Slab on Grade Foundations

At some home building sites, there may be circumstances that call for slab on grade foundations to be used when the home is built. This type of foundation is most often used in warm climates, as ground freezing and thawing is less common and there will be no need for heat ducting under the floor. They are used most often in warm climates that have expansive clay soil, such as Texas and California.

Slab on grade foundations are concrete slabs that are formed from molds set in the ground. Concrete is poured into the mold all at one time, with no space left between the ground and the home.

When the conditions are right for this type of foundation, it is a sturdy and relatively inexpensive option. Slab on grade foundations are also less vulnerable to termites, as there are no hollow spaces for them to hide and no wood channels present to lead termites from the ground into the home.

This foundation type does mean that utility lines can not be accessed from below, and the slab can transmit cold into the house when ground temperatures get low. It may also make a home vulnerable to flooding during moderate to heavy rains in low elevations.


Slab on grade foundations can make a home more vulnerable to radon gas in the home. Radon that is naturally present in the ground can radiate directly into a home when there is no crawlspace to separate the home from the ground. There are several radon mitigation techniques available that can lessen the amount of radon in a home, but they are expensive and can be difficult to install. Before a home is built on a slab, the amount of radon in the area can be measured to see whether the home should be built on a crawlspace instead.

Insulating a Slab on Grade Foundation

Some of the cold temperature conduction present with slab on grade foundations can be slowed by pouring the concrete directly over insulation such as Styrofoam. This creates a barrier separating the soil from the concrete, which can save on heating costs in the winter. There are also heating elements, such as hydronic heating, which can be built into the concrete. This is an expensive solution, but is a very effective one.

Insulating slab on grade foundations comes with its own set of worries, however. Termites have been known to tunnel through slab insulation to gain access to a home’s walls and wood framing. A termite infestation in this underground insulation would be undetected until the home became infested.

Other Issues

Because of this risk, some insurance companies will not cover homes with slab insulation for termites. Building codes in some southern states prohibit foam insulation that is in contact with the soil for this reason.

There are ways to keep the insulation around slab on grade foundations from attracting termites. This includes using a foam insulation that has been treated with a termiticide. These are usually made from boric acid, which is no more dangerous than any other termite treatment and is much less toxic than many other insecticides. It is also possible to install termite shields under the exterior walls of the home. This is not completely effective, but it can reduce the risk of a major infestation.