In my mind's eye a metal building can be a pretty porous structure, so even though you know where the bats CURRENTLY are entering and leaving it would be wise to take a pair of binoculars and walk the entire perimeter to see where else they may be able to get in. There also will often be multiple entry points, and you may not see this until you visit the site at early evening when the bats are leaving and find a way to observe all sides of the structure, perhaps with the help of some friends. Even if you block the preferred opening bats will attempt to find another way into this chosen roosting site if one is available, so all openings need to be permanently closed.
Some people have better results with evicting bats if they offer the bats an alternative outside but a little away from the structure, and bat "houses" are popular for this. Given the benefits of bats and their current plight with "white nose syndrome" we should do what we can to preserve the bats and allow them to continue to live in an area..........just not in our buildings. Once you manage to get these bats out and keep them out it would be good customer service to inspect the area where they have been living to see what kind of mess they are leaving behind. Their feces and urine are not healthy materials to leave in a structure inhabited by people, and some cleanup and sanitation may be needed.
This time of year one little concern you may have is that these bats may be breeding inside this structure too, and this could continue through the warm months. If you exclude the adult bats at this time you may be causing their young to be left behind to die, and this is not a good thing. So, if you can determine that young bats that cannot leave on their own are within this structure it would be advisable to hold off on the exclusion until early fall, or early September at the earliest. Repelling them from the interior with ultrasonic devices is completely ineffective and the use of moth balls is generally illegal, not to mention that you would have to use so much of the moth balls that it would create an awful odor for human inhabitants. The permanent solution is allowing them to leave willingly but not return.
Allowing bats to leave is often done with a one-way flap or sleeve that allows the bats to push their way out through that entry hole and past or through the material you have over it, but prohibit them from getting back through that material to re-enter. Not all bats leave every night, so whatever you place over their openings should be left there for at least 7 days. Bird netting of fine mesh can be used as a large flap over the opening that they can push aside to exit but cannot lift for re-entry, and hung at least 2 feet below the opening. A tube can be made from some material and also hung over that opening. Univar sells several configurations of "Bat Cones" that also work well, offering a cone that narrows toward the end so that the bats are unable to work their way back through the ultimate opening.
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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.