Wood Foundations

Summary

Permanent wood foundations have an excellent track record for durability and homeowner comfort. Over 300,000 US homes have them, and some systems have transferable warranties. They install quickly and offer good thermal qualities. Wood foundations are mostly used in cold weather installations where there might be concrete setting problems.

Details

Wood foundation walls are typically made of 2 x 8 studs at 16" on center and sheathed with 1/2" plywood on one side. Midwest Panel Systems produces a wood foundation system made of structural insulated panels. Crushed stone footings provide structural support and drainage; some systems use a concrete footing. Fasteners are stainless steel.

CCA (chromated copper arsenate) treatment permanently protects the wood from decay and termites, and is applied under pressure to achieve retention rates approximately 50% higher than codes require for many other wood/ground contact applications. CCA wood treatment is by far the preferred treatment, though there are a few other methods. ACZA treatment is used for species that are difficult to penetrate, such as Coastal Douglas Fir and some other western woods. ACQ Type B is sometimes used for its benign formulation, although it may be slightly less effective than CCA. ACA is more expensive, but its toxicity is so low it is used for food containers.

Properly installed, wood foundations are level, plumb, and square. Wood's superior R-value (compared to concrete) inhibits condensation, making the basement dryer. Wood foundations are also easier to modify if, for instance, an addition is built.

Installation

The first layer following excavation is gravel, coarse sand, or crushed stone. A concrete slab can then be poured (or a wood floor installed) over a vapor barrier. Wood floors are installed on sleepers. A sump pit with drain piping is usually recommended. The foundation panels are laid on a gravel, sand, or stone footing, and insulated, wired, plumbed, and finished like a typical stud wall. A layer of polyethylene film is applied to the outside before backfilling, and a treated plywood strip covers the exposed poly between grade and siding. The Southern Pine Council publishes a useful Permanent Wood Foundation Design Manual, which has detail construction drawings and photos.

Benefits/Costs

Wood foundation construction is considered easy and fast compared to standard poured concrete foundations. A trade can be eliminated from the job as there is no concrete work necessary, and installation is possible in virtually any weather. Walls conduct less heat out of the space and homeowners report that basements are less damp, musty, and cool than concrete basements. Some space is saved because the insulated wall thickness is less, and sleeving/furring for services is not needed. Cost is generally less than concrete, approximately $45 per foot of 8' wall, which includes foundation coating, 6 mil. poly vapor barrier, and hardware. Woodmaster offers a 75-year warranty.

Limitations

Some people may be skeptical about the durability or strength of wood foundations.

Code/Regulatory

Wood foundations are accepted as permanent foundations by all the model building codes as well as lending and insurance companies. Codes often refer to American Wood Preservers Association (AWPA) standards. They require certain species of Pine or Fir, limited to 20% heartwood, treated to 0.6 preservative retention (compared to 0.25 to 0.4 for regular treated wood), kiln-dried, and stamped from treater. The four treatment methods approved by AWPA are CCA, ACZA, ACQ Type B, and ACA.

Availability

Stock and custom sizes are available. Manufacturers tend to be in the northern states, so shipping to southern areas may not be cost-effective.

Contact(s)

Do you have a specific question about this technology and/or its 'real life' applications? Try the contacts listed below:

Southern Pine Council
Southern Forests Products Association
P.O. Box 641700
Kenner LA 70064-1700
504-443-4464
http://www.southernpine.com

American Forest and Paper Association/American Wood Council
1111 19th St. NW Ste. 800
Washington D.C. 20036
202-463-2700
http://www.awc.org

Hickson Corp.
1955 Lake Park Drive Ste. 250
Smyrna GA 30080
770-801-6600

Woodmaster Foundations Inc.
845 Dexter St.
P.O. Box 66
Prescott WI 54021
715-262-3655

Midwest Panel Systems
P.O. Box 38
Blissfield MI 49228
517-486-4844
http://www.insulspan.com

Hoover Treated Wood Products, Inc.
P.O. Box 746
Thomson GA 30824
http://www.frtw.com