Assessing, Repairing, and Controlling Moisture on Rebuilding Basements Concrete Floors

Moisture on the surface of concrete outlet. In most Iowa locations subsur- the laborers will not be tempted to add fl oors is common in Iowa homes. It can face drainage will be needed. excess water at the job-site, and then cause numerous problems, including puncture the water barrier to allow the mold and mildew growth, wet rugs and For more information, see Proper concrete to “dry.” carpet, damp and musty smells, rotting Drainage Around Your Home, Pm-1560, of wood, wet insulation, rusting of available at your local Iowa State Uni- Use air-entrained concrete with a 28- metal, and bacteria growth. versity Extension offi ce. day minimum compressive strength of 3,000 psi, a maximum slump of Water occurs on slab surfaces for a Provide a Moisture Barrier 5 inches, and a minimum cement number of reasons, including: To reduce moisture migration through content of 520 pounds per cubic yard • condensation concrete fl oors, place an impermeable unless experience demonstrates that • moisture migration through the water barrier over the rock and gravel less cement may be used to produce slab from wet soil under the slab before placing the concrete. Polyethyl- acceptable concrete. • broken or leaking pipes buried ene fi lm is low cost and easily installed, below or in the concrete but it is easily punctured and diffi cult Concrete curing for both walls and • surface water from leaks or to seal at the edges. More durable fl oors is an important step in produc- fl ooding collecting on the concrete products are polyethylene-coated kraft ing durable, long wearing moisture- paper and glass-reinforced waterproof resistant concrete. Concrete is cured To reduce the moisture problems in the paper, extrusion coated on both sides by applying water to the surface or by basement, the fl ow of moisture into the with polyethylene. Use 8 mil or heavier retaining water within the slab. Curing concrete from outside must be stopped. material and do not puncture the bar- should be started as soon as possible All leaks must be fi xed. rier. without damaging the surface. It should continue for 5 days in warm weather or You may have to combine several of the Use Control Joints 7 days in cooler weather. Do not allow following measures to control moisture Use control joints, construction joints, the concrete temperature to fall below in your home. Controlling moisture and isolation joints, and fi ll them with 50° F during curing. during new construction is inexpensive a suitable caulk. Joints are needed and effective. But in an existing house, where the fl oor abuts walls or columns, Use Insulating Concrete controlling moisture often is expensive, around fl oor and shower drains, around Insulating, or lightweight, concrete involves compromises, and may not plumbing openings, around electrical is made with insulating aggregate. It be effective. Obtain competent advice conduits, and around all other fl oor has warmer surfaces, which reduce before deciding what to do. penetrations. the amount of condensation. Check with your architect, engineer, designer, Provide Good External Drainage Use High Quality Concrete contractor, or concrete supplier for ad- Good external drainage consists of sur- Use a low-water concrete mixture in ditional information. face measures to remove water from the basement construction. Excess water surface of the ground and sub-surface decreases the strength of the fi nished Insulate the Floor measures to remove water from below concrete, causes it to shrink exces- To insulate concrete slabs, place insula- the surface. Surface measures include sively as it dries, and leaves voids as the tion around the perimeter walls. Use sloping the soil away from the house excess water evaporates. Weak concrete extruded polystyrene insulation, rigid and equipping the house with roof cracks more easily, shrinkage leaves fi berglass, or urethane foam. You can gutters. Subsurface measures include cracks that moisture can penetrate, and insulate crawl spaces and slabs with placing 4 to 6 inches of uniformly voids allow water to seep. Use only the exterior insulation on the walls or foot- graded washed rock or gravel under amount of water needed for curing. ings. The insulation must be continu- the basement fl oor, providing a drain Drier concrete requires more labor to ous and connect with the insulated outlet, and draining tile lines to a free place and fi nish. Make certain the labor walls above. is available before starting work, so

PM 1562 Reviewed January 2010 Most of the heat loss occurs at the by fl ooding, the subfl oor and insula- times needed to keep it cool may not be perimeter of a structure, and no ad- tion usually must be replaced. Exterior suffi cient to remove moisture. Cooler ditional insulation is needed under the moisture drainage and exterior insula- air cannot hold as much moisture, fl oor slab to save energy. However, a tion are preferred options. which condenses on cool surfaces. Air small amount of insulation (R-3) under conditioner supply ducts in a concrete the fl oor slab can help reduce summer The concrete fl oor must not leak or slab cool the slab and increase the risk condensation. be wet from anything except con- of condensation on the slab surface. densation. If exterior water leaks or Carpet the Floor seeps onto the fl oor, drainage must be