Basement and Crawlspace
Another area of a house, which must breathe for its well being, is the basement and crawl space. Lack of proper ventilation in a basement or crawl space is frequently a problem. The use of dehumidifiers during the warm months of the year is beneficial in assisting the removal of moisture from the air. Circulation of the air also helps reduce problems, assuming the volume of air being moved is consistent and sufficient.
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Crawlspaces can add considerable moisture to a house. A vapor barrier of 6 mil or greater polyethylene laid over the earth in the crawl space area with a minimum of joints (overlap joints a minimum of 24 inches) is generally recommended. To be effective, vapor barriers must be continuous. Installation of paper or foil-faced insulation between the floor joists will also retard infiltration of moisture into the house. The vapor barrier on the insulation should be placed against the heated side or the subflooring. If you use single-faced insulation, the exposed insulation should face the crawl space (fuzzy side down). Insulation with a vapor barrier facing on both sides is a good option for insulating a crawl space or basement.
In the summer months, the outside air will typically be 15 to 25 degrees warmer than the air in the crawl space. This will cause the humidity to rise in the crawl space, because warmer air has more ability to hold water than cooler air. In dryer climates, this may not be important. In coastal and northern climates, depending on the conditions in the crawl space, moisture may reach its dew point, which makes a case for ventilation. With proper ventilation, the saturating air in the crawl space will be diluted and the relative humidity controlled. If there is a dirt floor, a polyethylene vapor barrier should be installed to keep moisture from migrating out of the soil. A dirt floor should be looked upon as a large evaporator plate. In the winter months, the outside air will be cooler than the air in the crawl space. The warmer air in the crawl space will have more ability to hold water than the cooler outside air. The potential for condensation is remote at best in colder months, except in some coastal areas with excessive humidity. Generally, crawl spaces do not have to be ventilated in the winter, as long as there is no water penetration from other sources such as negative surface grades, which could create excessive moisture in the crawl space. Having warmer air in the crawl space is the key. Ventilating the crawl space is okay in colder months, however, the living space floors above the crawl space may be cold, uncomfortable and waste heat, even if they are insulated. For most climates, the recommendation is to ventilate the crawl space in the summer and close them in the winter. If there is a basement, a window or opening should be left open into the crawl space, even if it allows some heat loss from the basement to the crawl space.
Exhaust vents (kitchen, bath, dryer, etc.) should not terminate in a basement or crawl space. They should terminate to the exterior of the structure.
The space between the bottom of the floor joist and the earth under any building (except such space as occupied by a basement or cellar) should be provided with ventilation openings through foundation or exterior walls. Ventilation openings require a net area of ventilation not less than one square foot of clear air for each 150 square feet of crawl space area. One ventilating opening must be located within 3 feet of each corner of the building. The total area of ventilation openings may be reduced to one square foot for each 1500 square feet of under-floor area, when the ground surface has been covered with a continuous and properly lapped vapor barrier material. There is still a requirement for one ventilation opening to be located within 3 feet of each corner of the building. The vents may have operable louvers. Ventilation openings may be omitted on one side of the building (generally in the front).
Vapor Barrier and Vents
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Vapor barriers are intended to prevent the moisture vapor entering crawlspaces area. The vapor barrier is usually comprised of sheets of 6 millimeter polyethylene (plastic). The sheets are laid in rows over the soil. The edges of the sheets are overlapped and tapped to form a continuous barrier that will trap the rising moisture. In most locations, the vapor barrier is placed only over the soil and secured with weights. In some regions, the vapor barrier is continuous over the foundation wall and secured to the sill plate.
Crawlspace vents prevent moisture vapor from accumulating in the crawlspace by providing opening for the moisture to pass to the exterior. Vents are required on new homes. The general requirement is one square foot of vent opening for 150 square feet of under-floor area.