VENTED VERSUS UNVENTED CRAWL SPACES
The principal perceived advantage of a vented crawl space over an unvented one is that venting may limit radon and moisture-related decay hazards by diluting the crawl space air. Additionally, providing a vented crawlspace may make sense in flood-prone areas such as coastal zones subject to hurricanes. Venting can complement other moisture and radon control measures such as ground cover and proper drainage. However, although increased air flow in the crawl space may offer some dilution potential for ground source moisture and radon, it will not necessarily solve a serious problem. Vented crawl spaces are often provided with operable vents that can be closed to reduce winter heat losses, but also potentially increase radon infiltration. Although not their original purpose, the vents can also be closed in summer to keep out moist exterior air that can have a dew point above the crawl space temperature. This approach, however, requires a high level of informed occupant participation to be successful.
Unvented (conditioned) crawlspaces are generally preferred in most cases, except where flood risks are exceptionally high, as in coastal zones subject to hurricane flooding. The principal disadvantages of a vented crawl space over an unvented one are that (1) pipes and ducts must be sealed and insulated against heat loss (cooling loss in the summer) and freezing, (2) a larger area (the crawlspace ceiling typically is larger than the area of the crawlspace walls) usually must be insulated, which may increase the cost, (3) under hot humid conditions warm humid air circulated into the cool crawl space can cause excessive moisture levels in structural wood components (especially floor joists) that can cause mold and decay, and (4) an airtight, continuous thermal envelope at the crawlspace ceiling is very difficult to achieve in practice. It is not necessary to vent a crawl space for moisture control if it is open to an adjacent basement, and venting is clearly incompatible with crawl spaces used as heat distribution plenums. In fact, there are several advantages to designing crawl spaces as semi-heated zones. Duct and pipe insulation can be reduced, and the foundation is insulated at the crawl space perimeter instead of its ceiling. This usually requires less insulation, simplifies installation difficulties in some cases, and can be detailed to minimize condensation hazards.
Although unvented crawl spaces have been recommended, “except under severe moisture conditions,” by the University of Illinois’s Small Homes Council (Jones 1980), moisture problems in crawl spaces are common enough that many agencies are unwilling to endorse closing the vents year-round. Soil type and the groundwater level are key factors influencing moisture conditions. It should be recognized that a crawl space can be designed as a short basement (with slurry slab floor), and, having a higher floor level, is subject to less moisture hazard than a basement in most cases. Viewed in this way, the main distinction between unvented crawl spaces and basements is in the owner’s accessibility and likelihood of noticing moisture problems.
The major structural components of a crawl space are the wall and the footing (see Figure 3-2). Crawl space walls are typically constructed of cast-in-place concrete, concrete masonry units, or alternative systems like insulated concrete forms (ICFs). Crawl space walls must resist any lateral loads from the soil and vertical loads from the structure above. The lateral loads on the wall depend on the height of the fill, the soil type and moisture content, and whether the building is located in an area of low or high seismic activity. Because of the large number of variables involved in foundation structural design, final determination of wall thickness, concrete strength, footing dimensions, and reinforcing should be made after consultation of locally-enforced building codes or design by a licensed structural engineer.
In place of a structural foundation wall and continuous spread footing, the structure can be supported on piers or piles with beams in between. These beams between piers support the structure above and transfer the load back to the piers.
Concrete spread footings provide support beneath concrete and masonry crawl space walls and/or columns. Footings must be designed with adequate size to distribute the load to the soil. Freezing water beneath footings can heave, causing cracking and other structural problems. For this reason, footings must be placed beneath the maximum frost penetration depth unless founded on bedrock or proven non-frost susceptible soil or insulated to prevent frost penetration. Since the interior temperature of a vented crawl space may be below freezing in very cold climates, footings must be below the frost depth with respect to both interior and exterior grade unless otherwise protected.
Where expansive soils are present or in areas of high seismic activity, special foundation construction techniques may be necessary. In these cases, consultation with local building officials and a structural engineer is recommended.
