the ingress of insects and vermin,
(c)resist forced entry, and
(d)are easily operable.
(4)Storm doors, sliding doors and their components shall be designed, constructed and installed so that, when in the closed position, they
(a)resist wind loads,
(b)control air leakage to a minimum allowable 5 m³h/m and a maximum allowable 8.35 m³h/m,
(c)resist the ingress of insects and vermin, and
(d)are easily operable.
(5)Compliance with the performance requirements described in Sentences (1) to (4) shall be demonstrated by
(a)complying with the requirements in
(i)Subsection 9.7.4. or 9.7.5., and
(ii)Subsection 9.7.6., or
(b)design and construction conforming to Part 5.
Open OBC Div. B 9.7.3.1. in the code viewer
OBC Div. B 9.7.3.3. Thermal Characteristics of Windows, Doors and Skylights
(1)Metal frames and sash of windows, doors and skylights shall incorporate a thermal break.
(2)Windows and doors described in Sentence (1) do not require a thermal break where they are installed as
(a)vehicular access doors,
(b)storm windows and doors, or
(c)windows and doors that are required to have a fire-resistance rating.
(3)Windows, doors and skylights with or without storm doors or sash that are installed in buildings where the intended use of the interior space will not result in high moisture generation shall have a maximum thermal transmittance (U-value) or minimum temperature index (I) in accordance with Table 9.7.3.3.
(4)Windows, doors and skylights, with or without storm doors or sash that are installed in portions of buildings where the intended use of the interior space will result in high moisture generation shall be designed in conformance with Subsection 5.3. (See Note A-9.25.5.2.)
Open OBC Div. B 9.7.3.3. in the code viewer
OBC Div. B Table 9.7.3.3. Maximum U-value or Minimum Temperature Index (I) for Windows, Doors and Skylights[1][2][3]
Table 9.7.3.3. — Maximum U-value or Minimum Temperature Index (I) for Windows, Doors and Skylights[1][2][3] Forming Part of Sentence 9.7.3.3.(3) Component | max. U-value, W/m²K (2.5% January Design Temperature: Between −15°C and −30°C) | min. I (2.5% January Design Temperature: Between −15°C and −30°C) | max. U-value, W/m²K (2.5% January Design Temperature: Colder than −30°C) | min. I (2.5% January Design Temperature: Colder than −30°C) Windows and Doors | 2.0 | 68 | 1.7 | 77 Skylights | 3.0 | [2] | 2.7 | [2] Notes to Table 9.7.3.3.: (1) U-values for specific products can be determined according to measures referenced in AAMA/WDMA/CSA 101/I.S.2/A440, "NAFS - North American Fenestration Standard/Specification for windows, doors, and skylights." Temperature index (I) is determined according to the physical test procedure given in CSA A440.2/A440.3, "Fenestration energy performance/User guide to CSA A440.2-19, Fenestration energy performance." (2) There is no appropriate test procedure available for testing the condensation resistance of sloped glazing. (3) Where the U-value in this Table differs from the U-value provided in MMA Supplementary Standard SB-10, "Energy Efficiency Requirements" or MMA Supplementary Standard SB-12, "Energy Efficiency for Housing", the most restrictive U-value shall apply.
Open OBC Div. B Table 9.7.3.3. in the code viewer
OBC Div. B 9.7.6.1. Installation of Windows, Doors and Skylights
(1)The installation of windows, doors and skylights shall conform to CSA A440.4, "Window, door, and skylight installation," except that
(a)shims used to support windows, doors and skylights are permitted to be made of treated plywood, and
(b)protection from precipitation for walls incorporating windows or doors and for roofs incorporating skylights, and the interfaces of these walls with windows or doors and of roofs with skylights, shall also conform to Section 9.27.
(2)The installation of manufactured and pre-assembled windows, doors and skylights and the field assembly of manufactured window and door combination units shall conform to the manufacturer's instructions.
(3)Windows, doors and skylights shall be sealed to air barriers.
Open OBC Div. B 9.7.6.1. in the code viewer

Show all 21 articles

OBC Div. B 9.18.1.3. Heated and Unheated Crawl Spaces
(1)Crawl spaces shall be considered to be heated where the space
(a)is used as a warm air plenum,
(b)contains heating ducts or heating pipes that are not sealed and insulated to minimize heat loss to the space, or
(c)is not separated from heated space in accordance with Section 9.25.
