It is hardly possible to make a building completely pest proof, but on the other hand, it is foolish to just invite them in.
Proofing windows and other entry points, such as ventilation openings and holes, pipe ducts and vents with mesh can be a simple and effective measure. The mesh will of course have to be of a quality that can withstand the elements.
The denser the mesh, the more it will keep out light and air, so you should consistently choose the largest mesh aperture that can still stop the pests you want to keep out. Mesh aperture is usually given with a number that says how many loops there are per inch. If the mesh aperture is 10, it means that there are 10 loops per inch. With a wire thickness of approximately 0.3 mm, the opening (the distance from thread to thread) is approximately 2.25 mm. A mesh with an aperture of 10 can keep houseflies, blowflies, wasps and bees out. A mesh with an aperture of 18. meaning openings of 1.15 mm, keeps fruit flies and mosquitoes out too. Birds can be kept out by a wire netting.
Doors and gates should preferably be equipped with automatic closing mechanisms. If they necessarily must be open for the sake of customers or trucks, the insects can be slowed down and sometimes stopped completely by air blocking carpets. The system must usually be tailored to each case. A downwardly directed air flow, which point a little outwardly, at a rate of approximately 30 kilometres per hour will usually be sufficient. It will also reduce the risk of invasion of sparrows, but does not bother people who pass it. Excess pressure on the entire building is also an option. Excess pressure which allows an outward flow of air through the openings to be secured, at approximately 15 km per hour will for example keep fruit flies out.
Rat and mouse proofing and protection against birds pose special problems.