While the crawl space is not meant to be living space (such as a basement), it is still very important to control the amount of moisture that can build up in that space. High levels of humidity at relatively low temperatures can cause condensation on different surfaces within the crawl space. This condensation can cause wooden support structures to rot, decreasing their structural integrity. Condensation and high levels of moisture also create an environment that is conducive to mold growth, which can have adverse effects on the health of the home’s inhabitants.
In general, moisture management schemes must control water in two states. First, since the soil in contact with the foundation wall is always at 100% relative humidity, foundation walls must deal with water vapor that will tend to migrate toward the interior under most conditions. Second, liquid water entry must be prevented. Liquid water can enter from sources such as:
There are two main configurations for crawl spaces: vented and unvented. The vented crawl space has historically been the most widely employed design. It works by allowing outside air to flow through the crawl space, thereby, in theory, removing the excess moisture and allowing it to dry (Davis et al. 2005). Unvented crawl spaces (also known as closed or conditioned) do not have vents to the outside and rely on limiting moisture intrusion from the soil, along with mechanical drying mechanisms such as air conditioning or a dehumidifier to prevent moisture build-up (Dastur et al. 2005). For both vented and unvented designs, there are common techniques that are used to limit the moisture content in the crawl space. These techniques include methods for blocking moisture sources by providing proper drainage and vapor/air barriers. Additional methods for removing moisture build up in the crawl space are also employed.
The following construction practices will prevent excess water in the form of liquid and vapor from infiltrating the crawl space. These techniques are shown in Figures 3-3, 3-4, and 3-5.
Even after employing an effective crawl space drainage and vapor retarder system, it is still possible for moisture to find its way into the crawl space. In a vented crawlspace, cooler temperatures may cause moisture in humid air to condense on the walls, ceiling, and on the ground. Another possible source of moisture accumulation inside the crawl space is pipe leaks. These sources can create pools of water that need to be evacuated. This can be achieved by grading the crawl space floor and by installing a drain or sump pump at the low point. (Dastur et al. 2005). It is important to complete the internal drainage system early in the construction to prevent moisture buildup that can occur before the roof is completed.
Concrete foundation walls contain water from when they were poured which needs to be dissipated by allowing them to dry. In cases where the majority of the wall is below grade, it can only dry to the interior. The insulation material and the wall covers placed on the walls during the construction of the crawl space act as vapor retarders, not allowing the walls to dry to the interior. For this reason, it is recommended that these wall coverings be installed near the end of construction to allow for as much drying of the concrete as possible (BSC 2006).
In unvented crawl spaces it is important not only to have an effective vapor retarder, but also to have a complete air barrier. For this reason, all gaps between the foundation wall and sill plate, sill plate and band joist, and band joist and subfloor should be sealed. All gaps and penetrations in the foundation wall also need to be adequately sealed. A tight air barrier will prevent the influx of humid outside air through air transport, creating an interior space that is independent of outside moisture conditions. To further separate the conditions in the crawl space from those of the outside, mechanical drying systems such as a stand-alone dehumidifier should be used (Dastur et al. 2005). Alternately, the ductwork system can include the crawlspace in the supply / return cycle to effectively treat it as an interior space.
To further separate the conditions in the crawl space from those of the outside, mechanical drying systems such as a stand-alone dehumidifier should be used (Dastur et al. 2005).
Although a crawl space foundation is not as deep as a full basement, it is highly desirable to keep it dry. Good surface drainage is always recommended and, in many cases, subsurface drainage systems may be desirable. The goal of surface drainage is to keep water away from the foundation by sloping the ground surface and using gutters and downspouts for roof drainage.
Figure 3-3, 3-4, and 3-5 describe three different drainage techniques for crawlspaces. Figure 3-3 applies when the crawlspace floor is flush with (or above) the surrounding grade. In most cases, this type of crawlspace will not require perimeter drainage. On especially wet sites, or on sloping sites where part of the crawlspace floor is below-grade, it may still be wise to install a perimeter drain system, described below.
