Slab Foundations

UPDATED 11/15/2012

Slab-on-Grade Foundations Work Everywhere

Bird's eye view

A slab is less expensive and quicker to install than other types of foundations

Many of the same precautions and construction details used elsewhere in a house must be used in a slab foundation in order to make it long-lasting, energy-efficient and impenetrable to air, moisture, and water.

A combination of rigid foam insulation and plastic can isolate a slab from soil moisture and swings in outdoor temperature.

See below for:

Key Materials

Use expanded or extruded polystyrene

The usual insulation material for use under a residential slab is extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.), which has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 5 per inch. However, some green builders prefer to used expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.), which has an R-value of about 3.6 per inch.

Either material should have adequate compressive strength for a typical residential slab. Type 2 EPS has a compressive strength of 16 psi. (A concrete slab exerts a force of only 1 psi). Although manufacturers of XPS suggest that underslab insulation should have a compressive strength of 25 psi — a figure that happens to match the compressive strength of the product they sell — the suggestion is self-serving.

For special commercial applications where very high compressive strength is a necessity, XPS is certainly best. Some high-strength XPS foams have a compressive strength of 100 psi.

Design Notes

Slabs can be great multi-taskers

Not all concrete slabs are rough, cold, and drab surfaces that need to be tiled over or banished to basements and garages. With the right insulation underneath and the right finish on top, a slab can be a warm and attractive floor fit for the nicest home. If you keep it exposed and you don't need additional flooring materials — that's resource efficiency.

Finishing a slab
Concrete is a very flexible material when it comes to finishes. Depending on the texture you like, it can be screeded, ground, polished, and even stamped. Aggregates or objects can be set in and polished smooth or left as is. Dyes can create an endless number of hues and patterns; waxes and other sealers provide sheens ranging from flat to glossy.

Builder Tips

Isolate the slab from the ground

A waterproof capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break (typically polyethylene) is required between a concrete slab and the ground to keep moisture from working its way up through the slab. Builders in some parts of the country place several inches of “blotter” sand between the concrete and the polyethylene ground cover, in hopes that the sand will help the concrete cure evenly and remain flat. But the Building Science Corp., among others, points out that blotter sand can hold moisture for months. This practice can cause a variety of problems with floor finishes and floor coverings and should be avoided. The correct location for polyethylene sheeting is directly under a slab.

The Code

Put a vapor retarder on top of a capillary break

Section 506 of the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. covers the thickness, preparation and reinforcement of concrete slabs on grade. Generally speaking, the site must be prepped for a slab by first removing all topsoil and vegetation, adding a four-inch capillary break comprised of sand, gravel, or blast-furnace slag (above-grade slabs can omit the capillary break), and installing a 6-mil polyethylene vapor retarder with 6-inch seam laps on top of the capillary break. Any reinforcement should be supported so it remains in the center to the top third of the slab.

Section 403.3 and the associated Table 403.3 cover construction and insulation requirements for Frost-Protected Shallow Foundations which are essentially slab-on-grade foundations insulated to prevent frost movement and associated damage. Frost-protected shallow foundations are only approved for buildings heated to a minimum of 64 degrees F throughout the year.

This information is from Code Check Building 2nd Edition. Buy the book at the Taunton Press online store.


Slabs are the cheapest way to get out of the ground and into the framing.
Slabs can be the finished floor which saves time, money, and materials.
Slabs store heat well making them ideal for radiant floor heating.
Rigid insulation under a slab keeps it warm and dry inside.
You can green a slab three ways: by increasing the insulation thickness, by substituting fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info for cement, or by making it thinner than a standard slab.


Turned-down Concrete Slab Details
Concrete Stem Wall Slab Details


Slabs are typically 4 in. thick, but some builders suggest that thinner slabs can be just as effective and represent substantial savings in materials. Fernando Pagés-Ruiz, author of Building an Affordable House (The Taunton Press, 2005), says that with the right subgrade and highly stable soil a nonstructural slab as thin as 2 in. makes an adequate subfloor. He says there’s no reason to make interior slabs any thicker than 3 in.


LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H 1/2 point available for fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info or slag as replacement for 30% or more of Portland cement in foundation under MR 2.2 (Materials & Resources).

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Chapter 6 — Resource Efficiency: 3 pts. for use of foundation systems, such as slabs, that are considered resource-efficient (601.8); up to 4 pts. for recycled-content (fly ash or slag substitution for Portland cement in concrete) (604.1). Under Ch. 7 — Energy Efficiency up to 4 pts. for use of slabs in as part of a passive solar design (704.3.1.3, 704.3.1.4).


Concrete slabs are usually less expensive than full-basement foundations and crawl spaces. Slabs require less site work, can be installed in less time, and require less concrete. As long as they are detailed properly, they can be used successfully in even the coldest parts of the United States.

