Crawlspace Design & Construction
Designers, contractors and homeowners must understand the connection between the crawlspace of a raised floor system and the living space above. Every effort should be made to keep moisture out of the crawlspace. Provisions should also be made to maximize drying of any moisture that enters a crawlspace. Local climatic conditions will dictate the specific design and construction details for a particular raised floor system.
The subject of moisture control in crawlspaces is an area of ongoing research. Most crawlspaces today are constructed as unconditioned and vented systems, with building codes mandating minimum ventilation requirements. As an alternative, conditioned and unvented crawlspace systems are sometimes used and also recognized in the codes.
Unconditioned and Vented Crawlspaces
Figure 3 provides design detail considerations for unconditioned and vented crawlspaces. Important elements for the best moisture control include:
- Ground drainage
- Ground cover (vapor/gas retarder)
- Insulation installed within the floor cavity
- Plumbing located within the floor cavity, or well-insulated plumbing
- Air distribution ducts within the floor cavity or the interior of the structure
HVAC systems of many houses built over crawlspaces deliver conditioned air through ducts located in the crawlspaces. Whenever possible, ductwork should be located within the floor cavity (e.g. between joists, or between or through trusses or I-joists). When that is not possible, adequate clearance between the bottom of the ductwork and the ground should be provided to maintain proper ventilation. All ductwork should be meticulously sealed to avoid unnecessary energy losses from air leaks. Penetrations for plumbing, wiring and air ducts should also be sealed to minimize air exchange between the crawlspace and the living space. Ductwork should also be insulated to prevent condensation on the ducts. In addition, insulation should be carefully installed to the underside of the floor within the floor cavity. Never vent moisture or heat-producing sources (e.g. clothes dryers, kitchen or bath vents) into the crawlspace.
Building code requirements for ventilation openings through foundation walls are intended to reduce moisture levels in the crawlspace. Section 1203.3 of the 2003 International Building Code sets forth the underfloor ventilation openings and cross ventilation requirements for enclosed crawlspaces, such as within stem wall foundations. Open pier-and-beam foundations, commonly used with raised floor systems, already create a fully vented crawlspace.
Generally, building codes mandate that the minimum net area of ventilation openings required are not less than one square foot for each 150 square feet of crawlspace area. When an approved vapor retarder covers the underfloor ground, the minimum vent opening area can be decreased to one square foot for every 1500 square feet of crawlspace area. Vent openings are placed to provide cross ventilation of the underfloor space. These vent openings should be screened to inhibit pest entry into the crawlspace (see Pest Management). They also should not allow rain water or runoff to enter into the crawlspace.
Conditioned and Unvented Crawlspaces
Conditioned and unvented crawlspaces are only recommended when mechanical systems distribute conditioned air within the underfloor area. A conditioned and unvented crawlspace typically has insulated walls and can be thought of as a short basement. This type of crawlspace is designed to communicate with the living space. It should be dry, temperate, and have good air quality. Conditioned air spaces should not be ventilated with outdoor air.
Conditioned and unvented crawlspace systems should have a continuous ground cover sealed to insulated perimeter walls and any supporting piers. Care should be taken with all air-sealing construction details. This is necessary to minimize the unintentional introduction of unconditioned air, reducing the possibility of condensation on cold surfaces. In addition, extra care should be taken to prevent moisture from being trapped in the crawlspace. Any moisture that does get into the crawlspace should be remediated immediately.
For more information about proper construction of closed crawlspaces, click here.
Ground Cover (Vapor/Gas Retarder)
Draining storm water away from the foundation, preventing standing water beneath the crawlspace, and making provisions to remove excess moisture entering the crawlspace, are all important elements needed to provide a dry, trouble-free raised floor system. Control of ground moisture is also essential. One of the best ways to control this moisture is through the use of a ground-applied vapor retarder.
Exposed soil in crawlspaces and under porches and decks should be covered with an approved vapor retarder. A ground cover that retards transmission of water vapor from the soil into the crawlspace provides an effective way to prevent moisture and humidity problems. It should have a permeance of no more than 1.0 perm, complying with ASTM E1745, to resist alkali and other chemicals that can be contained in soils. It should also be rugged enough to withstand foot and knee traffic. The most commonly used ground cover material is a 6-mil (0.006 inch) polyethylene.
Before installation of the ground cover, the crawlspace floor should be smooth and free from sharp rocks and construction litter. Exact installation details will vary depending on the primary function of the ground cover (i.e. moisture control or radon gas control). For any crawlspace system, it is important to avoid standing water on top of the ground cover.
For unconditioned and vented crawlspaces, the edges of the cover should be overlapped 4″ to 6″. The cover does not need to extend up the face of the foundation wall, and no sealing is required. If the control of radon or other soil gases is not of primary importance, the ground cover may be cut in several low spots to provide drainage if needed.
Conditioned and unvented crawlspaces should have a continuous ground cover over all crawlspace soil. The ground cover should be sealed at the joints, as well as sealed to the perimeter wall and any piers. A thin layer of concrete added over the ground cover provides a better seal and further inhibits the entry of rodents.
In areas where radon gas is a concern, care should be taken to vent radon away from the building. By its very nature, an open pier-and-beam foundation readily dissipates radon gas. In enclosed, continuous wall foundations, the components of a passive, sub-membrane depressurization system are readily installed during construction. The soil within the crawlspace should be covered with a continuous layer of 6-mil polyethylene (minimum) soil-gas retarder. In addition, enclosed crawlspaces should be provided with tightly sealed pipes vented to the exterior of the building in accordance with the code. For more details, see Radon Reduction in Wood Floor and Wood Foundation Systems from the American Wood Council at www.awc.org.
Additional Moisture Control Requirements
In geographic areas where experience has demonstrated a need for more protective moisture control measures, the preceding general requirements should be modified to meet local climatic conditions.
1 Handbook of Fundamentals, American Society for Heating, Refrigerating and Air Conditioning Engineers, 2001.
2 The Case for Conditioned, Unvented Crawl Spaces by Nathan Yost, M.D., Building Safety Journal, International Code Council, May 2003.