Your Crawl Space Might Be Heated Without Anyone Deciding It Was

What counts as a crawl space in the first place

Section 9.18 of the BC Building Code covers crawl spaces, and it starts by defining which spaces it is talking about. Article 9.18.1.1 states that the Section applies to crawl spaces whose exterior walls have less than 25% of their total area above exterior ground level open to the outdoors.

That number matters on the North Shore, because so many houses sit on slopes. A space that is open on the downhill side, with a large area of wall exposed and uncovered, can fall outside Section 9.18 entirely. A space with small vents in each wall falls inside it. The test is the area of wall above grade that is open, measured against the total wall area.

Once a space is inside Section 9.18, the Code asks a second question before anything else: is the space heated or unheated. Every other requirement in the Section branches from that answer. Ground cover, ventilation, and the way the walls are built are all written twice, once for each case.

Homeowners tend to hear heated crawl space and picture a finished room with a vent blowing warm air into it. That is why the classification catches people. A heated crawl space, in code terms, can be a dirt-floored space you have entered twice in twenty years.

The three conditions that make a crawl space heated

Article 9.18.1.3 is where the classification is set, and it is short. Crawl spaces shall be considered to be heated where the space is used as a hot air plenum, contains heating ducts that are not sealed and insulated to minimize heat loss to the space, or is not separated from heated space in accordance with Section 9.25.

Read the middle condition again. Heating ducts that are not sealed and insulated to minimize heat loss to the space. A duct that runs through the crawl space and leaks warm air into it makes the space heated. Nobody signed off on that. The furnace installer was solving a routing problem, and the classification of the space changed as a side effect.

The third condition catches even more houses. If the crawl space is not separated from the heated part of the house by an air barrier and insulation built the way Section 9.25 describes, the space is heated. An open floor with no insulation between the joists, or insulation that was pushed aside years ago and fell down, means the crawl space and the living room are one connected volume of air.

The first condition is deliberate. Some older houses were built to blow warm air into the crawl space and let it rise through floor registers, which makes the whole space a duct. The Code has separate rules for that, covered further down.

So there are three doors into the heated category, and two of them are things that happen to a house rather than things somebody designed. That is the whole point of this article. Check which one applies to yours before you plan any work.

• The crawl space is used as a hot air plenum, meaning warm air is blown into the space itself and rises into the rooms above.
• Heating ducts pass through the space and are not sealed and insulated to minimize heat loss into it.
• The space is not separated from the heated part of the house in accordance with Section 9.25, which covers insulation, air barriers, and vapour barriers.
• Meeting any one of the three is enough. They are separate triggers, not a checklist you have to complete.

Everything that changes once the space is heated

The classification is not a label. It reaches into three separate requirements and rewrites each of them.

Ground cover is the clearest example. For an unheated crawl space, Article 9.18.6.1 accepts any of four materials: 50 mm of asphalt, 100 mm of 15 MPa Portland cement concrete, Type S roll roofing, or 0.10 mm polyethylene. Joints are lapped not less than 100 mm and weighted down. That is a cover laid on the ground to slow moisture coming up out of the soil.

For a heated crawl space, Article 9.18.6.2 asks for something different in kind. Polyethylene sheet not less than 0.15 mm, conforming to CAN/CGSB-51.34-M, installed as part of an air barrier system in accordance with Subsection 9.25.3. Joints lapped not less than 300 mm and either sealed and evenly weighted down, or covered with concrete not less than 50 mm thick. The perimeter sealed to the foundation wall. All penetrations sealed against air leakage.

Those two paragraphs describe different jobs. The first is a sheet on the dirt. The second is a continuous sealed surface tied into the air barrier of the house, with every pipe and post that passes through it sealed around.

Ventilation flips too. Article 9.18.3.1 requires unheated crawl spaces to be ventilated by natural or mechanical means, and where natural ventilation is used, not less than 0.1 m2 of unobstructed vent area for every 50 m2 of floor area. Those vents shall be uniformly distributed on opposite sides of the building, and designed to prevent the entry of snow, rain and insects. Article 9.18.3.2 sends heated crawl spaces somewhere else entirely: heated crawl spaces shall be ventilated in accordance with Section 9.32, which is the mechanical ventilation section that governs the rest of the house.

Walls change as well. Sentence (3) of Article 9.18.1.3 requires that insulation, an air barrier system and a vapour barrier be installed in the walls of heated crawl spaces in accordance with Section 9.25. An unheated crawl space has no such requirement for its walls, because the insulation sits in the floor above instead.

The half-and-half crawl space, and why it rots

The damaging case is the one nobody chose. A crawl space becomes heated because of the ducts running through it, and the perimeter vents stay open because they were there when the house was built and nobody had a reason to touch them.

Two things then happen at once. Warm air leaking from the ducts fills the space and leaves through the vents, so the furnace is heating outdoor air all winter. That part costs money and shows up on a bill.

The second part costs a building. North Shore air carries a lot of moisture for most of the year. When that air comes through the vents and meets a cool surface, the moisture in it condenses into liquid water on that surface. In a crawl space the cool surfaces are the foundation walls, the underside of the subfloor, the joists, and any metal ductwork. Water sits there and does not dry, because a crawl space has no sun and little air movement.

The result is what we find under floors on renovation projects: dark staining across the joists, insulation that has fallen out of the bays and is soaked through, rusted duct seams, and a smell that comes up through the floor registers into the bedrooms.

The vents themselves are doing exactly what they were designed to do. They belong to a crawl space that stays cold and dry and is separated from the house above. Once heat enters that space and the vents stay open, you are paying to warm air that leaves through the vents, while those same vents keep drawing humid outdoor air onto cool surfaces.

