6.7.3 In-service performance
Doors and windows shall be designed and specified to ensure adequate in-service performance. Issues to be considered include:
- weathertightness
- minimising thermal bridging
- continuity of the air barrier
- accommodating thermal movement
- operation and strength
- durability.
Weathertightness#
The following documents should show weathertightness performance classifications of doors and windows:
- CE/UKCA marking in accordance with BS EN 14351-1
- appropriate third-party product certification.
The performance classification levels should be appropriate for the site exposure conditions.
For doors and windows into low rise housing, the suitability of weathertightness performance classification levels for the site exposure conditions may be determined according to the procedure in BS 6375-1.
Higher levels of performance should be specified for doors and windows into medium and high-rise buildings as remedial works in the event of water leakage are likely to be more difficult. Windows in high rise buildings may also be exposed to higher levels of wind driven rain.
The satisfactory weathertightness performance of doors and windows also depends on appropriate detailing of interfaces with the surrounding building envelope. The interface between any window or door and its surround should be a robust detail that is easy to understand at the construction stage. It should be evident how it should be constructed and should be easy to inspect and/or test.
The head of frames should be set back from the edge of the lintel/cavity tray, and sills should project at least 25mm beyond the wall face.
Where doors and windows are incorporated into structural openings of timber framed walls, breather membranes should be used to provide a secondary water barrier to prevent entrapping water vapour and causing interstitial condensation within the wall.
Where accessible thresholds are required, the door should be protected from precipitation by a canopy, a drainage slot or channel adjacent to the cill provided and a platform with a slope of between 1:60 and 1:40 to ensure water run-off.
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.
Notes
1. Many manufacturers supply doors and windows which exceed the recommended classifications in BS 6375-1, commonly class 9A (600Pa) for watertightness and class 4 (600Pa) for air permeability. The use of doors and windows with higher levels of performance will reduce the risk of leakage.
2. Experience suggests that testing for watertightness at 25% of the design windload gives satisfactory performance provided that the installation is carried out correctly.
Figure 1: Water bar and weatherboard to external doors
Figure 2: Throating to window cill – cavity masonry wall
Figure 3: Throating to window cill – stone cill
Figure 4: Check reveal
Minimising thermal bridging#
The interface between any door/ window and its surround should be designed to minimise the effects of thermal bridging and risk of condensation.
In cavity masonry construction, insulated cavity closures should be built into the structural opening and cavity wall insulation tightly abutted to prevent gaps in the insulation.
Doors and windows should be positioned within the structural opening to maintain continuity of the insulation layer of the building envelope.
In cavity masonry walls, doors and windows should be positioned with an overlap between the inner face of the frame and the inner face of the outer leaf. The overlap should be between 30mm and 50mm for windows and 50mm for doors - so that the door or window is contiguous with the insulation layer of the external wall.
Notes
1. For doors, reinforced cavity closures may need to be used at thresholds.
Continuity of the air barrier#
To ensure continuity of the air barrier, door and window frames should connect to the primary air barrier and be appropriately sealed. For doors and windows incorporated into the structural openings of framed walls, the air barrier is likely to be formed by the air and vapour control layer. The air and vapour control layer should be taped and sealed in accordance with the manufacturer’s instructions.
Accommodating movement#
Expansion and contraction of frames is to be expected due to temperature fluctuations. To prevent frames from distorting, a gap around the perimeter of the frame and the structural opening should be provided.
The gap size depends on the frame material and the structural opening size and should be selected in accordance with Table 1.
Table 1: Recommended gaps between the frame and structural opening
The perimeter gap should be sealed with a suitable sealant which is able to resist water penetration and prevent air leakage whilst accommodating differential movement between the structural opening and the frame.
For doors and windows incorporated into the structural openings of timber frame walls, opening, and closing gaps should be provided around the frames in accordance with chapter 6.2 to allow for the thermal movement of frames and structural movement of the timber frame.
Silicone sealants should be applied to a closed cell polyethylene foam backing strip to control the depth of the joint, force the sealant against the sides of the joint during application and aid tooling of the sealant. The depth of sealant should be at least 5mm.
For checked reveals, the sealant should form a fillet with an overlap of 6mm to the frame and 10mm to the brick outer leaf.
When impregnated foam tapes are used for perimeter sealing, over sealing with a wet sealant is not usually required. Manufacturer instructions and product certifications should be followed.
Note
Further guidance on sealants is provided in the following standards:
- BS 6093 Design of joints and jointing in building construction - Guide
- BS 6213 Guide to selection of construction sealants
- BS EN ISO 11600 Building construction – jointing products – classification and requirements for sealants.
Operation and strength#
Doors, windows, and their fittings should be adequate to withstand operational loads. Characteristics and classes of performance should be in line with the relevant class of window or doorset as defined in Annex A of BS 6375-2.
Durability #
Subject to appropriate maintenance, doors and windows should have a minimum service life of 40 years. Component parts such as operating handles, gaskets and locking mechanisms may need replacing within the service life.
Last updated: 2nd January 2024