(2)Heating of heated crawl spaces shall conform to Section 9.33.
(3)Insulation, an air barrier system and a vapour barrier shall be installed in the walls of heated crawl spaces in accordance with Section 9.25.
Open OBC Div. B 9.18.1.3. in the code viewer
OBC Div. B 9.18.3.1. Ventilation of Unheated Crawl Spaces
(1)Unheated crawl spaces shall be ventilated by natural or mechanical means.
(2)Where an unheated crawl space is ventilated by natural means, ventilation shall be provided to the outside air by not less than 0.1 m² of unobstructed vent area for every 50 m² of floor area.
(3)Vents shall be
(a)uniformly distributed on opposite sides of the building, and
(b)designed to prevent the entry of snow, rain and insects.
Open OBC Div. B 9.18.3.1. in the code viewer
OBC Div. B 9.18.3.2. Ventilation of Heated Crawl Spaces
(1)Heated crawl spaces shall be ventilated in accordance with Section 9.32.
Open OBC Div. B 9.18.3.2. in the code viewer
OBC Div. B 9.18.7.1. Crawl Spaces as Warm Air Plenums
(1)Only crawl spaces under 1-storey portions of dwelling units shall be used as warm air plenums.
(2)Enclosing material in crawl spaces described in Sentence (1), including insulation, shall have a surface flame-spread rating not greater than 150.
(3)Combustible ground cover used as enclosing material in Sentence (2) shall be covered with noncombustible material. (See Note A-9.18.7.1.(3) and (4))
(4)Noncombustible material described in Sentence (3) shall
(a)extend not less than 300 mm beyond the projection of a register opening, and
(b)have turned-up edges. (See Note A-9.18.7.1.(3) and (4))
Open OBC Div. B 9.18.7.1. in the code viewer
OBC Div. B 9.19.1.1. Required Venting
(1)Except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the movement of air from the space to the exterior. (See Note A-9.19.1.1.(1))
Open OBC Div. B 9.19.1.1. in the code viewer
OBC Div. B 9.19.1.2. Vent Requirements
(1)Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area.
(2)Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area.
(3)Required vents may be roof type, eave type, gable-end type or any combination thereof, and shall be distributed
(a)uniformly on opposite sides of the building,
(b)with not less than 25% of the required openings located at the top of the space, and
(c)with not less than 25% of the required openings located at the bottom of the space.
(4)Except where each roof joist space referred to in Sentence (2) is separately vented, roof joist spaces shall be interconnected by installing purlins not less than 38 mm by 38 mm on the top of the roof joists.
(5)Vents shall comply with CAN3-A93-M, "Natural Airflow Ventilators for Buildings."
Open OBC Div. B 9.19.1.2. in the code viewer
OBC Div. B 9.19.1.3. Clearances
(1)Except as provided in Sentence (2), where venting is provided to a roof joist space, not less than 63 mm of space shall be provided between the top of the insulation and the underside of the roof sheathing.
(2)Where venting is provided at the junction of sloped roofs and exterior walls and where preformed baffles are used to contain the insulation, the baffles shall
(a)provide an unobstructed air space, between the insulation and the underside of the roof sheathing, that is
(i)not less than 25 mm in dimension, and
(ii)of sufficient cross area to meet the attic or roof space venting requirements of Article 9.19.1.2., and
(b)extend vertically not less than 50 mm above the top of the insulation.
(3)Ceiling insulation shall be installed in a manner that will not restrict the free flow of air through roof vents or through any portion of the attic or roof space.
Open OBC Div. B 9.19.1.3. in the code viewer
OBC Div. B 9.21.1.1. Application
(1)This Section applies to,
(a)rectangular masonry or concrete chimneys not more than 12 m in height serving fireplaces or serving appliances having a combined total rated heat output of 120 kW or less, and
(b)flue pipes serving solid fuel-burning appliances.
(2)Reserved.
(3)Except as provided in Sentence 9.21.1.3.(1), chimneys (other than those described in Sentence (1) and Sentence 9.21.1.2.(1)), gas vents and flue pipes serving gas-, oil- or solid fuel-burning appliances and associated equipment shall conform to Section 6.3.
