Slim 53mm thermally broken frame for side-hung, top-hung and fixed lights. WER A-rated and Kitemarked.
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Softline 70mm profile with an extended thermal break — U-values to 1.3 W/m²K and PAS 24 security.
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Our flagship system — U-values from 0.9 W/m²K, acoustic performance to Rw 45 dB and a 65mm frame sightline.
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Our aluminium casement windows are a popular choice for homeowners, architects, and trade professionals looking for a blend of modern design, long-lasting performance, and energy efficiency. At Direct Trade Windows, we manufacture every window in-house using market-leading aluminium systems from Smart and Cortizo. Whether you’re upgrading a traditional home, working on a new build, or delivering a commercial project, casement windows offer clean lines, flexible configurations, and excellent thermal performance.
We offer three premium casement window systems to suit a range of budgets and design requirements: the reliable Alitherm 300, the enhanced Alitherm 400 with improved energy ratings, and the sleek, high-spec Cortizo Casement. Each is built to order and backed by our technical expertise, responsive service, and a commitment to quality that’s earned us a strong reputation since 2002.
Our casement window range is also available with arched heads — see our arched and curved windows page for the full range of arch profiles.
The Alitherm 300 is Smart Systems’ established aluminium casement window, built around a 53mm thermally broken frame with a polyamide insulation barrier. It covers side-hung, top-hung and fixed lights, which makes it a straightforward specification for residential replacement work, extensions and light commercial schemes where clean sightlines and low maintenance are the brief.
Specified with the correct glass unit it achieves a U-value of 1.5 W/m²K and an A-rated WER, accepts 24mm to 36mm double or triple glazed units, and is Kitemarked and Document L compliant. Maximum sash sizes are 700mm x 1400mm side-hung and 1400mm x 1300mm top-hung.
Every Alitherm 300 window is fabricated to order in our own workshop, so sizes, opening configurations and finishes are set by the project rather than by a standard range.
The Alitherm 400 is the softline development of the Alitherm range — a 70mm frame with extended polyamide insulation zones and a more refined sash profile. It is the system to specify where Document L 2022 thermal targets, PAS 24 security or larger opening sashes drive the design, and the slimmer softline frame sits comfortably on period elevations as well as contemporary ones.
U-values reach 1.3 W/m²K with appropriate glazing. The system takes 24mm to 40mm double or triple glazed units, beaded internally or externally, and adds fixed lights and French casements alongside the standard side-hung and top-hung openings. Sash weights are supported to 28kg on standard hinges and 40kg on egress hinges.
Profiles are extruded in 6063 T6 alloy to BS EN 755-9 and EN 12020, and finished in single or dual RAL colours, anodised, textured and special finishes.
The Cortizo casement is our flagship window and the one we recommend for architect-led and premium residential projects. It is the highest-performing casement we manufacture — thermal transmittance down to 0.9 W/m²K and acoustic insulation up to Rw 45 dB, with glazing from 23mm to 44mm to reach those figures.
It is available side-hung or top-hung, in a standard version with a 65mm frame sightline or a flush version at 79mm. Heavy-duty sash options carry larger and heavier leaves than the slim sash where the opening demands it. Tested to air permeability Class 4, water tightness Class E1200 and wind resistance Class CE2400.
As a rule of thumb: Alitherm 300 for straightforward outward-opening replacement work, Alitherm 400 where you need a slimmer softline frame and stronger thermal and security figures, and the Cortizo casement where the project is chasing the best thermal, acoustic and sightline performance available. Send us your drawings or opening sizes and our technical team will recommend the right system and glazing specification for the project.
Technical Specifications
System Alitherm 300 / Alitherm 400 / Cortizo Casement
Material Aluminium with Polyamide Thermal Break
Frame Depth 53mm (Ali 300) / 70mm (Ali 400 & Cortizo)
U-Value (Double Glazed) From 1.3 W/m²K (Cortizo) to 1.5 W/m²K (Ali 300)
U-Value (Triple Glazed) From 0.9 W/m²K (Cortizo) to 1.2 W/m²K
Glazing Thickness 24mm – 45mm (double or triple)
WER Rating A-rated achievable across all systems
Security PAS 24:2022 approved
Weather Performance BS 6375-1:2009 compliant
Air Permeability Class 4 (600 Pa)
Water Tightness Class AE 2400
Wind Resistance Class 2000+
Max Sash (Side Hung) 700mm x 1400mm (Ali 300) / 750mm x 1750mm (Cortizo heavy-duty sash)
Acoustic Rating Up to 45 dB Rw
Document L Compliant Yes
All our aluminium casement windows are available with a choice of high-performance glazing to suit your project requirements:
We specify the optimal glass configuration for each project based on thermal performance, acoustic requirements and Building Regulations compliance.
Colours & Finishes
All our aluminium casement windows are powder coated using a Qualicoat-certified process and are available in the full RAL colour palette — over 200 standard colours plus bespoke colour matching for heritage and commercial projects.
A different colour can be applied to the internal and external faces of any frame — allowing the exterior to meet planning requirements or match the building's facade while the interior colour coordinates with the internal design scheme.
Dual-colour finishes are a popular choice for windows — anthracite grey or black externally with a clean white internal finish to match your interior.
Anodised aluminium provides a harder, more scratch-resistant surface than powder coat and is available in natural silver, bronze, champagne, and dark grey anodised finishes — commonly specified for commercial projects, high-traffic environments, and where a metallic appearance is required.
RAL and NCS colour matching is available for projects requiring an exact match to an existing material, manufacturer specification, or architect's colour schedule. Samples are available on request prior to order.
Explore common questions about our Aluminium Casement Windows, suitable for both residential and commercial properties. Discover how these windows offer durability, performance, and energy efficiency.
Get a Free Quote for Alitherm Heritage Casement WindowsAluminium casement windows are a versatile window system suitable for both residential and commercial properties. They can be installed in various settings, including homes, offices, and other commercial buildings.
These casement windows are available as fixed lights and single or double side-hung or top-hung sashes, all opening outwards — including our flagship Cortizo Casement system. They are thermally efficient thanks to a polyamide thermal break, built for long-term durability, and equipped with full PAS24 accreditation for security.
Yes, you can. Depending on your needs, you can choose from a variety of dimensions for side hung, top hung, and fixed casement windows, providing flexibility for different project requirements.
Yes, these casement windows, particularly using Smart's Alitherm models, are ideal for projects that require planning permission due to their versatile and high-quality design.