Slab foundations can be the finished floor
Because of the high mass of concrete, slabs can absorb and hold radiant energy in a passive solar design, and they’re ideal for radiant-floor heating systems. Concrete can be acid-etched or dyed to become a very attractive finish floor; in some cases, such floors may cost less than other flooring options.

There are, however, a few drawbacks. Concrete slabs are not the best choice for sloping sites. Plumbing and other in-slab utilities must be carefully planned. Wiring and ductwork are more difficult to install, and alterations after the fact are challenging and usually expensive.


Insulated slabs save energy

Install a continuous layer of rigid foam insulation under the slab, except in the hottest climates or in termite-infested areas where local codes may prohibit the practice. The insulation buffers the slab from outdoor temperature swings, keeps the slab warm and dry, and lowers energy bills.

The U.S. Department of Energy estimates that even in a mild climate, R-10 slab insulation saves 20% more than it costs (in 2008 dollars) — a pretty good return on your money. A continuous layer of foam is particularly critical if the slab includes in-floor radiant tubing. Rigid foam insulation is also a good capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break, helping to keep a slab dry as well as warm.

One potential disadvantage of foam-insulated slabs is that the insulation can shelter termites from view as they tunnel upward and into wood framing. Because of this risk, some jurisdictions prohibit the use of foam in ground contact. Where these destructive insects are common, termite shields installed beneath the sill plate, foam treated with an insecticide (usually a derivative of boric acid), or conventional termite control may be necessary.


Monolithic slabs
Also called thickened-edge slabs, these work well in milder areas of the country as a house foundation. But monolithic slabs can heave and crack in colder climates.

Cold-climate builders can choose two options: the slab can be placed inside a frost wall, or a frost-protected shallow foundation can be installed.

The frost-wall
This method requires excavation below the frost line for a perimeter footing. Once the footing is poured, stem walls are formed and placed. After the foundation is filled with compacted gravel, an above-grade slab can be installed inside the frost-wall. In cold parts of the country, where the frost line is 4 ft. or more below grade, going to these lengths negates some of the inherent advantages of a slab foundation. Many builders conclude that if they have to excavate below the frost line, they may as well put in a basement.

Frost-protected shallow foundations
Shallow frost protected foundations are more common in Europe than in North America. Designed to prevent frost problems, these foundations prevent subslab freezing by incorporating insulation to trap heat from the building and from the earth below the building. Of course, any insulation used for a frost-protected shallow foundation must be approved for below-grade applications.

Frost-protected slabs are accepted by code. A number of builders have adopted this approach to save both time and money. The construction details for frost-protected shallow foundations differ depending on whether a building is heated or unheated; builders may want to learn more about the “air freezing index” to design these foundations properly for a specific climate.

For more information on this type of slab, see Frost-Protected Shallow Foundations.

Insulated raft foundations
To avoid the problem of thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through concrete footings, many European PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. builders are now using “insulated raft” foundation systems. As typically installed, an insulated raft is a load-bearing flat slab on grade. The slab has a uniform thickness rather than a thickened edge. The concrete thickness and the rebar schedule are designed to support the loads imposed by the perimeter walls and any interior bearing walls.

The EPS forms for an insulated raft foundation resemble a big rectangular tray. There is a continuous horizontal layer of rigid foam under the entire slab, as well as vertical insulation at the slab perimeter; all of the foam locks together. After the concrete is placed, the foam forms stay put, just like the forms of an ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure. wall.

An insulated raft foundation differs from a frost-protected shallow foundation:

  • Insulated raft foundations have a uniform thickness rather than a thickened edge.
  • Unlike many frost-protected shallow foundations, insulated raft foundations always include a continuous horizontal layer of insulation under the entire slab.
  • Insulated raft foundations usually have no wing insulation, depending instead on a deep layer of crushed stone to address the problem of frost heaving.

For more information on insulated raft foundations, see Foam Under Footings.


Building Science Corp.
Primer on problems with concrete slabs and how to prevent them:

Explanation of air freezing index (AFI) from NOAA:

GBA: Frost-Protected Shallow Foundations

FHB: Frost-Protected Shallow Foundations

Image Credits:

  1. Charles Lockhart
  2. Daniel S. Morrison/Fine Homebuilding #160
  3. Brian Pontolilo/Fine Homebuilding #178
  4. Paddy Morrissey, Code Check Building 2nd Edition
Tags: , , , , , , ,
Oct 20, 2012 6:49 AM ET

Response to Jay Danielowski
by Martin Holladay

The vulnerability of any shallow foundation is susceptibility to frost heaving. If you are building in Hawaii, of course, you may have no frost worries.

A layer of crushed stone under a shallow slab ensures good drainage. If the material under a slab is well drained and dry, it won't be susceptible to frost heaving.

If you think that your climate and your soil conditions are such that you can omit the layer of crushed stone, you should probably talk to an engineer to confirm your hunch.

Oct 19, 2012 10:15 PM ET

insulated monlithic slab
by Jay Danielowski

Why do you need crushed rock under the perimiter of the shallow footing? I am building on sandy soil

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