Mechanical Barriers

Below-grade walls may be constructed of poured concrete, masonry blocks, or other materials such as all-weather wood or stone. This chapter discusses details for use of poured concrete and masonry foundation because these are the materials most commonly used for new construction. Recently, trade associations such as American Plywood Association (APA) and the National Forest Products Association (NFoPA) have issued publications on designing radon resistance permanent wood foundations. Information on these types of foundations can be found by contacting the appropriate trade association.21

The following is a list of recommendations that builders can use to utilize the foundation as a mechanical barrier to radon entry. Foundation walls and floor slabs are often constructed of poured concrete. Plastic shrinkage, and therefore cracking, is a natural function of the drying process of concrete. Many factors, such as the water/cement/aggregate ratio, humidity, and temperature, influence the amount of cracking that occurs in a poured concrete foundation. Cracking may be minimized by

1. Proper preparation, mixture, and curing of concrete.2223

2. Ferrous reinforcing (rebar rods and woven wire meshes).

3. Use of concrete additives to change the characteristics of concrete.

4. Water-reducing plasticizers, fiber-reinforced cements.24

To help prevent cracking in masonry walls or minimize the effects of cracks that develop,

1. Use correct thickness of unit for depth of soil.25

2. Use ferrous reinforcing (corners, joints, and top course).26

3. Coat the interior and exterior of the wall with dampproofing.

Cracks and joints in concrete and concrete blocks can be sealed using caulks. Polyurethane caulks have many of the properties required for durable closure of cracks in concrete. These features are

1. Durability

2. Abrasion resistance

3. Flexibility

4. Adhesion

5. Simple surface preparation

6. Acceptable health and safety impacts.

Typical points that should be sealed with caulks are

1. Plumbing penetrations (soil pipes and water lines as minimum).

2. Perimeter slab/wall crack and expansion joints (tool crack or use "zip"-off expansion joint material.

The open tops of concrete block walls are openings that should be sealed. This can be carried out by installing a row of solid blocks, lintel blocks, or termite cap blocks at the top of the wall.

Drainage details that leave openings through the foundation should be avoided or modified. Sump holes and French drains are widely used examples of this type of detailing. It is best to avoid them if possible, by using alternate drainage systems. When these design details are unavoidable, a little thought can allow the use of these details and still keep radon from entering the home. In many areas of the country, some type of dampproofing or waterproofing treatment is required by codes.

The application of dampproofing and waterproofing materials on the exterior, interior, or both sides of the foundation that can serve as a radon-resistant barrier is recommended to help control radon entry. It must be understood that a coating applied to a foundation intended to resist the flow of radon into the building is in addition to the normal waterproofing/dampproofing requirements.

Coatings are applied to the outside or inside of the foundation, creating a radon-resistant barrier between the source and the inside of the home. They come in a wide variety of materials including paint-like products that can be brushed on the interior of the foundation, tar-like materials that are applied to the outside, and cementitious materials that can be brushed or troweled on. They cannot be applied to the underside of the concrete floor slab for obvious reasons, so they must be applied to the inside surface of the slab. The effective life of an interior coating can be greatly diminished by damage; therefore, care must be taken to provide protection to the material used.

Membrane banners are applied to the exterior of the foundation and also beneath the floor slab during construction. Materials used for the membrane barriers range from coextruded poly olefin to polyvinyl chloride to foil sheets with many other materials in between. All membrane barriers must have the edges sealed to prevent radon from migrating around the edges and back into the building.

It is recommended that, as a minimum, a membrane be placed beneath the slab, and all foundation penetrations to the soil be scaled or otherwise dealt with in a manner that will prevent the entry of radon into the home.

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