Fixing, joints, sealing and final adjustment are part of the system.
Performance does not stop at the factory. A window has to be fixed properly, set level, sealed and adjusted so that it works the way it was designed to.
correct anchoring,
a check of the geometry,
a suitable joint and proper sealing,
attention to sills, thresholds and drainage,
final adjustment of the hardware.
The declared air permeability class applies to the window as a product; in the building it is the joint around it that counts. The principle is almost always the same: tighter inside than outside, so that vapour does not enter the joint from the room side and whatever does enter can dry outwards. What decides the installation is the support — fixing spacings and setting blocks carry the weight into the wall and not into the foam —, the position within the thickness of the wall, which affects the thermal bridge at the reveal, and the drainage at the threshold.
The joint is part of the window
The declared air permeability class applies to the window as a product. In the building, what counts is the joint around it. The principle is simple and almost always the same: tighter inside than outside. Water vapour then does not enter the joint from the room side, and whatever does get in can dry outwards.
What decides the installation
the support: fixing spacing and setting blocks carry the weight into the wall, not into the foam,
the position within the depth of the wall — it affects the thermal bridge at the reveal,
the drainage at the sill and the shelter from driving rain.
Damp in the corner of a reveal does not automatically mean a bad window. Often it means a cold internal surface at that point — a thermal bridge in the wall, not in the frame.
Frequently asked questions
Why is there a draught when the window is new?+
The declared air permeability class applies to the window as a product. In the building it is the joint around it that counts: tighter inside than outside, so that vapour does not enter from the room side.
Does damp in the corner of a reveal mean a bad window?+
Not automatically. It often means a cold internal surface at that point — a thermal bridge of the wall, not of the frame.
