Sealed Crawl Space

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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A sealed crawl space is part of the building. Get the water out first, then seal it by design.

The big idea

A sealed crawl space is an unvented space treated as part of the building. Instead of venting it to the outdoors, you cover the ground, insulate the walls, seal the air leaks and keep the space dry. Done right it cuts moisture, mould, rot and heat loss. Done badly it traps moisture, so the details matter.

A Building Science Corporation article by Joseph Lstiburek (March 2020) puts the idea simply: a crawl space should be either fully connected to the conditioned house or fully separated from it, and a partial connection causes moisture problems. He calls the conditioned approach the "mini basement" and the first choice. It is written for a US audience, and the code rules differ here.

What the code says

From Part 9, Section 9.18, as printed in the Vancouver Building Bylaw 2014 (a municipal reprint, possibly older than your province's edition):

• A crawl space is treated as heated if it is used as a hot-air plenum, has heating ducts that are not sealed and insulated, or is not separated from heated space under Section 9.25 (Article 9.18.1.3).
• An unheated crawl space needs 0.1 m² of unobstructed vent area for every 50 m² of floor area, spread evenly on opposite sides of the building (Article 9.18.3.1). Heated crawl spaces must be ventilated under Section 9.32 (Article 9.18.3.2).
• Ground cover for an unheated crawl space can be 0.10 mm polyethylene with joints lapped at least 100 mm and weighted down (Article 9.18.6.1).
• For a heated crawl space, ground cover shall be not less than 0.15 mm polyethylene as part of the air barrier system, joints lapped at least 300 mm and either sealed and evenly weighted down or covered with at least 50 mm of concrete, with the perimeter sealed to the foundation wall and all penetrations sealed against air leakage (Article 9.18.6.2).
• Water entering the crawl space shall be controlled by grading or drainage, unless shown to be unnecessary (Article 9.18.5.1).
• Access openings shall be at least 500 mm by 700 mm (19 3/4 in by 27 1/2 in) for a single dwelling unit (Article 9.18.2.1).
• Insulation and other enclosing materials shall have a flame-spread rating not greater than 150 (Article 9.18.7.1).

I found no sentence there that creates a "sealed, unvented crawl space" as its own category. How one is approved in your province is a question for your building department and designer, and it depends on your edition.

The parts of a sealed crawl space

• Water out first. Sealing does not fix a wet crawl space. Get the water out and keep it out (see Drain Tile, Gravel Beds and Sump Systems), then seal.
• Ground cover: a continuous, vapour-impermeable cover that also runs across posts and piers (Building Science), at least the code minimum above, with the seams lapped and taped and run up the walls. A heavier reinforced sheet is common practice. I dropped the old 0.25 to 0.5 mm range because I could not verify it. Ask your designer for the product.
• Gravel or a drainage layer under the cover where the site is wet.
• Wall insulation: airtight rigid foam or spray foam on the foundation walls, not on the floor above, with a covering where the code requires it. Building Science recommends this for a conditioned crawl space.
• Air sealing: close the vents and seal the gaps and penetrations.
• Conditioning: the space has to be dried and kept dry. Building Science lists options, including supply air with a transfer grille to the house, a return duct, a supply fan, an exhaust fan, and a dehumidifier. It says conditioning means controlling temperature and relative humidity, and it gives no single humidity target. I dropped the old 45 to 55% target because I could not verify it, so your designer sets it. That article cites a US code rate of 50 cfm per 1,000 ft² (about 25 L/s per 100 m²) of supply air or dehumidification, which is not a Canadian rule.
• Radon: Building Science says to address soil gas with a vent under the poly, as in basements. Check your province's radon requirements.

Ground cover installation

• Lap seams at least the code lap (300 mm / 12 in for a heated space in the text I read) and tape them with a compatible tape. Follow the maker's lap if it is larger.
• Run the cover up the wall and fasten it, and seal the perimeter to the wall (Article 9.18.6.2). The height up the wall comes from your design.
• Seal around posts, pipes and piers.
• Do not tear it. Lay the sheets so people walk on it as little as possible.

Example (numbers made up). A crawl space 8 m × 10 m (26 ft 3 in × 32 ft 10 in) with the cover running 0.3 m (1 ft) up a 36 m (118 ft 1 in) perimeter needs 80 + 36 × 0.3 = about 90.8 m² (977 ft²) of poly before laps and waste. For comparison, a conventional vented space of the same area would need 0.1 ÷ 50 × 80 = 0.16 m² (1.7 ft²) of vent area under the text above.

Common mistakes

• Leaving the vents open on a space that is meant to be sealed
• Thin poly that tears
• Unsealed seams and edges
• Insulating the floor above and not the walls
• No drainage under the cover
• No drying or conditioning
• Sealing a space with a fuel-burning appliance