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Aluminium Casement Windows


Klosen / Aluminium Windows / Aluminium Casement Windows

Heritage Windows with Klosen Van
New upstairs window in an old brick wall


Stunning to look at, durable and offering near-unbroken views out (and plenty of natural light in), aluminium casement windows are thoroughly modern and effortlessly stylish. Whether you’re building a new property or want to modernise an older one, these minimalist, flush-fit windows are hard to beat.

Like all our aluminium windows and doors, our casement windows are engineered to last and installed by an experienced team of specialist technicians. No matter how complex or unique your project is, we can carry out a top-tier installation that will last for years to come.

Download a brochure to explore the range of products we can offer or request your free quotation.

What Are Casement Windows

Casement windows are the UK’s most popular type. Typically side-hung (although top-hung versions are available), they can be fitted in a whole host of different configurations, making them extraordinarily versatile.

We can supply both double and triple-glazed panels, meaning you don’t have to compromise on thermal efficiency either. Learn more about the windows we supply below.

Why Aluminium?

Aluminium casement windows share the same benefits as regular aluminium windows (think durability, security and energy efficiency) but utilise a flush-fitting, ultra-modern design. If you want your property to look sleek and refined, these minimalist windows will certainly help you achieve that: you won’t believe what a difference they can make.

We’ve installed aluminium casement windows in properties across the UK, both for businesses and residential customers. From new builds to historic homes, our windows can transform the look of your property.

Brands We Work With

We’re not prepared to settle for anything less than the best here at Klosen. That’s why we only supply casement windows from Senior Architectural Systems and Cortizo, two of the industry’s leading brands.

Both companies are relentless in their pursuit of ultimate thermal efficiency, security, durability and – of course – aesthetics. Rigorously engineered and tested, the windows we supply come with lengthy guarantees and are offered in a range of finishes. Whatever your requirements, we’re confident we’ll have just the right product for you.

Download a brochure today to learn more about the windows we supply.

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Senior Architectural Systems Logo
Cortizo Logo

Our Customer Reviews

Great Price & Quick Delivery

This company have been amazing and came to our rescue when another company let us down repeatedly over a period of 4 months during our building work. We got a great price, a quick delivery time and the same product but this time with lovely fitters who knew exactly what they were doing. Now we will will have a warm house for christmas HOORAY !! We went for 3 large sliding doors and the quality of the product is excellent . Many thanks to Audrey Cieran and the team I would recommend highly
- Esther Young


Delighted with the product and the service provided by Klosen. The installation team were excellent and very helpful.
- Sara Hyem

Very Happy

We’ve been very happy with the work Klosen recently completed for us. They installed a variety of windows and sliding doors for us on our new build project. All of the information we requested early on was provided promptly and they were great with our architect. Very happy with the end result and the colour they helped us chose.
- Sarah Munton


Since 1972, Spanish company Cortizo has been leading the way in the manufacture of aluminium windows and doors. Working exclusively with aluminium, Cortizo has established itself as one of the industry’s major players.

All of its products are built to last, and all have been designed to beautifully combine function and form: in terms of aesthetics, very little else can come close.

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Cortizo Specifications

Transmittance: Uw ≥ 1.0 (W/m2K)

Acoustic insulation: Rw up to 45 dB

Air permeability: Class 4

Water tightness: Class E1200

Wind resistance: Class CE 2400

Security test: PAS24 passed

10 Year Guarantee

We offer a 10 year guarantee on all aluminium profiles and 25 years for the powder coating element

Thermal Efficiency

Stay warm in the winter and cool in the summer whilst cutting down on energy bills

Complete Flexibility

Our team are flexible throughout the entire process, including the design and installation of windows

25+ Years Experience

Klosen have over 25 years experience working with designers, architects and homeowners

Senior Architectural Systems


British manufacturer Senior Architectural Systems (SAS) has been around since 1991, and has risen to prominence as one of the nation’s largest and most well-respected suppliers of double and triple-glazed aluminium windows.

Consistently innovating and improving upon its products, SAS is known for its commitment to durability and thermal efficiency. The expanded polyurethane thermal barrier in its aluminium windows give them exemplary thermal performance and water resistance.

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double fronted house with feature stonework and driveway

Senior Architectural Systems Specifications

Typical sizes:
Width (mm)

Height (mm)
x 1500

Thickness: 28mm – 50mm

Air Permeability: 600 Pa
Water Tightness: 600 Pa
Wind Resistance: 2400 Pa

Residential CEN Standard:
1230 x 1480:
U-values as low as 0.73 W/m2K

Tested to PAS24, BS6375-2 and BS6375-3

Typical sizes:
Width (mm)

Height (mm)
x 1400

Thickness: 28mm – 44mm

Air Permeability: 600 Pa
Water Tightness: 600 Pa
Wind Resistance: 1600 Pa

Residential CEN Standard:
28mm Double Glazed Unit, 44mm Triple Glazed Unit
1230 x 1480:
1.5 W/m2K 1.1 W/m2K

Tested to PAS24, BS6375-1, BS6375-2 and BS6375-3


We’re proud to have supplied and installed Cortizo casement windows for a Grand Designs project in South Lincolnshire back in January of 2021. This stunning home was modelled on a historic Dutch barn but looks thoroughly modern – thanks in part to those contemporary and cutting-edge windows.

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Areas We Cover

Based in Market Deeping, Lincolnshire, we serve the entirety of Cambridgeshire and Lincolnshire, as well as London and the surrounding areas. Generally, we cover areas within a 60-mile radius of our base. Locations we frequently visit include Peterborough, Bourne, Stamford and Huntingdon.

Why Choose Klosen?

  • Over 25 years of experience
  • Five-star reputation
  • 10 year guarantee on all work as standard
  • No project is too big or small
  • Experience working on high-profile projects, including a Grand Designs home and the ABBA Arena in London
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How expensive are aluminium casement windows?

Although aluminium casement windows are pricier than the PVC or wooden equivalent, they are more durable and secure – not to mention more modern-looking. Prices vary depending on the size of the window and the finish you opt for, so please get in touch with us for an estimate.

How secure are the windows you supply?

Both our Cortizo and Senior Architectural Systems windows are very secure, having passed stringent PAS24 certification tests.

What colours and finishes are available?

A range of finishes can be specified, including optional powder coating finishes in any custom RAL colour – there’s guaranteed to be a shade that suits your property!

Get in touch with us today to receive your free quotation!