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Joseph Lstiburek’s BSI-020 describes wood foundations as a workable alternative to conventional concrete or masonry when every control layer is continuous and the wood is kept warm, dry, drained and structurally supported. The preferred basement detail still starts with concrete footings; other options include a conditioned crawlspace, a wood slab-on-grade and a frost-protected shallow foundation. None is a universal prescriptive design or proof that wood is the better choice for every site.
The practical lesson is straightforward: wood does not forgive gaps in drainage, membranes, insulation, air control, load paths or anchorage. A project needs current local code review, site-specific moisture and frost analysis, and structural design before construction.
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What BSI-020 means by a wood foundation
Building Science Corporation author Joseph Lstiburek published BSI-020 on January 25, 2021. He describes building wood foundations in the early 1980s, learning from details that worked and those that failed, and eventually favoring concrete and masonry because the extra effort did not seem worthwhile. He revisits the subject because clients want to reduce concrete and masonry use.
His central objective has not changed: “The plan was to keep the wood warm and dry.” That requires more than pressure-treated framing. Water must be directed away below and outside the assembly, the wood must be separated from soil, insulation must remain continuous, and the air-control and water-control layers must cross every joint.
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Lstiburek discusses four related assemblies:
- A below-grade wood basement that still uses concrete footings.
- A conditioned crawlspace treated as a small basement.
- A wood slab-on-grade foundation.
- A shallow frost-protected foundation with horizontal insulation at grade.
These are conceptual assemblies from an experience-based technical article, not stamped drawings, product approvals or current code determinations.
The control-layer strategy
Keep bulk water moving away
The original approach used drainage outside and underneath the foundation, with exterior insulation separating wood from wet soil. Historical materials included polyethylene sheet, grooved EPS and XPS, rigid fiberglass boards and fiberglass roofing insulation. Lstiburek describes an evolution from polyethylene toward fully adhered membrane as the primary water-control layer.
Exterior insulation is not merely thermal trim. It protects the membrane, keeps framing warmer, and forms part of the drainage strategy. Drainage material, protection boards and backfill must be selected for the actual soil, groundwater and loading conditions. BSI-020 supplies no site-specific drainage design or product rating.
Keep air, vapor and thermal layers continuous
Joints are the weak point in a panelized foundation. A sealant bead or gasket can fail, move, become contaminated or leave an unintended path. Lstiburek’s recommendation is to place waterproofing and air control in fully adhered membranes across the exterior of the panels, then install continuous exterior draining insulation. The membrane should not stop at a panel edge or depend on a joint seal alone.
Prevent insulation from becoming an insect highway
Exterior insulation can create a concealed route for insects into the building. The assembly therefore needs membrane continuity and deliberate insect protection at transitions, edges and penetrations. The article identifies this as a detailing obligation, not as a reason to omit exterior insulation.
How the recommended wood basement is assembled
Lstiburek’s preferred basement sequence combines wood framing with concrete footings and several layers of control. The order below describes the concept; an engineer and the authority having jurisdiction must adapt it to the site.
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- Prepare the bearing surface and drainage plane. Establish a stable, level base and provide drainage below the floor assembly. Remote sites can make concrete delivery difficult, while leveling crushed stone accurately enough for a foundation can also be challenging.
- Place rigid insulation below the floor slab. The insulation reduces heat loss and helps keep the wood floor framing warm. The required type, thickness and compressive capacity are project-specific and are not stated in BSI-020.
- Install a fully adhered membrane. Carry the water-control membrane over the floor assembly and out across the perimeter footings. The membrane should remain continuous at corners and transitions.
- Frame the floor and exterior walls. Build the wood structure on the prepared, isolated assembly. Design bearing, lateral resistance and connections for soil and building loads rather than treating the framing as ordinary above-grade construction.
- Apply the exterior water-control layer. Install adhered membrane over the exterior of the framed walls and connect it to the membrane under or over the floor as detailed by the designer.
- Add continuous exterior draining insulation. Keep the insulation unbroken around the foundation where possible, protect it from damage and provide a drainage path to the site system.
- Detail penetrations, corners and the top termination. Windows, service entries, the rim area and the connection to above-grade walls must preserve air, water and thermal continuity.
This sequence explains why a wood basement is not simply a concrete basement with timber substituted for concrete. The membrane, insulation, drainage and connection details are part of the structural durability strategy.
