Mildew thrives in damp areas, and can be removed with chemical treatment (bleach) and/or heat. Ventilation is another method of exhausting moist air and stunting mildew growth. Once killed, mildew odor disappears. If the mildew odor does not disappear the mildew has not been done away with.

When in good condition, the grout in your shower is capable of preventing mildew spores from traveling into the areas behind your tile. It seems apparent that while your grout was cracked they decided to get back there and homesteaded.

Solution: You probably won't need the builder to solve your problem. A small portable space heater should do the trick. You must not use the shower at all during this process. The idea is to kill the unwanted fungus with hot dry air. Simply place the heater in the shower (facing the problem area), turn it on, and leave it there for about three days. Regular (monthly) bleaching of your tile grout will help prevent mildew from returning.