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Performance Characteristics

Weather Performance

Experience has shown that steel windows are suitable for the majority of sites and exposure conditions in the UK, where an exposure rating of 1200 Pascals satisfies most locations. Extensive laboratory prototype and production audit testing give typical results tabulated below, classified in accordance with BS 6375-1. The extra weatherseals introduced in W40 to provide an external rainscreen, internal airseals and a pressure equalised cavity have significantly improved resistance to air and water penetration in severe exposure conditions, particularly for windows that open inwards.

Window Type Range BS 6375-1 Test Pressure Class BS 6375-1
Exposure Category
Air Water Wind
1989 edition 2009 edition 1989 edition 2009 edition 2009 edition 1989 edition 1989 edition 2009 edition
Fixed Light W40   4   300   C5   2000
W30 4 300 A5 2000
W20 600   300   2400   2400  
SMW 600   300   2400   2400  
Top Hung W40   3   300   C5   2000
W30   3   200   A4   1600
W20 300   200   2000   2000  
SMW 300   200   2000   2000  
Side Hung open out W40   4   300   C5   2000
W30   3   300   A4   1600
W20 300   200   2000   2000  
SMW 300   200   2000   2000  
Tubular Profile   2 (door)   900 (door)   C2 (door)   800 (door)
Side Hung open in W40   4   300   C5   2000
W20 200   100   1600   1200  
Tubular Profile   3 (door)   150 (door)   C3 (door)   1200 (door)
Bottom Hung W40   4   300   C5   2000
W20 200   100   1600   1200  
SMW 200   100   1600   1200  
Horizontal pivot W40   4   300   C5   2000
W20 200   100   1600   1200  
SMW 300   300   2400   2400  
Vertical pivot W40   2   150   C5   1200
W20 200   50   1600   1200  
Tilt & Turn Tubular Profile   4   750   C4   1600

BS 6375-1:1989

Air permeability was measured in terms of opening joint length (m³/h/m) against progressively increasing test pressures through 200Pa (class A), 300Pa (class B) up to 600Pa (class C). Class B, or a maximum value at 300Pa of about 16 m³/h/m, was the UK standard requirement. Watertightness was measured in resistance to leakage at progressively increasing test pressures, 300Pa being considered the most severe UK requirement. Wind load resistance entailed deflection and gusting tests at pressures ranging from 1200Pa to 2400Pa.

BS 6375-1:2009

Air permeability is measured in terms of both area (m³/h/m²) and opening joint length (m³/h/m) against progressively increasing test pressures through 300Pa up to 600Pa for classes 3 and 4. Class 2, or a maximum value at 300Pa of 13 m³/h/m, is the UK standard requirement. Watertightness is measured in resistance to leakage at progressively increasing test pressures, 300Pa being considered the most severe UK requirement. The values tabulated in Pascals can also be expressed in classes ranging from 2A (50Pa) through 8A (600Pa) to exceptional resistance up to a maximum of E1050. Wind load resistance entails a deflection test (at 2000Pa for class 5), a repeated pressure test of 50 positive and 50 negative gusts at half the deflection test pressure, and a safety test at 150% of the deflection test pressure (i.e. 3000Pa for class 5). The prefix C means that deflection of the longest frame member was less than 1/300 with less than 1/150 (prefix A) considered adequate in the UK.

The 2009 edition of BS 6375-1 clarifies and supersedes the 2004 edition but does not change either the test methods or the basis of classification. This means that test results declared in conformity with BS 6375-1:2004 remain valid according to BS 6375-1:2009.

Thermal Performance

Steel windows glazed with advanced insulating glass units can be shown to have a thermal performance which complies with the energy conservation requirements of the Building Regulations. Concern is sometimes expressed about the occasional risk of condensation through cold bridging. Given an extreme temperature differential from -5ºC outside to +20ºC inside and no ventilation air movement, condensation will form on internal frame surfaces when relative humidity is above 33%. Although this may be inconvenient, it does not harm the window and should encourage people to seek more ventilation as the appropriate means of control.

Thermal transmittance is expressed in typical U-values calculated in accordance with BS EN ISO 10077-2 for a standard reference window as defined in the European window product standard BS EN 14351-1. Some examples are tabulated below.

Product Specification U-value
W40 Double glazed with Argon filled low-E warm edge insulating glass units 2.0
Triple glazed with Krypton filled low-E alu spacer insulating glass units 1.7
W30 Double glazed with Krypton filled low-E warm edge insulating glass units 1.7
W20 Double glazed with Argon filled soft coat low-E insulating glass units 2.9
Tubular Profile Double glazed with Argon filled low-E warm edge insulating glass units 1.6
Triple glazed with Argon filled low-E warm edge insulating glass units 1.3


European harmonised testing methods are specified in the window and door product standard BS EN 14351-1, and guidance on their application in the UK can be found in BS 6375-2 Performance of Windows and Doors – Operation and Strength.

Strength is measured by subjecting casements to twisting with three corners held tight whilst the fourth is moved by a calibrated force. The idea is to simulate attempts to release a jammed sash. W40 casements achieve the maximum class 4, surviving a 350 Newton force on one corner with negligible deflection, exceeding the UK requirement of class 3 (the ability to resist a force of 300 Newtons).

W40 steel windows have also been subjected to weights hung from the handle jamb, designed to simulate an accidental vertical load. Casements barely moved out of square and promptly returned to their original shape when the load was removed. The UK requirement is class 3, resisting a load of 600 Newtons, and W40 steel windows satisfied the highest class 4 (a load of 800 Newtons).

Throughout the test programme for weathertightness and strength, units were continuously checked to ensure they satisfied ease of operation requirements by measuring the forces required to open and close them. All met class 1 requirements, a torque of no more than 10 Newton metres to turn the handle and a maximum force of 100 Newtons to push them open or pull them closed.