Panelized foundations: faster framing, harder joints
Factory or site-built panels can make a wood foundation easier to handle and assemble, especially where concrete logistics are difficult. The trade-off is joint control. Lstiburek reports that sealants and gaskets did not reliably produce the two-stage joints he wanted.
His corrective approach is to let the exterior fully adhered membrane provide the continuous water and air control across panel joints, followed by continuous draining exterior insulation. Panel edges still need accurate alignment, mechanical fastening and a designed load path. Membranes cannot compensate for a joint that is structurally misaligned or unable to resist backfill pressure.
At each joint, the installer should verify:
- The membrane overlaps or transitions exactly as specified.
- Substrate surfaces are clean, dry and compatible with the adhesive.
- Fasteners do not create unsealed penetrations.
- Insulation remains continuous and protected from construction damage.
- The air-control layer connects to the floor, rim and above-grade wall layers.
A conditioned crawlspace is a small basement
Lstiburek favors a conditioned crawlspace rather than a vented, damp enclosure. In building-science terms, it is a low basement: insulated, within the thermal and air boundary, and protected from ground moisture. Treating it this way avoids relying on outdoor air to dry a space that may be cooler than the surrounding soil.
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Omitting continuous concrete footings does not eliminate concentrated forces. Posts, beams, girders and roof reactions can create point loads that need a deliberate load path. Lstiburek’s example uses concrete pads and steel columns where those loads occur.
A crawlspace design should therefore map every concentrated reaction from roof and floor framing to soil, check bearing capacity, and provide access for inspection and maintenance. The article does not prescribe pad dimensions, column sizes, reinforcement, soil values or a universal pier layout.
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Wood slab-on-grade: the least risky option in Lstiburek’s judgment
Lstiburek calls wood slab-on-grade the least risky of the wood-foundation options in his opinion, while noting that he almost never sees it built. That is an experienced judgment, not a comparative failure study.
The conceptual build-up starts with a prepared gravel pad and a complete set of control layers:
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- Grade and compact the gravel so the surface is stable and accurately level.
- Install the specified water, air and vapor-control membranes.
- Form a perimeter wood footing with its required separation and protection from ground moisture.
- Place continuous rigid insulation at the perimeter and beneath or beside the assembly as designed.
- Frame the floor and install the subfloor over the insulated, protected base.
BSI-020 depicts two possible perimeter-footing placements, including one located over XPS. The correct arrangement depends on bearing, moisture exposure, thermal bridging and local requirements; the article supplies no thickness, strength rating or prescriptive detail.
Frost-protected shallow wood foundations
The frost-protection discussion uses horizontal insulation extending outward at grade to limit frost penetration beneath a shallow foundation. Lstiburek says a shallow frost-protected foundation can be wood, but he provides no climate-specific frost depth, insulation thickness, soil calculation or approved detail.
That information must come from the project’s jurisdiction, climate data, geotechnical conditions and structural design. Frost protection is not achieved by choosing a generic XPS board from a catalog. The designer must establish the required thermal resistance, dimensions, edge conditions, drainage and protection from damage.
Horizontal insulation also changes how water, snow, landscaping and future excavation interact with the foundation. Keep the skirt continuous, protect its outer edge, and coordinate it with the finished grade and drainage plan.
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Structural, wind and grounding issues that cannot be skipped
Backfill and point loads
Below-grade walls resist lateral soil and water pressure in addition to the building’s vertical loads. Floor diaphragms, wall framing, fasteners, connections and temporary bracing all need design for the actual backfill sequence. Concentrated reactions require pads, beams, columns or other supports that transfer force safely to competent soil.
Wind uplift and overturning
All-wood buildings need positive anchorage. Lstiburek writes: “You need to anchor these puppies to the ground just like mobile homes….and don’t forget to ground your electrical system.” The article does not specify anchor type, spacing, embedment, design wind speed or uplift and overturning loads. Those values must be calculated for the site and building geometry.
Electrical grounding
An all-wood structure still requires a code-compliant grounding and bonding system. Because BSI-020 gives no grounding method, electrode arrangement or conductor sizing, the electrical designer must coordinate the foundation and utility details with the current electrical code.
Insects and physical damage
Membranes and insulation need protection from termites, carpenter ants, rodents, tools, sunlight and backfill operations. Specify inspection zones and durable protection at exposed edges. Do not bury a damaged membrane or assume that insulation alone is an insect barrier.
