Doors, windows and glazing

6.7.3In-service performance

Doors, windows, and glazing shall be designed and specified to ensure adequate in-service performance. Issues to be considered include:

  1. weathertightness
  2. fire safety
  3. thermal break
  4. strength
  5. accommodation of thermal movement.

Weathertightness

Doors and windows shall be tested and classified for air permeability, watertightness, and wind resistance in accordance with the following standards:

  • BS 6375-1,
  • BS EN 1026 and BS EN 12207,
  • BS EN 1027 and BS EN 12208,
  • BS EN 12211 and BS EN 12210.

The classification levels should be suitable for the site exposure conditions and selected in accordance with BS 6375-1.

Coupled door and window assemblies contained within a single storey should:

  • be supplied as a system using products and materials fully specified by the system supplier to achieve the required performance,
  • be designed as an engineered system to support its self-weight and imposed loads safely to the supporting structure,
  • include dual sealed coupling joints comprising outer seals as the primary water barrier, drained cavity to remove any water that by passes the outer seals and inner seals providing air and secondary water barriers,
  • include suitable seals within coupling joints such as impregnated foam or flexible polymer gaskets (it is not sufficient to rely solely on wet sealants),
  • have accredited type testing to verify weathertightness performance of coupling details.

Further guidance on assemblies is also provided in CWCT TN 95.

Windows and external doors should be constructed and detailed to ensure they remain weathertight, including at interfaces with the structure. When placing external frames into openings ensure:

  • the opening is suitably dimensioned to provide an accurate fit for the frame and gap for sealing,
  • proprietary cavity closures are fitted in accordance with manufacturer’s instructions and third-party certificates,
  • there is sufficient clearance to allow positioning without distortion and to accommodate in-service thermal movement,
  • the head of the frame is set back from the edge of the lintel and cavity tray,
  • frames are positioned a minimum 30mm into the cavity to maintain continuity of thermal insulation at jambs and sills,
  • sill projects at least 25mm beyond the wall face,
  • the junction between the wall and front and back of the frame is sealed with suitable mastic sealants to maintain weathertightness.

In Scotland, Northern Ireland, and areas of very severe exposure, ‘check reveals’ should be used. The frame should be set back behind the masonry outer leaf with a minimum 12mm overlap to the frame.

Bay windows should be linked to DPCs at reveals to maintain weathertightness and detailed to minimise thermal bridging.

Fire safety

Fire-resisting doors and positive self-closing devices should be fitted where they are required by building regulations.

Thermal break

Metal frames should incorporate thermal breaks to increase their thermal resistance.

Strength

Door frames, windows and their fittings should be adequate to withstand operational loads. BS 6375-2 provides guidance on characteristics and classes of performance appropriate for the UK.

Structural loads should be carried on lintels, beams or appropriate structural elements. Where frames are required to carry structural loads, they should be designed accordingly.

Resistance to thermal and moisture movement

Doors and windows should be installed with a perimeter gap around the frame of between 5mm and 10mm to allow for thermal expansion.

Where openings are wider than 3.0m for larger units such as patio doors the gap can be up to 15mm.

Gaps 5mm wide or more should have a backing strip to the sealant and the depth of sealant should be at least 5mm.

Where frames are tight to the opening, such as with a checked reveal, the sealant should form a fillet with an overlap of 6mm to non-porous surfaces and 10mm to porous surfaces.

Construction sealants should be selected to suit; the construction and frame materials, joint size, anticipated joint movement, and exposure to weather. Further guidance is provided in the following standards:

  • BS 6093 Design of joints and jointing in building construction - guide,
  • BS 6213 Guide to selection of constructional sealants,
  • BS EN ISO 11600 Building construction – Jointing products – Classification and requirements for sealants.