The inherent strength of steel ensures maximum rigidity once the window frames have been installed and glazed. The most secure steel window is a regular, standard window fitted with key operated locking devices and divided into small panes of glass, with vertical and horizontal bars tenon-riveted or welded into the frame and strong intersecting joints. Even if the glass is broken, entry cannot be gained through this type of window. Specifying laminated glass makes it even more resistant to intrusion.

A high measure of security can be achieved with large pane steel opening windows by using multi-point locking devices. A supplementary ventilator, mounted in the top glazing rebate, will give permanent or controlled ventilation with locked window security.

Following successful tests for enhanced security to BS 7950, W40 products, both open in and open out, fitted with concealed multi-point locking bolts and friction stay hinges or brass butt hinges, satisfy the Secured by Design standard. Doors of cold-formed tubular steel have been tested in conformity with EN 1627 for burglar resistance to classes 1, 2 and 3.

Safety in Use

To ensure that a window holds its glass in place and remains closed even under impact from a heavy body, it can be tested to BS EN 13049 where a heavy impactor, consisting of two lorry tyres enclosing a steel cylinder and weighing 50kg, is swung into the window. Five classes are defined for this test, ranging from dropping the impactor from 200mm (class 1) to raising the impactor to 950mm before letting it go (class 5). The full range of W40 steel windows has been submitted for this test, including fixed lights, hinged and pivoted casements, and they survived the maximum impact to satisfy class 5.

It is important to verify that safety restrictor devices are sufficiently robust to withstand abuse. Those fitted to steel windows by the manufacturer have been tested to ensure that they remain engaged when subjected to a horizontal load in excess of 350 Newtons (35 kg).

Safety in Fire

Where there is a requirement for fire safety combined with maximum daylight and vision, steel framed fire resistant glazing is the preferred choice. The SWA has developed a hot-rolled steel framing system offering 60 minutes fire resistance, which complements adjacent hot-rolled steel windows and doors that may not be fire-rated. The SWA system can be used either internally or externally where its weather and corrosion resistance qualities will be important.

Alternative fire resistant glazing systems using cold-formed tubular steel profiles, all backed by authenticated test evidence, are offered by a number of SWA members for both integrity only and integrity with insulation. For details, please contact the SWA.

Operation and Control

The long life of steel windows is supported by the fittings that are used. The fittings are not only durable; they complement the elegant and timeless appearance that is unique to the steel window. Made from high quality noble materials such as brass and stainless steel, the fittings are highly sustainable and available in a range of hard-wearing, attractive finishes to suit any application.

There are matching styles of casement fasteners and stays for commercial and domestic applications. A selection of handles, stays, folding openers, hinges, pivots, catches and bolts enable the window to open in a variety of ways. Concealed friction stay hinges and multi-point locking bolts present a clean uncluttered appearance, enhancing the slender lines of several product ranges.

The concealed multi-point locking system provides higher security for the W40 range, enabling it to meet the Secured by Design standard. Its handle can be locked in the cracked open out position to give background trickle ventilation. Key lockable bolts, face-mounted or concealed in double leaf applications, are available in extra long lengths for high reach applications.

The combination of established durable materials and modern manufacturing methods offers a range of fittings, providing traditional appearance, high security and long life.

Surface Finishes

A key part of BS 6510, adhered to by all SWA members, is positive protection from rust. Frames of hot-rolled solid steel are hot-dip galvanized after fabrication to BS EN ISO 1461. During this process, the windows are thoroughly cleaned and then dipped in a bath of molten zinc. The zinc reacts with the iron in the steel to form alloy layers which are then covered with pure zinc as the window is withdrawn. This combination of soft zinc over hard alloy layers produces a highly durable protective coating that will not flake or peel. In the event of accidental damage, the galvanized coating will corrode in preference to the steel which prevents rusting.

Frames of cold-formed tubular steel can be assembled in one of two ways. The profiles can be either hot-dip galvanized before cold-forming or fabricated first and then stoved with an epoxy zinc primer, hot metal zinc sprayed, or hot-dip galvanized.

Galvanized steel windows do not require painting for protection but their appearance is enhanced by the addition of a factory-applied polyester colour coating. Polyester coatings have a life expectancy of at least 15 years and a comprehensive range of colours is readily available, with the matt finish proving most popular because of its surface enhancing properties.



Steel windows are suitable for fixing direct to brickwork, concrete, stone or into subframes. Windows and integral sills must be installed in accordance with good building practice, allowing for damp proof course and finishes. No load should be applied to the head of any window. A perimeter joint design gap shall be provided, generally of not less than 2mm and not greater than 8mm once the frame is centralised in its opening, that allows for thermal movement, fabrication size variance and aperture construction tolerance.


Steel windows can be single glazed or fitted with insulating glass units. Glazing practice should comply with BS 8000-7 and reference is also recommended to the Glass and Glazing Federation’s data sheets on glazing techniques, particularly the use of setting, location and distance blocks and clips. Specific glazing compounds are available for use with factory finished steel windows.

Aftercare and Maintenance

The durability and high performance of steel windows can be enhanced even further if good maintenance practices are observed.

Frames should be cleaned at regular intervals using a mild, non-alkaline detergent in warm water, applied with a soft cloth or sponge.

Glazing and perimeter sealants should be inspected on an annual basis and appropriate maintenance action taken.

Galvanized steel windows, which are not factory finished with a polyester colour coating, are manufactured with clearances to allow for up to three or four coats of paint on the meeting surfaces. When the repainting programme has exceeded this, the paint should be stripped off all meeting surfaces prior to repainting.

All hinges, pivots, handles, stays and other mechanical parts should be checked for operation, kept free of excessive paint build up and lightly lubricated.