Crawl Spaces in the Home

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 problems 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. To prevent this, be sure perature within the crawl space ap to vent the clothes dryer to the outside. proaches that of the outside air. This ac Be sure that all dryer-vent pipe connec commodates situations where the fur tions are tight, and the pipe is supported. nace resides in the crawl space, which in To prevent condensation within the vent that case must be left open to meet there pipe, keep the pipe as short as possible. quirements for combustion air for the Also be sure to discharge condensate heating system. Of course, furnaces water from central cooling systems into should be connected to a flue or chimney An open crawl space is used storm or sanitary drains. Often, the con so that combustion products are not dis where the foundation vents must densate from mechanical cooling systems charged into the crawl space, where they be kept open the year around due is drained through a tube into the crawl could taint the air via seepage into the to moisture problems or to pro space and allowed to either drip on top house. vide combustion air to a furnace of the ground cover or run into a drywell In cold areas, water pipes in open located in the crawl space. beneath the ground cover. Drainage crawl spaces must be insulated to protect should always be made to sewer lines if against freezing. It is also important to the soil is silt or clay. Condensate drain insulate heating and cooling pipes and age on top of the ground cover is never ducts to prevent excessive energy loss. acceptable. The standard for insulated ducts is only A common source of water is a one inch of insulation or less, which is humidifier. Humidifiers should not be not adequate in the northern half of the used if they lead to condensation run United States. ning down windows. Where air ducts also handle cool air in summer, the insulation must have a va por retarder on its outside. Any joints in VENTILATION that vapor retarder should be sealed with Before the effectiveness of polyethylene a vapor-retarder tape to prevent conden ground covers was proven, large ventila sation inside the insulation during the tion openings were required in the foun cooling season. dation wall to allow moisture to escape In cold climates, insulating the floors from the crawl space. With the use of a of the house is critical to comfort. To polyethylene ground cover to prevent maintain a desirable surface temperature moisture from the ground rising into the and reduce energy loss, the floor should crawl space, a minimum level of ventila be insulated to limit the heat loss through tion may still be recommended. the floor to five British Thermal Units per 4 Building Research Council

hour (Btu/h) per square foot. If the out duct work during the summer may still door temperature is -30 F, the resistance be necessary with a vapor retarder on the of the insulation (R-value) should be at outside of the insulation. Ventilation of least 19; for 0 F, R-13; and 30 F, R-7. the crawl space in the summer may in These resistances serve as a guide in se crease the problem of condensation on lecting the insulation material. If flexible ducts carrying cooling air. The use of a insulation is installed in the space be dehumidifier in a closed crawl space may tween the joists underneath the floor, it be appropriate during the summer should be supported by hardware cloth, months. wire netting, or sheet material fastened to If the temperature in the crawl space the bottom edge of the joists, or arched can be maintained within 5 degrees Fahr wires sprung into place to hold the insu enheit of the thermostat temperature set lation against the bottom of the subfloor. ting in the house, the surface tempera Batt insulation is available with nominal ture of the floors in the rooms will also be R-values of 7, 13, and 19. Closed-cell comfortable. The heat loss into the foam insulation board such as polysty ground through the earthen floor of the Batt-type insulation is often held rene, polyurethane, or polyisocyanurate crawl space is negligible. Vagrant heat in place under the floor with has an R-value of approximately 4-per from non-insulated warm-air ducts or lengths of wire sprung into place inch of thickness. hot-water heating pipes will usually keep between joists. Closed-cell foam insulation panels are an insulated, closed crawl space near that vapor retarders, so no additional vapor desired temperature range. retarder is needed. Insulate the walls enclosing a crawl In open crawl space construction, the space to reduce heat loss and help main undersides of the joists should be cov tain the crawl space temperature. To ered when this material is used, and not properly insulate a closed crawl space, left exposed to the cold air and ground of the box-sill headers and the end joists of the crawl space. Leaving the bottoms of the floor must be insulated, preferably the joists uncovered can lead to rotting of with dose-fitting panels cut from rigid • exposed surfaces. foam insulation such as extruded poly The undersides of the joists can be cov styrene, polyurethane or polyisocyanu ered with closed-cell insulation panels, as rate. Alternately, use a flexible-type insu described above, or by the kraft or foil lation (batt or blanket) that provides va flanges of faced insulation, or polyethyl por protection on one side. The vapor re ene sheet, or roofing felts. Because such a tarder should face the interior of the Closed Crawl Space covering could constitute a "cold-side va crawl space. por retarder", there should be holes or For exterior walls, insulation in sheet joints to allow any condensed water to or block form is most easily applied. drain out. Floor insulation should fit Tempered nails or an adhesive mastic is snugly around insulated pipes and ducts suggested. Insulation should be of a type that penetrate it. not affected by termites or dampness. Ex panded polystyrene foam is the most Closed Crawl Spaces common type of insulation used for The moisture control provided by a crawl space walls. Note: Exposed plastic polyethylene ground cover is so effective foam boards of all types do present Warm Crawl Space that crawl-space ventilators can usually some fire hazard, which may be signifi be kept closed during both the heating cant if the furnace is in the crawl space and cooling seasons. It provides the or if the entire crawl space is used as a A closed crawl space is one that maximum comfort on the floor above heating plenum. can be heated. The use of a ground with a minimum expenditure for insula Cover the ground surface of the crawl cover is essential for this type of tion. This "closed" crawl space construc space with polyethylene film. Do not ex crawl space. tion is recommended except under severe tend the film over the insulation, or the moisture conditions. moisture vapor below the ground cover Insulation around water pipes and heat could flow upward into the insulation. If ducts is usually not needed in the closed batt-type insulation is used, either sus crawl space. However, insulating cooling pended from a nailing strip on the plate ducts to prevent condensation on the 5 Crawl Spaces in the Home

