Crawl Spaces

Crawl Spaces. That Space Under Your House

Proper preparation of the crawl space is critical to the long-term success of a building. A properly prepared crawl space will function to keep a home sound for generations to follow. Crawl spaces are associated with perimeter foundations or post and pier foundations. A perimeter foundation has the appearance of a continuous concrete wall above grade with openings to allow air into the crawl space. Pier and post foundations involve the use of concrete piers at regular intervals with posts extending off the pier to a floor system above. A post and pier system will have a skirting extending to earth to give the house a finished appearance.

Behind either of these exterior walls lies the transitional zone between the soil and the house above. The following will aid in a better understanding of the crawl space and its elements, design and the areas where problems can occur.

The Uniform Building Code Regarding Crawl Spaces

  • The Uniform Building Code (UBC) sets out several rules based on generations of experience. These rules apply to all crawl spaces.
  • All crawl spaces must have an entry 20" x 24". In order to move about in a crawl space, the Code requires a minimum of 18" of clearance below the large 2" X 8", 10" or 12" boards called joists. There must be 12" below the large beams called girders.
  • To provide for ventilation and air circulation, the crawl space must have openings or vents in a ratio of one square foot for each 150 square feet of floor above. These openings are covered with a 1/4 inch screen mesh. The soil in a crawl space must be covered by a vapor barrier which is simply a plastic sheeting. Last, the Code requires wood and earth to be separated by 6". Any discussion of the Uniform Building Code must be prefaced with the understanding that the standards in the code are minimum standards. The code makes no effort to establish maximum standards, hence one must realize that compliance with the Uniform Building Code may not be adequate because of unique characteristics in each home.
  • A house constructed on soil which allows for good migration of water such as a sandy formation at the top of a hill may be well served by a crawl space 18" deep where the water evaporating into the air and being carried out through the vents is minimal. Conversely the same house at the bottom of a hill or a house with a reverse slope (soil sloping toward the house rather than away) may be burdened with substantially more water coming from the soil into the air in the crawl space and the minimum clearance and vent openings may not keep the crawl space dry.
  • Similarly, a home built thirty years ago would not have a crawl space with insulation and the addition of insulation will require an increase in air circulation to keep the moisture held by the insulation at a safe level.

Vapor Barriers

A vapor barrier is a durable plastic sheeting placed over the ground once all the wood debris has been removed. It functions by causing water rising from the soil to collect in beads on the underside of the plastic and fall back into the soil. This reduces the relative humidity of the air in the crawl space and hence the moisture content of the construction lumber above.

Vent Screens

The code contains an unusual provision on the venting of crawl spaces; code enforcement officials are allowed to deviate from the standards if they believe conditions are such that the ventilation standard is too great. The result of this discretion has been tremendous variation in crawl space ventilation around the community. The critical question is: Does the crawl space ventilation keep the crawl space dry enough to prevent deterioration?

In considering vent screens the critical concept is net free area. Simply, net free area is calculated by using length times width less the obstruction created by the wire mesh and louver. It is never useful to install louvers over a screen mesh and it is very common to see 1/8 inch screen mesh. The use of 1/8 inch mesh and louvers is never acceptable because the reduction in free area is too great to allow the vent to perform. Vents are located near corners to eliminate inactive air spaces and cross ventilation is essential to positive movement of air by outside breeze or convection.

Minimum Clearance

The minimum clearance between joist and earth has two functions. The first enables a worker to move about in the crawl space and the second and most important is that the clearance between joist and earth limits the cubic volume of air in the crawl.

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