Alitherm 400 Windows
Superior energy efficiency, security, and style for modern homes

The Alitherm 400 Windows are a cutting-edge range of aluminium windows that deliver superior energy efficiency, security, and style for modern homes and commercial projects. Developed by Smart Systems and fabricated by BM Bifolding Doors, Alitherm 400 is an up-to-date window system designed specifically to meet the latest UK Building Regulations (including the stricter 2022 insulation requirements). These windows are highly versatile – available in various configurations such as fixed frames, side-hung or top-hung casements, tilt & turn, and even flush sash and steel-look options within the same suite. With slim profiles and an elegant finish, Alitherm 400 windows allow maximum daylight while keeping your property comfortable and secure. They truly “frame the future” by combining outstanding value and quality with class-leading performance.

Made to measure, made to lastExpert support at every stageFast lead times, nationwide deliveryCompetitive pricing, transparent quotesComplete product range, one trusted supplierInstallation-ready with full documentationBacked by MKMs nationwide branch networkMade to measure, made to lastExpert support at every stageFast lead times, nationwide deliveryCompetitive pricing, transparent quotesComplete product range, one trusted supplierInstallation-ready with full documentationBacked by MKMs nationwide branch network

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Exclusive to MKM branches.

Instant Online Quoting & Ordering

Trade access to view live pricing.

Fast-Track Lead Times & Delivery

Pre-set Fast Doors for quick dispatch.

Technical & Installation Support

Downloadable guides & documentation.

  • High Energy Efficiency: Alitherm 400 is engineered to achieve excellent thermal performance. Thanks to deep thermal breaks and the ability to accommodate high-spec glazing, these windows can attain U-values around 1.4 W/m²K with double glazing – even hitting Band A Window Energy Ratings with the right glass. This means warmer homes and lower heating bills, fully compliant with (and exceeding) current Document L requirements. The system’s advanced design maximises insulation without bulk, so you get slim, efficient windows.
  • Multiple Styles in One System: Whether you need standard outward-opening casements, flush sash windows for a sleek look, heritage steel-look windows for character, or tilt & turn functionality, Alitherm 400 can do it. It offers a choice of standard, slimline, flush, and even industrial-style sashes all within the same frame depth. This flexibility is great for projects that mix window styles – everything still matches in sightlines and finishes, giving a consistent aesthetic.
  • Enhanced Security: Security is built into the Alitherm 400 range. It has been tested and certified to PAS 24:2022 for enhanced security on windows and doors. Multi-point locking systems, strong hinge-side protection, and internallly beaded glazing are standard, making it extremely difficult for intruders to force open. You can opt for Secured by Design versions if needed, but even standard spec provides a very secure window that meets Document Q for new builds.
  • Excellent Weather Performance: These windows keep the British weather out. They feature dual weather seals, integrated drainage, and clever design details to ensure high levels of air-tightness and water-tightness. For example, they achieve Class 4 air permeability and up to 2400 Pa water resistance in testing, meaning even in severe conditions, they won’t leak or draft. You’ll notice the difference in comfort – no rattling in wind or whistling gaskets, just a snug, quiet interior.
  • Modern, Low Maintenance Quality: Alitherm 400 windows have slim, attractive profiles that enhance your property’s appearance – slimline mullions and a clean chamfered (or flat) design. Extruded in high-grade aluminium and finished with durable powder coat, they require virtually no maintenance apart from occasional cleaning. Unlike older aluminium or steel windows, they won’t corrode or need repainting, and the operation stays smooth thanks to precision fabrication. They are truly a “fit and forget” product providing decades of reliable service.

For Trade Professionals

Alitherm 400 is a dream system for fabricators and installers due to its versatility and performance. As a fabricator, BM Bifolding Doors has fully adopted this system, meaning we can supply windows that meet the new lower U-value targets without extra fuss (for example, using co-extruded glazing gaskets and thermal foam inserts to hit 1.2–1.4 W/m²K easily). We provide trade customers with any configuration needed: need a bunch of top-hung vents for a school? Done. A mix of flush casements and dummy sashes for a heritage-look home? No problem. The system’s components are modular, allowing us to mix sash types in an order while keeping consistent outer frames. Installers will appreciate the thoughtful design: Quick-Glaze beads with pre-inserted gaskets make glazing faster and profiles are designed with adjustability and easy-fitting cleats for bays or coupling. All Alitherm 400 windows come with the necessary trickle vents and fire egress hinges if required by regs (we’ll ensure compliance aspects are covered). The system is also light enough for one-person handling of moderate-sized windows, yet strong enough to support large spans (fixed lights can be quite large). Because it’s widely used, getting additional parts or hardware is straightforward – but when you buy from us, we include everything needed for installation down to packers and fixings if requested. With BM Bifolding Doors as your supplier, you’re also accessing our technical support; we keep up-to-date with any system improvements or regulation changes (like Document L 2022, which Alitherm 400 was specifically developed for). This means you can confidently sell and install these windows knowing they are future-proof and high quality, leading to happy customers and fewer call-backs.

