Craw-l Spaces in the Hotne Council Notef4.4 Building Research Council School of Architecture University of Illinois at Urbana-Champaign Crawl Spaces in the Home spection purposes and to service equip­ CRAWL SPACE CONSTRUCTION Crawl space houses should ment, such as a sump pump or furnace. Many houses without basements are and can be constructed so built over crawl spaces. That is, the floor that they are free from the of a house is built over an excavation MOISTURE SYMPTOMS AND SOURCES deep enough for someone to gain access Obvious symptoms of excessive moisture problems of moisture. to the underfloor space by crawling. The passing up through the floors and walls in­ minimum depth of the crawl space clude: should be 24 inches under the floor joists, A a musty odor; mold on the walls near the or 18 inches under any beams or sup­ floor, in corners, and in closets; ports. An 18-inch clearance beneath duct A moisture condensation on windows work is also desirable. which are equipped with insulating glass or storm windows; A moisture or frost on the underside of roof sheathing; and A peeling paint on walls with moisture condensation. In the crawl space itself, moisture prob- lems may be indicated by: Repair and modernization A toadstools or furry mats on lumber; problems are simpler in A damp earth; crawl-space houses than in a house with a slab foundation. A mold on the ground; A condensation on masonry foundation walls; and A mildew or condensation on band joists and subfloor. These are more likely to appear along the north side of the house. The exterior walls of the house and the outer edges of the floor framing com­ Sometimes there are no visible symp­ monly rest on a foundation wall extend­ toms of moisture problems within the ing around the house. Sometimes, how­ crawl space itself. ever, the exterior walls and the floor are The only satisfactory way to avoid supported by beams that rest on piers lo­ moisture problems is to prevent moisture cated around the edge of the house. In from entering the crawl space. Moisture such construction, non-load-bearing problems that show up within a house as condensation or frost on the underside of The information contained in this skirting usually encloses the crawl space roof sheathing, or as condensation or publication was produced by the excavation. School of Architecture-Building The interior ends of floor joists are gen­ frost on windows in the living area may Research Council (BRC) at the erally supported on wood or steel beams be the result of dampness in the crawl University of Illinois at Urbana­ that rest on masonry or concrete piers, space. An uncorrected moisture problem Champaign. For a set of 27 but support can be provided by load­ can cause decay of the wood-floor fram­ Council Notes for $29.95 call bearing walls. ing and ultimately structural failure of 1-800-336-0616. Floors overhead are usually of wood the house. with conventional joist construction. Moisture problems in crawl spaces may be However, concrete, masonry, metal, or a due to: combination of these materials can be A the construction of a crawl space in an used. Normally, the crawl space contains area of high water tables; only plumbing pipes, heating and cool­ A improper grading of the lot for drain - ing ducts or pipes, and electrical wiring. age; or Easy access should be provided for in- A the absence of moisture control devices such as a ground cover or ventilation openings. Crawl space houses should and can be constructed so that they are free from the problems of moisture, they resist termite 2 Building Research Council infestation, and the floor and rooms pressure against the crawl-space walls. above the crawl space can be kept at Rainwater falling on the ground or from comfortable temperatures with a mini­ the roof can pass through or under the mum of heat loss from the structure. foundation walls. To avoid such prob­ lems, grade the lot so that drainage runs Ground Water away from the house and no water is al­ Sometimes the level of the water in the lowed to stand on the site. ground (water table) is raised above the Minimum grading specifications will suf­ bottom surface of the crawl space. fice for most conditions: Water may penetrate the foundation A the slope of the grade should be at least walls or rise through the ground surface 6 inches in 10 feet; in the crawl space, allowing water to A the ground should fall at least 6 inches stand in the crawl space. on all sides of the house; and This is commonly due to: A the slope should extend at least 10 feet A heavy or prolonged rains; out from the foundation, except where A a spring which appears during wet sea - side yards are narrower. sons only and was not discovered when In new construction, it will usually be the house was built; necessary to provide additional fill next A water flowing along an impervious layer to the house the first year or two after within the soil; or construction to compensate for the set­ A an overlooked farm drain tile. tling of the backfill around the founda­ Crawl space construction is not recom­ tion. Failure to do so may result in a mended if the ground-water level in the "moat" around the house. area is so high that it can flood the crawl To divert rainwater, use wide over­ space periodically. hangs and/ or gutters and downspouts Where a spring or drain tile is discovered extending several feet from the house. after construction, or where unexpected Where gravel beds are used as a drip flooding occurs: area substituting for gutters, the ground A detour the drain around the house; level is really at the bottom of the gravel A install a footing tile around the outside of and the grading must be done so that the crawl space at foundation level with water drains from the bottom of the the bottom of the footing; gravel bed. Make sure that downspout A backfill with coarse gravel to within 8 extenders or splash blocks are used to inches of the finished grade; discharge rainwater past the backfill onto Dampness within a crawl space A install field tile or perforated plastic the undisturbed soil. In a crawl space can usually be avoided if the lot drain lines within the crawl space; house, this is usually 2 feet to 3 feet away is graded correctly and a A grade the bottom of the crawl space so from the foundation. Do not connect moisture-control device, such as a that any water will drain to the tile; downspouts to footing drains. plastic ground cover, is used. In areas of high water tables, install a A cover the crawl space area with coarse gravel (fine gravel or sand tends to footing drain or field tile with open joints block tile); and or perforated plastic drainage tubing. Connect the tile to a sump pump, or to A provide either a gravity drain or sump pump to remove the water collected by "daylight." Waterproof the outside walls the drainage lines. with a bituminous coating. Costs and installation problems inhibit Capillary Rise of Ground Moisture the use of other effective methods of Moisture travels upward from lower lay­ groundwater control, such as a mem­ ers of certain soils by capillary action and brane waterproofing system. evaporates within the crawl space (as Surface Water much as 13 gallons per day have been noted to rise under a 1,000 square foot Surface drainage problems usually occur house). Capillary rise occurs in nearly all because the floor of the crawl space is crawl spaces built in areas where the soil two to three feet below the finish grade is composed of clay or silt. The moisture outside the house. Heavy soils may re­ is present even though the ground in the tain surface drainage and create water 3 Crawl Spaces in the Home crawl space may seem dry, dusty, and Provide at least four corrosion-resistant even cracked. foundation vents that can be closed dur­ Moisture from capillary rise can be ing the heating season and during humid kept out of the crawl space by using a summers. The bottom of the vent open­ vapor-retarding ground cover that is not ing should be at least 2 inches above the susceptible to damage by fungi. Polyeth­ soil grade to prevent rainwater from en­ ylene film, 6 mils thick is effective. To in­ tering the crawl space through the vent. stall this, grade and smooth the ground, Insulated covers should be cut from and remove any trash or debris. Turn up panels of rigid foam insulation. The vent the ground cover four to six inches on the openings should be screened with eight­ wall of the crawl space. Hold the poly­ by-eight corrosion-resistant mesh (64 ethylene in place against the walls with openings per square inch) to keep out in­ sand or bricks to help achieve a neat in­ sects. If the crawl space is dry, opening stallation. Where more than one piece of the vents may not be necessary, even in polyethylene is needed, lap the edges the summer; but it is much easier to four to six inches. Sealing the edges is not block and insulate vents that were not necessary. Use wide rolls of polyethylene needed than to install vents that were not to reduce the number of edge joints. originally provided. Open Crawl Space Moisture From Mechanical Systems Open Crawl Spaces O ften moisture is introduced into the O pen crawl spaces are those that are crawl space from the mechanical systems ventilated to such an extent that the tem­ within the home.