Basements and crawlspaces can have a big impact on the performance of wood flooring. Some of these spaces are finished and conditioned, while others are not. Understanding how these spaces can affect wood floors is critical to achieving a successful wood flooring installation.
The first thing to consider when evaluating basements and crawlspaces is the foundation walls. Foundation issues translate to flooring issues. Cracks in foundation walls may be a larger sign of settling, structural damage, or water infiltration. These are structural issues that will need to be fixed by a qualified contractor before flooring installation can begin.
A basement can be partly or entirely below-grade. Basements normally are constructed to keep both liquid and capillary water from finding its way into the structure using vapor barriers on the foundation walls, surface drainage systems, below-grade drainage systems, perimeter drainage systems, and capillary breaks.
The ambient conditions of the basement will change from season-to-season and may affect the flooring above. These ambient conditions will differ depending on the space being finished or unfinished. In a finished basement, the walls normally are insulated, and the space is heated and cooled similarly to the living space above it. This is considered a conditioned space.
In unfinished basements, the walls are not insulated, and the heating and air conditioning often is not turned on, or is maintained differently than the upstairs living spaces. Unfinished basements are considered unconditioned spaces. Unconditioned basements typically are cooler and have higher RH levels than the living space above. If an unfinished basement becomes finished, the conditions below the floor will change, which could affect a wood floor that is already installed.
A crawlspace is an open area of a building that is partly or entirely below-grade. It is not a complete floor level. It typically is just deep enough to allow a person to gain access to the under-floor area by crawling.
The distance from the earth to the underside of the floor joist must be a minimum of 18” and a minimum of 12” from the earth to the underside of the beams. If the crawlspace is built on piers or stilts, the pier or stilt should be set on the footing evenly.
The temperature and moisture conditions in a crawlspace will differ from the temperature and moisture conditions in the living space of the structure. In general, unconditioned crawlspaces are cooler and have higher relative humidity levels than in the living space above.
Humidity levels in crawlspaces are elevated by the evaporation of moisture from the soil. Evaporation generally is greatest during Summer, when the soil is warmer, and less during the Winter, when it is cooler. A Class I vapor retarder installed over the ground greatly reduces evaporation from crawlspace floors, thereby lowering crawlspace humidity levels.
The temperature gradient from the cooler underside of the subfloor system in the crawlspace to the indoor living side of the subfloor system can be drastic. This temperature gradient may result in condensation forming on the underside of the subflooring due to the dew point.
A crawlspace can be classified into three general categories: open crawlspaces; ventilated crawlspaces; and enclosed and conditioned crawlspaces.
Open crawlspaces include open pier-and-beam foundations. These are considered unconditioned spaces. Open crawl spaces may have a continuous wall on just one side and be open on the other sides.
Skirting these types of crawlspaces to form an enclosed crawl space and then adding venting could result in moisture issues, especially in hot and humid areas of the country.
Ventilated crawlspaces are regulated by the International Residential Code, section R408. The regulations are as follows:
- The underfloor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement) should have ventilation openings through foundation walls or exterior walls.
- The minimum net area of ventilation openings should not be less than 1 square foot (0.0929 square meters) for each 150 square feet (14 square meters) of under-floor space area, unless the ground surface is covered by a Class I vapor retarder material.
- Where a Class I vapor retarder material is used, the minimum net area of ventilation openings should not be less than 1 square foot (0.0929 square meters) for each 1,500 square feet (140 square meters) of under-floor space area.
- One such ventilating opening shall be within 3 feet (914 mm) of each corner of the building.
Enclosed and conditioned crawlspaces meet all requirements as detailed in International Residential Code, section R408.3, and are conditioned and maintained at the same temperature and humidity levels as the above interior living space. These crawlspaces present the ideal circumstances to create a balanced condition below and above the flooring system.
International Residential Code, section R408.3, states that ventilation openings in crawlspaces are not required where the following items are provided:
- The exposed earth is covered with a continuous Class I vapor retarder. Joints of the vapor retarder should overlap by 6” (152 mm) and should be sealed or taped. The edges of the vapor retarder should extend not less than 6” (152 mm) up the stem wall and should be attached and sealed to the stem wall or insulation.
In addition to the International Residential Code, section R408.3 Class I vapor retarder requirements, one of the following requirements also must be met.
- Continuously operated mechanical exhaust ventilation must be provided at a rate equal to 1 cubic foot per minute (0.47 liters per second) for each 50 sf (4.7 square meters) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grill), and perimeter walls insulated in accordance with International Residential Code, section N1102.2.22, or
- Conditioned air supply sized to deliver at a rate of equal to 1 cubic foot per minute (0.47 liters per second) for each 50 sf (4.7 square meters) of crawlspace floor area, including a return air pathway to the common area (such as a duct or transfer grill), and perimeter walls insulated in accordance with International Residential Code, section N1102.2.22, or
- Plenum in existing structures complying with International Residential Code, section M1601.5, if the crawlspace is used as a plenum, or
- Dehumidification sized to provide 70 pints (33 liters) of moisture removal per day for every 1,000 sf (93 square meters) of crawlspace floor area.
In some cases, vapor retarders are installed on the underside of the joists in crawlspaces. According to the International Residential Code, section R408.8, in hot and humid climates, it is a standard requirement for the builder to provide installation of a continuous Class I or Class II vapor retarder to be installed on the exposed face of air-permeable insulation installed between the floor joists, and exposed to the grade in the under-floor space. This vapor retarder is not required in unvented crawlspaces constructed in accordance with the International Residential Code, section R408.3.
Insulation plays an important role in crawlspaces as well. In unconditioned spaces, such as open and ventilated crawlspaces, insulation impacts the temperature gradient and moisture migration from an unconditioned space into a conditioned space. Insulation installation should be completed by a qualified professional.
Building codes in many climate zone regions dictate construction methods related to insulation and moisture control systems installed in crawlspaces. Common insulation and vapor retarding systems used below the subfloor include fiberglass batt insulation, closed-cell spray foam insulation, and foil-faced rigid insulation panels.
The National Wood Flooring Association has detailed information about basements and crawlspaces, and how they impact wood floors, available through its Wood Flooring Installation Guidelines. These Guidelines are available for free to NWFA members. More information is available at nwfa.org/technical-guidelines.