We had occasions when we were plagued with them in the house. We would zap one then ten minutes later another one would buzz by. We have nets on all windows and all doors, and were paranoid about keeping them shut at all times. However whilst visitng one of the bathrooms I noticed four in there. I had only previously cleared this room half an hour ago. It was the extractor fan, which extracted into a large chimney like cavity. I removed the front cover of the fan and placed netting over the inside face. The fan itself is situated a good 6 inches away so there is no way of fouling. Havnt had 1 mosquito in the place for over a week now.