Mobile Home Skirting: At a minimum, skirting must be vented in all four corners. A good rule of thumb is at least one square foot of open vent area per 150 square foot of floor area. Keep in mind that vents with insect screens, louvers etc reduce the effective open area of the vent by 30-50%. Since you'll want insect screens and louvers on your skirting vents, be sure to calculate the added 30-50% when determining the number of vents you will need to install in your skirting.
Older trailers built before 1976 conform to different building codes than those built later. Because of this, many of these older homes require repair and remodeling to be brought up to local building codes. While repairing mobile homes is possible, determining the cost and time of repair depends on the age and condition of the mobile home.