• Warm air escaping unsealed ducts raises the temperature and the moisture content of the crawl space air.
• Open perimeter vents pull in humid outdoor air and let the heated air out.
• Incoming moist air condenses on cool joists, subfloor, foundation walls, and metal ducts.
• Wet insulation stops insulating and holds water against the wood it is touching.
• Air from the crawl space moves upward into the house, carrying the smell and the humidity with it.

Pick one and build it all the way

There are two complete answers, and each one works when it is built completely.

Keep it unheated. Seal and insulate every heating duct in the space so it stops losing heat into it. Insulate the floor above and complete the air barrier between the crawl space and the house per Section 9.25, so the space is genuinely separated. Keep the vents open and sized per Article 9.18.3.1: 0.1 m2 of unobstructed vent area for every 50 m2 of floor area, uniformly distributed on opposite sides of the building, protected against snow, rain and insects. Lay ground cover per Article 9.18.6.1.

Make it heated on purpose. Close the vents. Insulate the foundation walls and install the air barrier and vapour barrier per Section 9.25. Lay 0.15 mm polyethylene as part of the air barrier system, lapped 300 mm at joints, sealed and weighted or covered with 50 mm of concrete, sealed to the foundation wall around the whole perimeter, with every penetration sealed. Then ventilate the space per Section 9.32, which means it joins the mechanical ventilation system serving the house.

The second option costs more up front and it is the one we recommend on most North Shore houses where ducts or plumbing already run below the floor. The crawl space joins the conditioned part of the house, the ducts stop losing heat to a space that is outside the insulation, and the floors above stop being cold.

What fails is doing half of either one. Closing the vents without laying a sealed ground cover traps soil moisture in a space with no way out. Insulating the walls without sealing the poly perimeter leaves an air path from the ground into the wall assembly. Sealing the ground but leaving the vents open means paying to heat the outdoors.

Water has to be dealt with before anything else

None of the above survives a wet crawl space, and the Code puts that requirement first for a reason. Article 9.18.5.1 states that except where it can be shown to be unnecessary, the ingress of water into a crawl space shall be controlled by grading or drainage. Sentence (2) sends drainage of foundation walls to Article 9.14.2.1, and Sentence (3) sends drainage of the ground cover or floor-on-ground to Subsection 9.16.3.

Grading is the ground around the house sloping away from the walls so rain runs off instead of soaking in beside the foundation. Drainage is the perforated pipe at the footing that collects water in the soil and carries it away. Both fail quietly on older North Shore houses. Perimeter drain pipe from the 1960s is often crushed, full of roots, or plugged with silt, and the downpipes that were meant to feed it have been disconnected during past work.

Many lots here also sit over ground where the water level rises close to the surface in winter. Water finds the crawl space, and a sealed polyethylene ground cover then holds it where you can see it. That is a useful outcome, because the alternative is water moving up through the soil as vapour into a space you never look at.

Our order of work is fixed: fix grading and perimeter drainage first, confirm the space stays dry through a wet stretch, then build the ground cover and the air seal. Doing it the other way around means opening the work again.

Getting in, and getting to the equipment

Two articles in Section 9.18 deal with physical access, and they are the ones most often missed on renovations because nothing about them shows in a finished room.

Article 9.18.2.1 requires an access opening of not less than 500 mm by 700 mm to each crawl space where the crawl space serves a single dwelling unit, and not less than 550 mm by 900 mm for other crawl spaces. Sentence (2) requires the opening to be fitted with a door or hatch, except where the crawl space is heated and the access opening is from an adjacent heated space. That exception follows the same logic as the rest of the Section. Once the crawl space is inside the heated envelope, a hatch separating it from the heated room above has nothing left to separate.

Article 9.18.4.1 covers getting to equipment. Where equipment requiring service such as plumbing cleanouts, traps and burners is located in crawl spaces, an access way with a height and width of not less than 600 mm shall be provided from the access door to the equipment, and for a distance of 900 mm on the side or sides of the equipment to be serviced.

That 600 mm clear path is a real constraint when you are adding new insulation to the underside of the floor or building up a slab over the ground cover. Both eat headroom. On a crawl space with 900 mm of clearance to start with, the plan has to account for the path before the insulation is specified.

It also decides where you can place equipment. A cleanout tucked behind a beam with 400 mm of crawl clearance in front of it is a plumber's call-out that costs several hours, every time.

• Access opening: 500 mm by 700 mm minimum for a single dwelling unit, 550 mm by 900 mm otherwise.
• Door or hatch required, unless the crawl space is heated and the opening is from an adjacent heated space.
• Access way to serviceable equipment: 600 mm high and 600 mm wide, from the access door to the equipment.
• Plus 900 mm of clearance on each side of the equipment that has to be serviced.

If your crawl space is the duct

A small number of older houses here were built with the crawl space itself acting as the warm air plenum. The furnace blows into the space, the space pressurizes, and warm air rises through registers cut into the floor. Article 9.18.7.1 governs those, and its limits are narrow.

Only crawl spaces under one-storey portions of dwelling units may be used as warm air plenums. A two-storey section of the house cannot be heated this way.

The enclosing material, including insulation, shall have a flame-spread rating not greater than 150. That rules out several common insulation and sheet products that would be acceptable in an ordinary crawl space, because the space is now carrying the air people breathe and any fire in it feeds directly into the rooms above.

Combustible ground cover has to be protected beneath each register opening with noncombustible material extending not less than 300 mm beyond the projection of the register opening, with up-turned edges. That detail exists for anything dropped or falling through a floor register.

If you have this system and you are renovating, treat it as a decision point. Converting to ducted supply removes a set of constraints that will otherwise govern every material choice you make down there.