Figure 3-4 and 3-5 describe foundation drain systems, which are recommended for all crawlspaces where the floor is below the level of the surrounding grade. On especially dry sites, it may be possible to eliminate the drainage system and not experience moisture problems. Figure 3-5 describes the recommended best practice. It consists of two independent loops of perforated foundation drain, one inside the footing and one outside. These drain independently, either to daylight or to an internal sump. Figure 3-4 shows another option that is appropriate when site drainage conditions are good. There is no provision for drainage of the space inside the footings. Its single loop of foundation drain is on the outside of the footing, and drains to daylight or to an internal sump.
Another important factor to consider when managing moisture in a crawl space is the way it is to be insulated. Crawlspaces can be insulated at the exterior walls, or vented and insulated at the crawlspace ceiling (Figure 3-6). Insulation not only plays a role in the thermal efficiency of a home but also in the way that moisture behaves. Cooler surfaces in a crawl space can cause moisture from the air to condense on the surfaces. For unvented crawl spaces, the best approach is to treat the crawlspace as a short basement, placing insulation on the exterior or interior surface of the crawlspace walls. Research has shown that closed crawl spaces with wall insulation perform better than wall-vented crawl spaces with ceiling insulation (Dastur et al. 2005).
A key question in the design of an unvented crawl space is whether to place insulation inside or outside the wall. In terms of energy use, there is not a significant difference between the same amount of insulation applied to the exterior versus the interior of a concrete or masonry wall. However, the installation costs, ease of application, appearance, and various technical concerns can be quite different.
Rigid insulation placed on the exterior surface of a concrete (Figure 3-6a) or masonry wall has some advantages over interior placement in that it can provide continuous insulation with no thermal bridges, protect structural walls at moderate temperatures, and minimize moisture condensation problems (Figure 3-7). If the exterior insulation value is high enough, the joists and sill plates can be left open to inspection from the interior for termites and decay. On the other hand, exterior insulation on the wall can be a path for termites and can prevent inspection of the wall from the exterior. If needed a termite barrier should be installed through the insulation where the sill plate rests on the foundation wall. This option is shown in all drawings that depict exterior crawlspace foundation insulation. Vertical exterior insulation on a crawl space wall can extend as deep as the top of the footing and, if desired, be supplemented by extending the insulation horizontally from the face of the foundation wall. Insulation that is exposed above grade must be protected with a coating to prevent physical damage and degradation. Such coatings include fiber cement board, parging (stucco type material), treated plywood, or membrane material (Baechler et al. 2005).
Exterior wall insulation must be approved for below-grade use. Typically, three products are used below grade: extruded polystyrene, expanded polystyrene, and rigid mineral fiber panels. (Baechler et al. 2005). Extruded polystyrene (nominal R-5 per inch) is a common choice. Expanded polystyrene (nominal R-4 per inch) is less expensive, but it has a lower insulating value. Below-grade foams can be at risk for moisture accumulation under certain conditions. Experimental data indicate that this moisture accumulation may reduce the effective R-value as much as 35%-44%. Research conducted at Oak Ridge National Laboratories studied the moisture content and thermal resistance of foam insulation exposed below grade for fifteen years; moisture may continue to accumulate and degrade thermal performance beyond the fifteen-year time frame of the study. This potential reduction should be accounted for when selecting the amount and type of insulation to be used (Kehrer, et al., 2012, Crandell 2010).
Rigid fiberglass and rigid mineral wool panels (R-4 per inch) do not insulate as well as extruded polystyrene, but are the only insulations that can provide a drainage space for foundation walls because of their porous structure. Insulation that is exposed above grade must be protected with a coating to prevent physical damage and degradation. Such coatings include fiber cement board, parging (stucco type material), treated plywood, or membrane material (Baechler et al. 2005).
Interior crawl space wall insulation (Figure 3-6b) is more common than exterior, primarily because it is less expensive since no protective covering is required. On the other hand, interior wall insulation may be considered less desirable than exterior insulation because it (1) increases the exposure of the wall to thermal stress and freezing, (2) may increase the likelihood of condensation on sill plates, band joists, and joist ends, (3) often results in some thermal bridges through framing members, and (4) may require installation of a flame spread resistant cover. Interior insulation is not recommended on non-core filled masonry block walls, due to an increased risk of moisture accumulation within the assembly.