Open OBC Div. B 9.21.1.1. in the code viewer
OBC Div. B 9.22.1.4. Combustion Air
(0.1)Every solid fuel-fired fireplace, including a factory-built fireplace, shall have a supply of combustion air from outdoors in accordance with Sentences (0.2) to (1).
(0.2)The combustion air shall be supplied by a noncombustible and corrosion-resistant supply duct.
(0.3)The supply duct shall have
(a)a diameter of not less than 100 mm or equivalent area, and
(b)an exterior intake for entry of air from the outdoors.
(0.4)The supply duct shall contain a tight-fitting damper that shall be located close to the interior outlet and be operable from the room containing the fireplace.
(0.5)The operating mechanism shall clearly indicate the actual position of the damper.
(0.6)The interior outlet shall
(a)be located as close as possible to the opening in the face of the fireplace, and
(b)be designed to prevent embers from entering the supply duct.
(1)Where a supply of combustion air is provided directly to the fire chamber of a fireplace, including a factory-built fireplace, the installation shall comply with the "Outdoor Air Supply" requirements provided in CAN/CSA-A405-M, "Design and Construction of Masonry Chimneys and Fireplaces."
Open OBC Div. B 9.22.1.4. in the code viewer
OBC Div. B 9.22.9.4. Heat Circulating Duct Outlets
(1)The clearance of combustible material above heat-circulating duct outlets from those outlets shall be not less than
(a)300 mm where the combustible material projects not less than 38 mm from the face, and
(b)150 mm where the projection is less than 38 mm.
Open OBC Div. B 9.22.9.4. in the code viewer
OBC Div. B 9.25.4.3. Installation of Vapour Barriers
(1)Products installed to function as the vapour barrier shall protect the warm side of wall, ceiling and floor assemblies.
(2)Where different products are used for the vapour barrier and the insulation, the vapour barrier shall be installed sufficiently close to the warm side of the insulation to prevent condensation at design conditions. (See Note A-9.25.4.3.(2) and A-9.25.5.1.(1))
(3)Where the same product is used for the vapour barrier and the insulation, the product shall be installed sufficiently close to the warm side of the assembly to prevent condensation at design conditions. (See Note A-9.25.4.3.(2) , A-9.25.5.1.(1) and A-9.25.5.2.)
Open OBC Div. B 9.25.4.3. in the code viewer
OBC Div. B 9.25.5.1. General
(1)Except as provided in Sentences (2) to (4), sheet and panel-type materials incorporated into assemblies described in Article 9.25.1.1. shall conform to Article 9.25.5.2. where
(a)the material has
(i)an air leakage characteristic less than 0.1L/(s × m^2) at 75 Pa, and
(ii)a water vapour permeance less than 60 ng/(Pa×s×m²) when measured in accordance with ASTM E96 / E96 M, "Standard Test Methods for Water Vapor Transmission of Materials," using the desiccant method (dry cup), and (See Note A-9.25.5.1.(1) (a)(ii))
(b)the intended use of the interior space where the materials are installed will not result in high moisture generation. (See Note A-9.25.5.1.(1))
(2)Where the intended use of the interior space will result in high moisture generation, the assembly shall be designed according to Part 5.
(3)Wood-based sheathing materials not more than 12.5 mm thick and complying with Article 9.23.17.2. need not comply with Sentence (1). (See Note A-9.25.5.1.(3))
(4)Where a material has a water vapour permeance not less than 30 ng/( Pa × s × m^2 ) and a thermal resistance not less than 0.7 ( Pa × s × m^2 ) and the heating degree-days of the building location are less than 6000, the assembly need not comply with Sentence (1).
Open OBC Div. B 9.25.5.1. in the code viewer
OBC Div. B 9.31.6.1A. Hot Water Temperature
(1)Where a hot water supply is required by Article 9.31.4.3., equipment shall be installed to provide to every dwelling unit an adequate supply of service hot water with a temperature range from 45°C to 60°C.
(2)An electric storage-type service water heater shall have a minimum set storage temperature of 60°C.
Open OBC Div. B 9.31.6.1A. in the code viewer