Air tightness in new buildings has been discussed for several decades. But the knowledge in the subject is low. With tougher requirements from the Swedish government about energy consumption for a new building required competences how these shall be met. It's not only the thick mess of insulation that are important, it's also the air tightness. This report is about air leakage in Swedish houses. How these leaks can be measured/detected and what the consequences may be of them. Both theoretical and practical tips and advice will be given. The author wants to increase the knowledge in this subject for people in the Swedish building industry.