Even with the best thermal insulation, leaky joints in the building shell can cause heat loss and consequently higher energy costs as well as damage to the structure. 

Air tight means that air from the inside cannot escape to the outside. Wind tight, by contrast, means that wind cannot force itself into the insulation from outside.

By the way, please forget the outdated notion that leaky windows and doors provide the fresh air that you need.

Importance of a tight building shell

• large heat loss due to leaks because of construction errors, especially with low temperatures and strong wind
• often a creeping phenomenon that becomes noticeable only in the next heating period
• undesirable draft (cold feet or cold breeze from above)
• structural damage due to condensed humidity
• danger of construction errors with multiple-layer structures and complex structures
• dry air in winter due to increased air exchange (wood shrinkage).
• reduced noise protection

What to watch for

Roof construction

• If an attic is extended to serve as living space, then a vapor barrier is required on the inside (must be air-tight to avoid heat loss).
• Work must be done properly and cracks and spaces avoided .
  
  Particular weak points
  
  
  • transition from roof to wall, between roof and attic floor, and roof and end walls
  • installation of dormers
  • building elements such as rafters that penetrate the insulation
  • integration of attic steps
  • installation of roof windows
  • penetration of ventilation pipes, chimney, etc.
  • Aufsparrendämmung in den Randzonen

Windows and doors

• Adjust doors and windows
• Cracks often occur between window or door frames and wall structures as well as window sills.
• Seal the joints between windows and the neighboring structure: diffusion resistant inside and wind-tight to the outside.

Outside walls

• Wood frame constructions require sealing with a vapor barrier inside and außenseitig mit wind paper.
• Mortar joints for hollow brick structures must be fully sealing in peripheral areas.

Electrical installations

• Caution: If vapor barriers are penetrated on installation of pipes and other lines, the openings must be sealed.