System 4-20

Thermally Broken Casement Window

The Metal Technology thermally broken casement window has been designed to offer the specifier the advantages of polyamide thermal break technology in meeting the latest thermal requirements of the current building regulations.

Performance Overview

Weather Performance - Independently weather performance tested to BS 6375-1

Air Infiltration BS EN 1026 Class 4
Watertightness BS EN 1027 Class E900
Wind Resistance BS EN 12211 Class E2400

Average air permeability’s under positive and negative air pressures at 50pa 0.07m3/h.m2


  • BS EN ISO 140-3
  • BS EN ISO 717-1


This system has passed BS 7950 incorporating Amendments 1, 2 and 3 "Specification for Enhanced Security Performance" as generally accepted on Secure by Design products.

Size Limitation

Vent Section Vent Height Vent Width
Top Hung Casement 2000mm 2000mm 100kg
Side Hung Casement 838mm 1800mm 47kg

Note that maximum height and maximum width cannot be achieved simultaneously. For complete details of maximum/minimum sizes, handle positions and weight restrictions, see the size limitation charts in Metal Technology’s technical literature.

Thermal Performance

Metal Technology's System 5-20, in conjunction with the correct glass specification, is designed to aid compliance with the latest thermal requirements of the current building regulations.

System 4-20 U-Frame Values
Fixed light outer frame 2.94 W/m2K
Outer frame and vent 2.89 W/m2K

The following table, based on a standard commercial GGF window configuration and warm edge spacers, demonstrates how such improved U-frame values then contribute to improving the overall thermal performance of a complete window

Metal Technology can provide tailored U-value calculations using their dedicated estimating software to calculate overall project average window U-values for their full range of systems.


Aluminium profiles are extruded fromaluminium alloy 6060T6, T5 or T4 complying with the recommendations of BS EN 12020-2 / BS EN 755 Parts 1 to 9. Polyamide thermal breaks are produced from glass reinforced nylon sections designed to withstand temperatures in excess of 200°C, allowing the sections to be powder coated after thermally breaking.


Frame members are mitre cut at 45°, corners are reinforced with extruded aluminium crimping cleats and corner braces, and a secure joint is formed by pneumatically crimping into the extruded crimping cleat. Mullion and transom bars are square cut shaped and fixed securely to the frame by means of stainless steel screws and fixing cleats. All frame joints are sealed during construction against entry of water using Metal Technology’s two-part adhesive. Metal Technology recommend that only A2 or A4 Austenitic (300 series/class 70) stainless steel fixing screws are used in the assembly of their products.


The range of sections can be provided in either of the following ranges of finishes:

  • Anodised to BS 1615 or BS 3987 or BS EN 12373-1
  • Powder organic coated to BS 6496 or BS EN 12206-1
  • This system can accommodate a different colour finish internally to that used externally. Metal Technology's powder coating is produced to Qualicoat standards.


The system can be glazed internally or externally and can accommodate glazing units from 24mm to 34mm.

Read More »