Foundation vent screens keep the bugs out and allow proper ventilation of the crawl space below a house. They are absolutely necessary and essential to a home's well being, but sometimes the vents rot and age and need replacing. It is important to learn how to spot a bad screen, and, after doing so, remove it and replace it.
Check the screen for holes and gaps where rodents and bugs may enter. Check to see if the screen will slide or open. If problems exist, you must replace the screen.
Use a hammer and brick chisel to carefully chip away the mortar that was applied to the outside edges of the screen. It will almost always chip right off and into single chunks. Be careful not to chip the concrete or concrete block opening in which the screen sits.
Remove the screen from the opening. It may be necessary to pull it from the hole, using the claw of your hammer. Don't worry about any damage to the old screen, you will discard it anyway. Use a wire brush to clean any loose mortar from the opening.
Gently slide the replacement vent into the opening with the screen facing out. There will usually be an arrow pointing out if the screen is installed correctly.
Using the small pointing trowel and a bucket of premixed mortar mix, repack mortar around the vent. Pay special attention to the way the original vent was packed and try to match the angle of the mortar. Make sure it is smooth and as straight as possible. Allow the mortar to harden over 24 hours before attempting to use the vent.