or supported by furring strips, a continu Composed mainly of earth, wood ous vapor retarder should supplement particles, and excreta, these small tubes any vapor retarder incorporated in the have been found on the surface of batt or blanket to reduce the number of concrete slabs, on concrete piers in crawl joints in the vapor retarder. spaces, on one or both surfaces of foun Conditions may be ripe for mold dation walls, and in the cavities of hol growth on the underside of the plastic low masonry. ground cover. This might seem to be a Termites can also enter through or cause for concern; however, there is no bypass improperly installed termite indication to date that this poses a prob shields. Cracks, expansion and construc lem in the house. The benefits of a tion joints in concrete slabs and other ground cover as the principal means of concrete work also provide excellent pas moisture control on the crawl space floor sageways. Untreated wood embedded The most common heating system have been firmly established. or left in the concrete may permit direct in crawl-space houses uses a access to the interior partitions of the down-flow warm air furnace Heating Systems structure. If a colony is present below the with the plenum and complete Heating systems that supply heat near excavated area, easy entry to the duct system located in the crawl the floor or along the exterior walls of the space. structure can occur. house are very effective for a crawl space Several common construction practices house, because they eliminate or raise the probability of termite attack. reduce uncomfortable drafts along the These include: floor. Therefore, baseboard heating or .._ leaving scrap pieces of wood, fiber - perimeter heating is recommended with board, paper, or other wood items un the heat distributed through ducts in the der or near the foundation or in the crawl space for the perimeter system. backfill around the foundation walls; All furnaces have filters that require .._ construction details that make future in frequent changing or maintenance. A spection difficult; horizontal furnace in the crawl space is .._ leaving untreated form boards and difficult to service and requires that the grade stakes embedded in the concrete; crawl space be left open to provide com .._ failing to treat the soil where wood is bustion air. A furnace in the garage at the close to the soil and termites can travel end of the house requires longer duct from the soil to wood unseen. The latter Sometimes the entire crawl space work, resulting in reduced air flow and is especially critical in masonry-veneer is used as a plenum for a down lower temperatures reaching the far end. construction when the masonry extends flow warm air furnace. Stub Also, lost heat from the furnace jacket below grade; ducts point toward heat outlets, warms the garage instead of the house. A .._ chemical treatment has been inade - which are holes cut in the floor down-flow furnace installed in a closet or quote. and covered with register grilles. utility room in the house is the preferred Termite Shields heat source. Termite shields provide a metal barrier between the termite nest and food sup TERMITE CONTROL ply. A shield should be made of a durable Periodic inspection for subterranean metal, such as zinc, copper alloy, copper, termites is a good practice in areas where terneplate, or galvanized iron or steel. infestations occur. Termites may enter a Copper-plated paper products are not ac structure at any of several points, most ceptable. often through untreated wood in contact A termite shield does not prevent infesta with the soil. Common entries are tion. A termite shield forces termites to wooden steps and supports, dirt fills un build shelter tubes so that the infestation der porches, and around wood-framed can be seen and further treatment ap access openings with wood sills at or be plied. A heating system using a hori low grade. The key to termite shield effectiveness zontal warm air furnace located When the wood is some distance above is proper installation and continuous in in the crawl space is difficult to the ground, termites may spection. This is feasible only where both service and reqttires an open construct a shelter tube to enable moving edges of the shield are open to inspec crawl space. between the nest and untreated wood. tion, which means a crawl space must be