Operating both a dehumidifi er and air Basement carpet furnishes both a provided before insulating. Typically, a conditioner will remove moisture and moisture retarder and some insulation. concrete fl oor is insulated as shown in keep the space comfortable, but at a The room and the top surface of the table 1. higher energy cost. carpet will be slightly warmer after the carpeting is installed, which can reduce Control Winter Moisture Open Spaces and Circulate Air surface condensation on the carpet. Excess winter humidity levels in homes You can open spaces by leaving doors can be caused by moisture evaporating open and putting louvered doors on Because carpet retards the fl ow of heat from the fl oor slab surface, even when closets. Circulate air with the heating and moisture, moisture can easily be the concrete appears dry. For more and air conditioning blower, ceiling trapped between it and the concrete information, see Controlling Winter fans, or small circulating fans. Place and require longer to dry than on a Moisture Problems in Houses, Pm-947, furniture on small blocks and away fl oor without carpeting. Keep carpeted available at your local Iowa State Uni- from walls so air can circulate. rooms dry and warm. Never install versity Extension offi ce. carpeting over fl oors that leak or have Ventilate the Basement moisture migrating through them. Do Seal the Concrete Surface Ventilating basements to reduce con- not carpet wet areas. Commercial products can seal concrete densation requires a constant watch on against small amounts of capillary weather and temperatures. On hot dry Insulate Above the Concrete, moisture migrating through the slab. days, outdoor air circulated into the Construct Another Floor Another option is to place a 6 mil vapor basement will help dry the basement In existing homes, interior measures retarder on the fl oor, and place a new and heat the walls, reducing moisture. may be the only alternative. Insulating 2-inch slab on the existing slab. Con- above the concrete and constructing sult with your architect, engineer, con- On hot humid days, outdoor air might another fl oor above the insulation may tractor, or concrete supplier about this condense on the concrete depending be a solution to cold, wet fl oors with option and the concrete mix needed. on the temperature and moisture levels. moisture condensing on the surface. This option will not reduce condensa- If the amount of moisture on the walls tion, nor stop moisture from entering increases after you open windows and However, a wood fl oor system and insu- when the water table rises above the ventilate, close the windows and oper- lation above the concrete can trap mois- fl oor level. ate a dehumidifi er and/or air condition- ture and cause problems with mold, er to reduce moisture levels. mildew, wood rot, and odor. If water does Eliminate Water Leaks enter the home through the concrete or Check for broken water pipes in and Ventilating on cool dry days can cool below the slab, and in walls. A small the concrete, increasing surface con- leak can discharge hundreds of gallons densation when the air temperature and Table 1. Insulating a Concrete Floor of water over time. Check for roof, ap- humidity rise. Ventilate cautiously. Do 1. Seal the concrete fl oor surface. pliance, and other leaks. not ventilate on cool wet days. 2. Attach preservative-treated (nomi- Dehumidify the Area Reviewed by Tom Greiner. Prepared by Tom nal) Greiner, former extension agricultural engineer, 2-inch x 2-inch (actually 11/2 x 11/2) Dehumidifi ers remove moisture from and Laura Sternweis, extension communication wood sleepers 24 inches on center to the air, and heat the air in the space. specialist. the concrete fl oor. Warm air can hold more moisture. This material is based upon work supported by the Extension Removing the moisture and heating Service, U.S. Department of Agriculture, under special project 3. Place 11/2 inch closed cell extruded number 93-EFRA-1-0002. polystyrene rigid insulation between the space helps reduce condensation problems, but may make the basement . . . and justice for all sleepers. The U.S. Department of Agriculture (USDA) prohibits discrimina- uncomfortably warm. Dehumidifi ers tion in all its programs and activities on the basis of race, color, 4. Cover the fl oor with a 4 or 6 mil national origin, gender, religion, age, disability, political beliefs, vapor retarder. usually will reduce summer condensa- sexual orientation, and marital or family status. (Not all prohib- tion problems. Check with appliance ited bases apply to all programs.) Many materials can be made 5. Screw plywood fl ooring to the wood available in alternative formats for ADA clients. To fi le a complaint suppliers for the proper size dehumidi- of discrimination, write USDA, Offi ce of Civil Rights, Room sleepers. 326-W, Whitten Building, 14th and Independence Avenue, SW, 6. Cover the plywood with fl ooring. fi er for your space. Washington, DC 20250-9410 or call 202-720-5964. Air conditioners remove moisture as Issued in furtherance of Cooperative Extension work, Acts of May If headroom permits, use thicker wood 8 and June 30, 1914, in cooperation with the U.S. Department of they cool the air. Since the basement Agriculture. Jack M. Payne, director, Cooperative Extension Ser- vice, Iowa State University of Science and Technology, Ames, Iowa. already is cool, the short operation