Materials named in BSI-020 and their roles
| Material or category | Role discussed | What the article does not establish |
|---|---|---|
| Fully adhered membrane | Continuous water and air control across floors, walls and panel joints; evolved from earlier polyethylene use. | Product brand, thickness, adhesion test, warranty or code approval. |
| XPS or EPS rigid insulation | Below-slab, exterior draining and frost-protection insulation in the described concepts. | Compressive strength, R-value, thickness, fire performance or site suitability. |
| Rigid fiberglass, fiberglass roofing insulation and mineral wool | Insulation options discussed across historical and contemporary assemblies; modern mineral wool can serve functions formerly handled by fiberglass board, EPS or XPS. | Comparative performance ratings or a product-specific recommendation. |
| Concrete footings and pads | Continuous support for the basement concept and concentrated support in the crawlspace example. | Dimensions, reinforcement, soil bearing value or mix specification. |
| Steel columns | Support for point loads where a crawlspace omits conventional footings. | Section size, corrosion protection, base connection or allowable load. |
| Cellular PVC or other protection boards | Potential physical protection for membranes and insulation where required by the assembly. | Exact placement, thickness or approved product. |
Wood versus concrete or masonry: a qualitative decision
BSI-020 does not report costs, carbon totals, measured failure rates or a head-to-head performance study. The useful comparison is therefore qualitative.
| Decision axis | Questions for a wood foundation | Questions for any alternative |
|---|---|---|
| Moisture control | Can drainage, membrane continuity and exterior insulation keep framing warm and dry? | How will the wall resist groundwater, leaks and drying limitations? |
| Air and thermal continuity | Can membranes and insulation cross every panel joint and transition? | Where are thermal bridges, joints and air leaks? |
| Structural support | How are backfill, point loads, uplift and overturning transferred to soil? | What footing, wall and connection design handles the same loads? |
| Site and climate | Are groundwater, soil, frost and exposure compatible with the proposed details? | Does the alternative simplify or complicate those site conditions? |
| Construction logistics | Will panel handling and membrane work be easier than concrete delivery and precise aggregate leveling? | Which method fits access, labor, weather and sequencing? |
| Durability coordination | Can the project coordinate insects, protection, drainage and grounding? | What maintenance and inspection obligations remain? |
A preconstruction checklist
- Confirm the foundation type is permitted by the current local building code and zoning requirements.
- Obtain site information on soil bearing, groundwater, drainage, frost conditions and termite exposure.
- Have a qualified structural designer size footings, pads, walls, columns, fasteners and anchorage for actual loads.
- Draw one continuous control-layer line for water, air, vapor and thermal protection through every section and joint.
- Specify membrane compatibility, substrate preparation, protection boards, drainage outlets and repair procedures.
- Coordinate windows, utilities, stairs, decks, porches and below-grade penetrations before framing.
- Plan inspections before backfill, including membrane integrity, insulation continuity, fasteners and drainage.
- Provide a code-compliant electrical grounding and bonding design.
- Document backfill material, lifts, compaction limits and equipment access so walls are not overloaded during construction.
- Keep photographs and as-built dimensions for concealed membranes, insulation, anchors and supports.
What the article does—and does not—prove
BSI-020 demonstrates a way to think about wood foundations: treat moisture, air, heat, insects and structure as one connected system. It does not establish that wood foundations cost less, have lower embodied carbon, last longer, or comply automatically with a particular jurisdiction’s code. It also does not provide a universal detail for a two-story house, a specific soil, a particular frost zone or a retail insulation product.
A Green Building Advisor episode overview reports 1937 as the first documented use of permanent wood foundations, but that historical statement comes from the 2020 secondary page rather than BSI-020 itself. Reader discussions asking about uplift, overturning and how to start as a first-time builder identify legitimate design questions; comments are not engineering authority.
Bottom line for a project team
Choose a wood foundation only when the project can deliver the detailing discipline it demands. The strongest concept in BSI-020 pairs concrete support where needed with fully adhered membranes, continuous exterior draining insulation, deliberate drainage, protected joints and engineered anchorage. A conditioned crawlspace, slab-on-grade or frost-protected shallow foundation can be viable in the right conditions, but each requires its own load path, moisture strategy and code review.
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