Deteriorated Ductwork in a Slab House


Slab houses utilize a single layer of concrete that is several inches thick for the foundation. The slab rests on a bed of crushed gravel for drainage. On some older houses, the ductwork for the home may be under this concrete layer. This placement inevitably causes problems as the years pass and deterioration of the ductwork begins to affect conditions within the home.

Ductwork Beneath the Slab

  • Slab homes are generally built in areas where the ground does not freeze, though they are sometimes found in colder areas. During the 1950s, slab houses were common because they could be built quickly and inexpensively. The ductwork was often installed before the concrete was laid, with the assumption that it could last for many years without needing to be replaced. However, moisture from the ground causes the steel to corrode, requiring expensive break-up of the concrete slab surface and replacement of the ducts.

Signs of Deterioration

  • Homeowners may find a number of problems in their homes when the ductwork below the concrete slab begins to deteriorate. They may find signs of moisture coming from the ductwork. A musty, moldy smell may develop in the house from the intrusion of water into the ducts. Energy bills may be high from poor duct efficiency. These signs call for inspection by a heating and air-conditioning contractor who has experience with ductwork below the concrete. In some cases, the ductwork can be repaired without a large expense. If the ductwork is badly deteriorated, however, it may be time to completely replace the ductwork with an updated system that is easier to repair.

Installing New Ductwork

  • If problems with ductwork below the concrete slab are extensive, it may not be economically feasible to repair the old system. Generally, a new system of ductwork is installed in the attic area below the ceiling. This placement allows for easy access when repairs are needed, as well as faster detection of leaks that can cost the homeowner money in energy costs.

Sealing Off Under-Concrete Ductwork

  • When ductwork under the slab has been replaced with overhead ducting, the outlets must be sealed over. A good method is to fill the old ducts with an expanding foam insulation material. Then, use pre-mixed concrete to seal off the duct. Fill the hole so that it is about 1/16 inch below the floor and allow the concrete to dry. Then, fill the remainder with floor leveling compound and trowel to a smooth, level surface, according to the MosbyBuildingArts website.

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