PWF Perfection: FPL Demo House 15 Years Later

The following blog has been adapted from The Wood Handbook, Wood as an Engineering Material.

Light-frame buildings with basements are typically supported on cast-in-place concrete walls or concrete block walls supported by footings. This type of construction with a basement is common in northern climates.

Another practice is to have concrete block foundations extend a short distance above ground to support a floor system over a “crawl space.” In southern and western climates, some buildings have no foundation; the walls are supported by a concrete slab, thus having no basement or crawl space.

The Research Demonstration House was built in 2001 for research and education at FPL.

The Research Demonstration House utilizes a wooden basement, and was built in 2001 for public education and scientific studies at FPL.

But treated wood can also used for basement foundation walls. Basically, such foundations consist of wood-frame wall sections with studs and plywood sheathing supported on treated wood plates, all of which are preservatively treated to a specified level of protection. To distribute the load, the plates are laid on a layer of crushed stone or gravel.

The walls, which must be designed to resist the lateral loads of the backfill, are built using the same techniques as conventional walls. The exterior surface of the foundation wall below grade is draped with a continuous moisture barrier to prevent direct water contact with the wall panels. The backfill must be designed to permit easy drainage and provide drainage from the lowest level of the foundation.

basement

The wooden foundation was constructed during the brutal Wisconsin winter, where conventional masonry would have been difficult.

Because a foundation wall needs to be permanent, the preservative treatment of the plywood and framing and the type of fasteners used for connections are very important. A special foundation (FDN) treatment has been established for the plywood and framing, with strict requirements for depth of chemical penetration and amount of chemical retention. Corrosion-resistant fasteners (for example, stainless steel) are recommended for all preservative-treated wood.

This construction technique is exemplified at the Forest Product Laboratory’s (FPL) Research Demonstration House (take a virtual tour here). The basement walls and floor are constructed of pressure-treated Southern Pine lumber and plywood to create the permanent wood foundation (PWF). The PWF is designed to resist and distribute earth, wind, and seismic forces and resist termite attack.

Thanks to insulated walls, the basement of the Research Demonstration House stays warm year-round.

Thanks to insulated walls, the basement of the Research Demonstration House stays warm year-round.

The walls consist of nominal 2- by 10-inch treated lumber supports and nominal 3/4-inch-thick plywood sheathing. The basement of the Research Demonstration House was constructed in freezing temperatures during the middle of winter when construction of a masonry or poured concrete foundation would have been very difficult.

For more than a decade, the basement of the Research Demonstration House has stayed dry, warm, and structurally sound thanks to expert engineering and it’s PWF. It stands as an example of how wood, usually reserved for above-ground construction, can be a valuable subterranean asset, and pushes the boundaries of what can be done with mankind’s most important building material.

For more information on the use of wood in buildings and bridges, please see Chapter 17 of The Wood Handbook, Wood as an Engineering Material.