Prices From £600

Alitherm 400 Windows

Slimline, flush, secure, highly energy efficient

National Insntallation Teams

Why Choose Alitherm 400 Windows?

The Alitherm 400 window system is a modern, highly efficient aluminium window designed to suit both contemporary and traditional UK properties.

A Smarts Aluminium system, it delivers durability, slim sightlines and excellent thermal performance — all while providing a clean, modern aesthetic.

System depth – 70mm
Sightlines – Contemporary slim frame
Thermal performance – Low U-values (dependent on glazing)
Glazing – Double or triple glazing options
Security – PAS 24 option / Secured by Design
Operation – Side / top hung, fixed, tilt & turn
Finishes – 150+ RAL colours, dual colour available
Hardware – Modern & discreet handle options

KEY FEATURES & TECHNICAL SUMMARY

System depth – 70mm
Sightlines – Contemporary slim frame
Thermal performance – Low U-values (dependent on glazing)
Glazing – Double or triple glazing options
Security – PAS 24 option / Secured by Design
Operation – Side / top hung, fixed, tilt & turn
Finishes – 150+ RAL colours, dual colour available
Hardware – Modern & discreet handle options

Typical Applications

The Alitherm 400 Window is ideal for:

Residential properties (modern or traditional)
Extensions and glazed refurbishments
New-build developments
Coastal / exposed environments needing resilience
Homes upgrading from uPVC or timber
Mixed-material façade systems pairing with bifolds/sliders

Nationwide supply & installation for aluminium sliding doors

We operate across the entire UK — including London, the South East, Midlands, North West, Yorkshire, Scotland and Wales. Wherever your project is, we deliver:

Local survey & measurement
Tailored door design and specification
Professional installation by certified teams
Quality checks, warranty & aftercare

Supply and installation of premium Aluminium Sliding doors across the UK. Sleek, thermally broken designs ideal for modern homes.

The Fenestration Network

4.8

Based on 23 reviews

review us on

Caera Mahoney

19:17 05 Dec 25

We were originally going to order our patio doors supply only and get our builders to fit them, but after reading some of the advice from Nick at doorandwindowexperts we realised it was too big a risk for such an important and expensive purchase. Nick recommended Tom at The Fenestration Network and from the very first contact we felt really looked after; no question/query was too much trouble and we received a truly individual service. We opted for the full survey, supply and fit even through it stretched our budget and I'm glad we did. The communication was great, the lead time was reasonable and the fitters were really lovely. We are very happy with our new doors! 😊

Simon Bates

08:46 02 Dec 25

Fantastic service from start to finish Tom was great to deal with along with the Survey meeting the lads before they started to double check everything. The fitting team turned up on time today with the delivery also was almost finished for 11 o’clock very impressive., Considering the size of the units and the Power needed to do this job was very difficult and had to be thought out and planned correctly they nailed it, the trimming and silicone’s skills were second to none . Will look forward to using this business again for any window door installation , large profile glass installation.Etc

tracy wake

17:44 14 Oct 25

Great company to deal with professional and extremely knowledgeable exactly what we as the supplier and the end user (customer) needs

Allan Barker

18:18 10 Oct 25

Fenestration network have supplied and fitted some top class bi-fold doors in our extension project.
A clear payment plan which is in line with all top window/door suppliers
Good communications throughout the process
The company was found with assistance of an online expert who is keen to advise on reliable, expert supply and fit companies at a reasonable price, not expensive like some other companies I visited.
If you have a building project I would recommend Fenestration Network, make contact and speak with Tom.
They will be able to advise on different types of doors depending on your requirements.
I would highly recommend you contact Febestration Network for stunning, top quality, High end doors.

Matthew Hall

16:25 01 Oct 25

I have really enjoyed working with The Fenestration Network. Every job I have been involved in has been exactly as described, with the team always being friendly and professional. When I have been on site to capture footage of properties with new glass and glazing supplied by them, the installations have looked amazing both in person and on camera.

Customers I have spoken to while on site have also told me how impressed they are with the quality of the products and services provided by The Fenestration Network.

From a drone filming and marketing perspective, the aluminium glazing products, windows, doors, and bi-folds have been fantastic. They come in a wide range of colours, which not only gives each home a unique look that perfectly matches and complements the property, but also makes the promotional videos stand out with variety and style.

The Fenestration Network clearly delivers exactly what their customers want, both in quality and in choice.

Paul Craddock

11:37 19 Sep 25

We had windows and doors supplied and fitted by Tom and his team and apart from their prices being very competitive, the quality, look and service was excellent. To add to this, Tom gave us a discount for allowing him to have a drone take pictures which will happen once all the rendering is finished. I would highly recommend them to anyone. Thank you Tom and the team

Robert Knight

10:21 29 Aug 25

I contacted Tom Evans of The Fenestration Network for my recent, significant, aluminium window supply and install project. I found Tom easy to work with and thorough on endeavouring to get everything right before the project started. There was no pressure to rush things, but he was quick to action anything needed. The installation team he used in my area of the country was polite, hard working and personable. Overall, I valued his high personal integrity displayed thoughout the project.

Stuart Wilson

08:10 15 Aug 25

From the first contact with Tom from the Fenestration Network they were very professional and very responsive.. They had been passed our project idea and I gave approximate sizes for bi-fold doors/windows for our outside terrace…

We wanted to fully enclose it, within 48 hours Tom came back to me with a priced quote which was exceptional…. Full detailed drawings of each view suggesting top openers on one side…. My experience has been first class, I have previously been in the Architectural industry for 12 years manufacturing these type of products and was impressed how this company worked.

I agreed the price and within 1 week Tom had arranged for a site survey to finalise the dimensions…. The installation team that Tom used for the survey and installation were very good and very keen to do a fantastic job, the 2 guys (Matt & Gary) couldn’t do enough for us.

The end product was a great addition you our property giving us a fantastic room which we can use all year round.

I would have no reservations in recommending this company for any projects you are planning in the future…

See attached video of the finished project.

Matthew

15:19 13 Aug 25

Excellent from start to finish very professional company to deal with and great products

Bradley Roberts

12:14 08 Jul 25

We used The Fenestration Network for the replacement and installation of all our windows and patio doors as part of our renovations. We ran into some difficulties during the installation but the team were dedicated to getting us back on track! Would recommend to others looking for window and door installers.