6 Building Research Council

deep enough to permit easy access for consuming, and sometimes practically such inspection. impossible. The termite shield is a strip of metal The principle of termite control with • placed between concrete or brickwork chemical barriers is to isolate the struc and wood to force the termites to build ture from the termites. Chemical applica shelter tubes where they can be seen. tions to existing structures require rein The main weakness of termite shields spection in subsequent years. For exten is improper installation. Termites can sively infested buildings, some termites, move through a hole as small as 1 I 32 of including the ones that can reproduce, an inch. Thus, the shield should be sol may be trapped in the wood above the dered completely at all seams, at over chemical barrier. On occasion these have lapped corners, and at other breaks or found a new path back to untreated soil, holes in the metal. Openings around an where they reestablish a colony. This is chor bolts, plumbing and heating ducts, increasingly the case as the trend in and other breaks should be soldered or chemical control has been to treat only filled with coal-tar pitch. For best results, around or near the current termite in the shield should be at least 8 inches fested area. above the ground with a 2-inch lip ex tending horizontally on each side of the foundation or pier and an additional 2 PIER CONSTRUCTION inches bent downward at a 45-degree an Where the outside walls and the outer gle. For appearance and safety reasons, edge of the floor of a house rest upon a the extension on the outside of the foun beam supported by piers, the crawl space dation is often bent down or omitted. is usually enclosed by non-load-bearing However, these practices impair the ef (curtain) walls placed between the piers. fectiveness of the shield . Sometimes the curtain wall may support the masonry veneer walls above it. The Soil Treatment footing for the curtain wall should be Although soil treatment was devel poured integrally with the footings of the • oped primarily to control infestations in piers, unless the bottom section of the existing structures, the most control curtain wall is reinforced to span be effective time to apply the chemicals is tween piers. Common size limitations for during the construction of the founda pier construction are given tion. After construction has been com below: pleted, treatment is more expensive, time

Table 1. Common Size Limitations for Pier Construction Minimum Limit on Height Dimensions Piers 10 tinnes the least Concrete 8-il1chdianneter dinnension 10 tinnes the least Solid Masonry 8 il1ches x 12 il1ches dinnension 4 tinnes the least Hollow Masonry 8 il1ches x 16 il1ches dinnension Curtain Walls Concrete 4 il1ches 14 tinnes thickness* Solid Masonry 4 il1ches 14 tinnes thickness* Hollow nnasonry 4 il1ches 10 tinnes thickness*

7 Crawl Spaces in the Home

correspondingly thicker. The foundation wall should extend at least 8 inches above grade and remain exposed at least Typical Crawl Space Wall Section 6 inches. C- Masonry wall cap. A hollow masonry foundation wall must be capped with a course of solid masonry or a 4-inch poured concrete cap reinforced with #14 wire mesh 2 inches x 2 inches. D - Ground cover. 6-mil polyethylene film. E - Metal termite shield. Use corrosion-resistant metal that is stiff enough to retain the form shown - 26 gauge galvanized iron or 16-ounce cop per. Where anchor bolts penetrate the shield, the area should be well-sealed with coal-tar pitch or tight lead washers. Need not be installed if soil poisoning is used. F- Sealer for the sill plate Use material similar to expansion joint or a fiberglass sill sealer. G - Sill plate 2 x 6, anchored with lh-inch bolts, 8 feet on center - minimum of two bolts to each piece of sill. Lumber that is pressure-treated with KEY TO ILLUSTRATION preservative is strongly recommended A- Poured concrete continuous wall for sill plate 1) to prevent decay in lum footing either 12 inches by 6 inches or 16 ber likely to be caused by condensation, inches by 8 inches, preferably with two and 2) to discourage termites. round, lh-inch steel reinforcing rods. H - Band joist. Size of footing depends on bearing I- Joists. value of soil. Footings specified above are J- Foundation-wall insulation, may be suitable for most conditions. rigid foam insulation or mineral fiber in B - For frame houses, 8-inch masonry sulation with a vapor retarder against the wall or 6-inch poured concrete wall. wall. (Check local building code.) K - Band joist insulation. Where the exterior walls to be sup L - Vapor retarder. ported by the foundation are thicker (as M - Footing tile. in solid masonry or masonry veneer N - Coarse gravel backfill. house), the foundation wall must be 0 - Parging and waterproofing.

@ 1996, 2002 by The Board of Trustees of the University of Illinois Material in this publication by Rudard A. Jones with Revisions by Henry R. Spies and William B. Rose Editor:BRC Staff Graphic Arts: Selah Peterson Illustrations: Selah Peterson

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