For airtightness in passive houses, the minimum requirement is an ACH₅₀ of at most 0.6 per hour. This means that at a pressure differential of 50 pascal only 60 percent of the air inside a building may be exchanged per hour.

You can mention to a customer mainly the following benefits:

• Thermal comfort: cold air coming in through leaks leads to draughts, pools of cold air resulting in cold feet, and an uncomfortable vertical layering of temperatures both in individual rooms and throughout the building.
• Reduced thermal energy requirement: for the reasons given, making a building airtight leads to significant savings in energy and cost.
• Structural damage is avoided: if air flows through permeable building elements from inside to outside, water vapour cools down and condenses within the structure, forming droplets inside the elements, which results in structural damage.

The main aspects of the air seal strategy are:

• Select the simplest possible shape for the building envelope, with little variation in material.
• Define the position of the windproof and airtight layer, with unheated areas such as the basement clearly separated.
• Minimize the length of junctions, make surfaces as homogeneous as possible.
• Choose simple structures, avoid penetrations.
• Reduce penetrations for building services; if appropriate, plan a service layer.
• Define materials and installing techniques that seal areas and joints.
• Precise planning of details and coordination with tradesmen are vital.