Materials that are resistant to moisture damage are recommended for use in contact with concrete foundation components. Rigid foam plastic is typically used to insulate the walls of unvented crawlspaces (Figures 3-7 and 3-8). In areas not prone to termite infestation, rigid foam should be installed and sealed at the rim joist to prevent entry of moist air into the wood structural components. This air barrier is especially critical in cold climates, and when exterior insulation is not installed. Batt insulation should only be used at the rim joist where access is required for termite inspections. Expanded or extruded polystyrene rigid foam insulation should be used to cover the walls and attached with mechanical fasteners. A three inch wicking gap should be left between the wall insulation and the ground, and a three inch termite inspection gap or continuous termite shield should be present at the top of the wall and the sill plate (Marshall 2008). An ignition barrier or fire barrier may be required, based on code jurisdiction and occupancy.
It is possible to eliminate the ignition barrier requirement. This has been done by using foil-faced polyisocyanurate insulation panels, which are rated for exposure in basements and crawlspaces in some jurisdictions. Note however that the unperforated foil facing is completely vapor-impermeable, and very little drying will occur through it.
Interior insulation retrofits carry additional risks: capillary breaks may not be present, either at the top of the wall or between the foundation and the framing; insulating on the interior will tend to increase moisture accumulation in the framing in that case. A capillary break may not be present between the footing and the wall, potentially increasing the presence of moisture due to capillary wicking. Since waterproofing and drainage systems are often not present or not working on older houses, bulk water penetration is possible. For description of a robust retrofit interior insulation strategy see Ueno (2011).
Insulation placed horizontally around the crawl space floor perimeter can provide additional thermal protection for sealed crawlspaces with interior or exterior insulation on the foundation walls. However, it may also create additional paths for termite entry. In cold climates, insulation of the entire floor area to prevent heat loss may be desirable.
In a vented crawl space, the insulation is always located in the ceiling (Figures 3-6e and 3-9). There are two recommended approaches to crawlspace ceiling insulation:
These systems are the only ones capable of preventing mold and decay due to high humidity conditions that may occur in the crawlspace in most climates (Lstiburek 2008). Impermeable floor finishes like vinyl flooring and some types of ceramic tile must be avoided to allow the floor to dry upward into the home.
In addition to more conventional interior or exterior placement covered in this handbook, there are several systems that incorporate insulation into the construction of the concrete or masonry walls. These include (1) rigid foam plastic insulation cast within concrete walls, (2) polystyrene beads or granular insulation materials poured into the cavities of conventional masonry walls, (3) systems of concrete blocks with insulating foam inserts, (4) formed, interlocking rigid foam units that serve as a permanent insulating form for cast-in-place concrete, and (5) masonry blocks made with polystyrene beads instead of aggregate in the concrete mixture, resulting in significantly higher R-values. However, the effectiveness of systems that insulate only a portion of the wall area should be evaluated closely because thermal bridges around the insulation can impact the total performance significantly.
Techniques for controlling the entry of termites through residential foundations are advisable in much of the United States (see Figures 3-10 and 3-11). The following recommendations apply where termites are a potential problem. Consult with local building officials and codes for further details.
Plastic foam and batt insulation materials have no food value to termites, but they can provide protective cover and easy tunnelling. Insulation installations can be detailed for ease of inspection, although often by sacrificing thermal efficiency.
In principle, termite shields offer protection, but should not be relied upon as a barrier. Termite shields are shown in this document as a component of exterior insulation systems. Their purpose is to force any insects ascending through the wall out to the exterior, where they can be seen. For this reason, termite shields must be continuous, and all seams must be sealed to prevent bypass by the insects.
These concerns over insulation and the unreliability of termite shields have led to the conclusion that soil treatment is the most effective technique to control termites with an insulated foundation. However, the restrictions on some traditionally used termiticides may make this option either unavailable or cause the substitution of products that are more expensive and possibly less effective. This situation should encourage insulation techniques that enhance visual inspection and provide effective barriers to termites. For more information on termite mitigation techniques, see NAHB (2006).
Construction techniques for minimizing radon infiltration into a crawl space are appropriate if there is a reasonable probability that radon is present (see Figures 3-12 and 3-13). To determine this, contact the state radon staff.