The requirements for the installation are defined by the ČSN 74 6077 standard, which specifies that the installation must not impair the functional characteristics of the entire filling of the opening. What is essential is the proper design of the contact of the filling of the opening and the structure or the connection joint which must be air-tight, water-tight, must eliminate thermal bridges, exhibit sufficient sound attenuation values and must enable expansion movements of the window.

Parts of the connection joint

  • inner closure of the connection joint (vapour barrier) in the form of plastic foil, bituminous strip, foam tape or sealant
  • anchoring elements of the window (strip anchors, sliding pins, brackets)
  • pads (supporting, spacing, rectifying)
  • thermal insulation filling of the joint (PU foam)
  • outer closure of the connection joint (vapour permeable/diffusion layer) in the form of plastic foil, foam tape or sealant


Application method

  • preparation of the opening or base (solid, free of dust and impurities)
  • installing the window into the openings on support pads (remaining permanently in the structure)
  • levelling the window in the opening with the use of spacing pads to delimit the distance of the window from the window reveal (remaining permanently); if necessary, temporary rectifying wedges are used that are removed after the installation
  • anchoring the window by means of suitably selected anchor type
  • removing the rectifying wedges
  • wetting the base e.g. with a water spray
  • filling the connection joint with PU mounting foam in such a way as to fill the joint evenly and sufficiently, i.e. the foam should exceed over the window frame level
  • trimming the PU foam after hardening
  • applying the inner closure of the joint (vapour barrier) and the outer closure of the joint (vapour permeable foil)
  • masonry trimming


Most frequent application errors

  • unsuitable or previously untreated (not wetted) surface delimiting the connection joint cause insufficient cohesion of the foam with the substrate, impaired expansion and non-homogeneous structure of the foam in its cross-section
  • insufficient filling of the connection joint – light showing trough the joint (fig. 4)
  • insufficient protection from UV radiation if the joint is closed after a time period


Frequently asked questions

  • Does the vapour barrier (inner closure of the connection joint) need to be applied? What method is the most suitable?

Without the inner closure of the joint there is a risk of condensation in the PU foam layer, causing degradation of its function, especially its thermally technical properties and there is an increased risk of the occurrence of mould around the connection joint  

  • Does the outer closure of the connection joint (vapour permeable layer) need to be applied? And what method is the most suitable?

Without the outer closure of the joint rainwater may penetrate through the connection joint to the inside and infiltration of air through the joint might occur.   Related documents

  • ČSN 74 6077 Windows and exterior doors – Installation requirements

Fig. 1 – Diagram of connecting joints


Fig. 2


Fig. 3


Fig. 4 – Light showing trough the joint

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