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Concrete-Slab Foundations

By HHI Staff

Concrete slab foundations are relatively easy to construct in a healthy manner. They are often used in healthy houses because they are easy to cover with a low-tox floor covering such as ceramic tile. While it’s possible to install a wood floor over a concrete slab, it generally involves much more work and the results are not always successful.

Construction
A concrete floor slab can be an integral part of the house foundation or built separately from the footings and foundation wall. Neither approach is better than the other from a health point-of-view as long as the slab is constructed and insulated with care.

 

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To minimize cracking in a floor slab, the concrete should typically be mixed with the correct amount of water and be reinforced with steel. Wooden grade stakes used during the construction of a slab should be removed because they can become pathways for radon and other soil gases as the wood deteriorates.

Any joints between the slab and any walls, pipes or wires should be well-sealed with caulking or aerosol-foam insulation. If a concrete floor slab is made in one piece, and all cracks and gaps have been sealed, it can be effective at blocking the air-pressure differences that move radon, ground moisture, lawn chemicals and other contaminants from the soil into the living space. If control joints are required in a multi-piece floor slab, they should be fitted with flexible water stops during construction to prevent soil gases from passing through the gaps.

Heating ducts embedded in concrete slabs should be avoided because they can be difficult to clean, and if there is a water spill, they can remain damp and harbor mold growth. Plumbing supply lines under slabs should be minimized because they are difficult to repair if a leak develops. Plumbing drain lines under slabs should have cleanouts in appropriate places so they can be maintained.
Insulation
An uninsulated slab is somewhat cooler than the air in the room, especially in the winter, so there will be a microclimate of higher relative humidity near the floor and possible mold growth.

From a thermal standpoint, an insulated floor slab is more connected to the living space than to the ground. The most effective use of insulation is around the perimeter because it’s in close proximity to the extremes of air temperature. The center of a floor slab is further removed from the extremes of air temperature so, in moderate climates, it may not be necessary to insulate under the center of a floor slab. Besides being energy saving, warm floors are more comfortable to walk on in winter.
Drainage
Perforated drainage tiles should be used around the perimeter of a concrete-slab foundation to divert any excessive ground water from the structure. A 4"-layer of gravel under the slab can act as a capillary break to prevent moisture from wicking up through the slab into the living space.

A perforated drainage tile is often placed under the center of a house within the gravel for a future radon-removal system—should one be needed later on. The perforated tile should be connected to a length of solid pipe that is stubbed up through the slab and capped. If high levels of radon are detected when house construction is complete, a fan can be connected to the capped pipe to suck radon from under the slab and blow it outdoors.
Damp Proofing
Polyethylene plastic sheeting should be laid under the slab to minimize the migration of moisture, radon and other soil gases through the floor itself. The sheeting functions as both a diffusion retarder and, if well-sealed, an air pressure retarder should the concrete develop cracks. If there is no gravel under the slab, the plastic acts as a capillary break to prevent moisture from wicking up through the slab into the living space. The sheeting should be 4 to 6 mil in thickness and seams lapped at a minimum of 12 inches.

Some builders are now installing plastic sheeting under concrete footings or a damp proof coating on top of them, to prevent moisture from wicking up through them into the foundation wall and then evaporating into the living space.

Plastic sheeting might be a problem for sensitive persons if they are exposed to it directly. When separated from the living space by concrete it’s almost never a problem—especially when the floor system is airtight.

A damp proof coating on the outside of the foundation wall will prevent moisture from moving horizontally through the foundation wall by capillary suction and diffusion.
Termite Treatment
Metal termite shields can be placed under wood-framed walls with a slab foundation, but this is often difficult to do effectively. The area beneath a concrete-slab foundation is inaccessible once a house is complete, and this is often where termites reside. If a slab is crack-free, they won’t be able to pass through it. But termites can pass around electrical wires, plumbing drains and other barriers that penetrate the slab by chewing their way through caulking and sealants.

For these reasons, toxic chemicals are often sprayed under slabs to deter termites. In typical leaky houses, these chemicals can easily enter the living space because of air-pressure differences and diffusion. In new, healthy construction, all measures taken to block the entry of radon and ground moisture will also prevent termite chemicals from entering the living space. If chemicals are used, and they are later found to be moving from the soil into the house, a radon-removal system can also function as a termite-chemical removal system.

 

 

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Concrete-Slab Foundations:  Created on May 8th, 2010.  Last Modified on May 16th, 2010

 

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