A permanent wood foundation (PWF) is an engineered construction system that uses load-bearing exterior light-frame wood walls in a below-grade application. A PWF consists of a stud wall and footing substructure, constructed of approved preservative-treated plywood and lumber, which supports an above-grade superstructure. Besides providing vertical and lateral structural support, the PWF system provides resistance to heat and moisture flow. The first PWF examples were built as early as 1950 and many are still being used today.
A PWF is a strong, durable and proven engineered system that has a number of unique advantages:
- energy savings resulting from high insulation levels, achievable through the application of stud cavity insulation and exterior rigid insulation (up to 20% of heat transfer can occur through the foundation);
- dry, comfortable living space provided by a superior drainage system (which does not require weeping tile);
- increased living space since drywall can be attached directly to foundation wall studs;
- resistance to cracking from freeze/thaw cycles;
- adaptable to most building designs, including crawl spaces, additions and walk-out basements;
- one trade required for more efficient construction scheduling;
- buildable during winter with minimal protection around footings to protect them from freezing;
- rapid construction, whether framed on site or pre-fabricated off-site;
- materials are readily available and can be efficiently shipped to rural or remote building sites; and
- long life, based on field and engineering experience.
PWFs are suitable for all types of light-frame construction covered under Part 9 ‘Housing and Small Buildings’ of the National Building Code of Canada (NBC), that is, PWF can be used for buildings up to three-storeys in height above the foundation and having a building area not exceeding 600 m2. PWFs can be used as foundation systems for single-family detached houses, townhouses, low-rise apartments, and institutional and commercial buildings. PWFs can also be designed for projects such as crawlspaces, room additions and knee-wall foundations for garages and manufactured homes.
There are three different types of PWFs: concrete slab or wood sleeper floor basement, suspended wood floor basement and an unexcavated or partially excavated crawl space. Lumber studs used in PWF are typically 38 x 140 mm (2 x 6 in) or 38 x 184 mm (2 x 8 in), No. 2 grade or better.
Improved moisture control methods around and beneath the PWF result in comfortable and dry below-grade living space. The PWF is placed on a granular drainage layer which extends 300 mm (12 in) beyond the footings. An exterior moisture barrier, applied to the outside of the walls, provides protection against moisture ingress. Caulked joints between all exterior plywood wall panels and at the bottom of exterior walls is intended to control air leakage through the PWF, but also eliminates water penetration pathways. The result is a dry basement that can be easily insulated and finished for maximum comfort and energy conservation.
All lumber and plywood used in a PWF, except for specific components or conditions, must be treated using a water-borne wood preservative and identified as such by a certification mark stating conformance with CSA O322. Corrosion-resistant nails, framing anchors and straps that are used to fasten PWF-treated material must be hot-dipped galvanized or stainless steel. Exterior moisture and vapour barriers must be at least 0.15 mm (6 mil) in thickness. Dimpled drainage board is often specified as an exterior moisture barrier.
For further information, refer to the following references:
Permanent Wood Foundations (Canadian Wood Council)
Permanent Wood Foundations – Durable, Comfortable, Adaptable, Energy efficient, Economical (Wood Preservation Canada and Canadian Wood Council)
Wood Design Manual (Canadian Wood Council)
Wood Preservation Canada
CSA S406 Specification of permanent wood foundations for housing and small buildings
CSA O322 Procedure for certification of pressure-treated wood materials for use in permanent wood foundations
CSA O86 Engineering design in wood
National Building Code of Canada