Measure the perimeter of your home along the base of the foundation with a measuring tape device. Consult your local building code book for specific details on how far apart each crawl space vent must be. This data varies wildly from county to county and state to state. In most cases, building code will require 150 square feet of ventilation for every 1,000 square feet. Standard crawl space vents are four-inches by 10-inches.
Select a starting point on the rim joist between the two sub floor joists. This will probably be concealed by siding and plywood, so you will have to use a beam locater device to determine the space between the joists. Measure and mark the vent hole on the first bay just above your foundation line. The starting point should be in one of the four corners of your home. You should place a vent in every third bay. The bay is the 16-inch gap between two joists above the foundation. Use your beam locater to determine this gap.
Drill a 1/4-inch hole in one of the four corners of your marked out vent hole. Insert a powered jigsaw into the drill hole, and cut along the marked line until the vent hole is cut out. Place your vent inside the hole to see how it fits.
Apply a thick strip of silicone caulk to the back side of your crawl space vent's faceplate. Slide the vent into your newly cut out hole, and press the faceplate firmly up against the house.
Locate the pre-threaded rods and locking nuts that came with your vent. Thread a locking nut into one of the pre-threaded rods so the nut is sticking out of the rod by about 3/4-inch. Repeat this for the other threaded rod in the back of mounting hardware.
Grasp one of the assembled threaded rods and locking nut. Reach inside the vent with the locking nut facing toward you. Feel around the faceplate to locate the screw hole on the inside of the faceplate. Place the locking nut into screw hole on the faceplate, and thread the locking nut into the faceplate. Tighten the nut with a 7/16 inch wrench. Repeat this to install the other rod on the other side of the vent.
Grasp the backing bar piece that came with your vent's hardware, and reach it into the vent's opening. Situate the backing bar so it spans the width of the vent hole. Align the two holes on either side of the backing bar with the two rods extending from the faceplate inside the vent hole.
Push the two rods through the holes in the backing bar. Fasten the backing plate to the rods by threading the two included nuts to rods poking through the holes on the backing bar.
Place the vent's screen door inside the vent, and attach it to the vent's faceplate with the fastening nut that came with the vent. Tighten the nut as tight as possible with your fingers.
Repeat this procedure to install every crawl space vent.