Basements, Crawlspaces, and Slabs

Basements

The basement is the default in most of Wisconsin: footings have to be 4 feet down anyway, so a few more feet buys a full story of space for mechanicals and storage.

Walls. A basement wall is a retaining wall: soil pushes on it, harder the deeper it goes. The UDC gives lateral soil pressures by backfill type, from 30 psf per foot of depth for clean gravel and sand to 60 for clays (more for rigid walls), and some soils, like organic clays and high-plasticity clays, are unsuitable as backfill ¹. Wall thickness then comes from the code’s tables by material and the height of unbalanced fill (outside grade minus basement floor), and is never thinner than the wall it carries ¹. The wall needs lateral support at the bottom (the slab) and at the top (anchor bolts and the floor framing) ¹: backfilling before the first floor is framed is how basement walls get pushed in.

Floor. A concrete floor must be at least 3 inches thick; in clay soils it sits on a 4-inch base of clean sand, gravel, or crushed stone, and with drain tile it needs that base regardless ¹. A vapor retarder goes directly under the slab or under its base: at least 6 mils or reinforced, lapped 6 inches and taped ².

Insulation. Basement walls in the thermal envelope get R-15 continuous insulation or R-19 in a framed cavity (or R-13 cavity plus R-5 continuous) ².

Crawl spaces

A crawl space lifts the floor framing above the ground on short foundation walls. Wisconsin’s energy code treats it as part of the house ²:

• Vapor retarder: all exposed earth covered by a continuous vapor retarder, after removing topsoil and other decayable organic material; joints overlapped 6 inches and taped, edges run 6 inches up the wall and sealed to the wall or its insulation.
• Insulation: crawl space walls insulated to the energy table, fastened permanently and covering the full height.
• Frost: if the crawl floor is less than 48 inches below grade, the foundation must be frost-protected like a frost-protected slab; otherwise footings go below frost.

The construction code adds an access opening of at least 14 by 24 inches for crawl spaces with 18 inches or more of clearance ¹, and wood within 18 inches of crawl-space soil without a vapor retarder has to be treated or decay-resistant ¹.

Slabs on grade

A slab on grade puts the floor right on the ground. In Wisconsin it still has to deal with frost:

• Thickened edge on frost footings: the slab’s perimeter rests on footings and walls below frost or 48 inches.
• Frost-protected shallow foundation (FPSF): rigid foam around and under the slab edge keeps the ground below from freezing, so the edge can be shallow. It must be designed to ASCE 32 ¹.
• A house on a floating slab with no frost footing must be designed by structural analysis ¹.

A heated slab needs insulation under the whole slab and at the edge (R-10 under and R-15 at the edge in the prescriptive table), and an unheated slab near grade needs R-10 edge insulation running 48 inches down or under ². Garage slabs are at least 4 inches thick on 4 inches of granular fill, sloped to the door or a floor drain ¹.

Choosing

• Basement: most space per dollar where footings are deep anyway; needs good drainage (later lesson).
• Crawl space: shallower digging, easy access to plumbing and wiring; must be sealed and insulated, not a damp, vented hole.
• Slab: least excavation, no stairs down; plumbing is buried in concrete; in Wisconsin, an FPSF or deep thickened edges.

What to take from this

Basement walls are retaining walls sized by soil and backfill height and braced top and bottom; their floors are 3 inches minimum over a 6-mil taped vapor retarder, and walls get R-15 continuous or R-19. Crawl spaces need a sealed vapor retarder over all earth, insulated walls, a 14 by 24 access, and frost protection if shallower than 48 inches. Slabs need frost footings or an ASCE 32 frost-protected design.