For Homeowners & Business Owners

Choosing Alitherm 400 aluminium windows for your home or business means investing in the latest window technology and enjoying all the benefits it brings. Homeowners will notice the improved comfort immediately – these windows eliminate the cold drafts you might have felt with old windows, thanks to their excellent seals and insulation. Even standing next to the window on a winter night, you’ll find less chill, since the frames and glass are doing their job keeping heat in. If you’re replacing old PVC or timber frames, you’ll also likely gain more glass area due to slimmer frames, boosting your daylight and view. Aesthetically, Alitherm 400 offers clean lines that can either stand out as modern or blend with traditional styles depending on configuration. For example, in a modern new extension you might have flat, minimalistic frames, while in an older house’s front you might opt for a flush sash with faux sash bars to mimic original windows – all achievable within this one range. Business owners upgrading to these windows will enjoy not only the energy savings (important for climate control in offices or storefronts) but also the curb appeal: the slim and uniform look of these windows can really freshen up a building’s facade. Noise reduction is another perk – the system, combined with appropriate acoustic glazing, does a good job at cutting down outside noise, giving you a more serene interior (something both families and office workers appreciate). And let’s not forget durability: unlike some old windows that can rot or distort, aluminium windows are extremely stable. Alitherm 400’s finish will look as good as new years down the line; you won’t have to worry about repainting or swelling frames that make windows hard to open. Maintenance is basically a quick wash of the glass and wipe of the frames occasionally, and maybe a drop of oil on moving parts yearly. They’re also environmentally friendly in that aluminium is recyclable and long-lived, plus the thermal efficiency means lower energy usage. Overall, for any homeowner or business owner looking to upgrade windows, Alitherm 400 provides a top-of-the-line solution balancing style, strength, and performance – a smart investment into your property’s future.

Everything you need to know about Smarts Visofold 1000 Bifolding Doors

Can I order both custom and set-size products online?

Yes. Our platform offers a wide range of set-size bifolding and sliding doors for fast-track delivery, alongside custom-size products tailored to your exact measurements. Simply choose the product type, select the size option, and configure your specification.

Each product page includes detailed descriptions, specification options, and recommended use cases. Whether you need corner bifolds for an extension, heritage sliders for a period home, or Alu Elite internal screens, our team is on hand to advise and help you choose the most suitable product.

Set-size products like our Fast Track bifold and sliding doors are typically delivered within 3–5 working days. Custom-size and specialist items such as gable windows or luxury garden rooms usually have lead times of 3–6 weeks, depending on complexity and volume.

Most external aluminium products (bifolds, sliders, French doors, windows) are supplied pre-glazed and partially assembled for ease of installation. Internal systems like Alu Elite doors and screens may be delivered in kit form with full fitting instructions.

Yes. All of our aluminium systems meet or exceed UK standards, including Part L for thermal efficiency, Part Q for security, and PAS 24 certification on selected models. Certificates and technical documentation are available on request.

Absolutely. The site is designed for both trade professionals and self-managed domestic customers. You can purchase any product online, and if you need help with measuring or specification, our team is happy to assist.

Yes. We offer supply-only as standard, but installation services can be arranged in many areas via our approved installer network. Alternatively, products come with detailed fitting guides suitable for competent installers or builders.

All products are backed by BM’s manufacturer warranty:

  • 10 years on aluminium frames

  • 5 years on double glazing

  • 25 years on powder-coated finishes
    Full warranty terms are provided with your order.

Yes. MKM trade customers can apply for a dedicated trade account, unlocking exclusive pricing, bulk discounts, and faster quoting tools. Speak to your local MKM branch or apply directly through the site.

You can visit selected MKM branches or our Middlesbrough showroom to view popular products in person. We also offer brochures, virtual product tours, and sample finishes on request.

  • Configurations: Alitherm 400 supports a wide range of window types. Standard outward-opening casements (side-hung or top-hung) are the most common. The system also offers Tilt & Turn windows for those who need inward opening functionality (great for high-rises or for easy internal cleaning). A Flush Casement option means the opening sash sits flush with the outer frame for a sleek look – very popular in contemporary designs or conservation areas mimicking timber flush sash. For heritage projects, a steel-look variant is available, using a slim sash with dummy transoms and mullions that replicate traditional steel windows (often called Alitherm Heritage, which Alitherm 400 can align with). There’s also the ability to create French casement windows (dual sash opening without center mullion) using flying mullion fittings. In summary, almost any window style you require can be done within this one system.

  • Frame Depth & Sightlines: The window frame depth is 70 mm, matching common UK window rebates and allowing easy coupling with doors or fixed lights. Sightlines are impressively slim: the standard mullion is around ~50–55 mm visible and the sash sections can be as slim as 59 mm for the steel-look option or slightly more for standard (but still slender compared to many competitors). This means more glass and a sleeker appearance. Importantly, even with slim profiles, the system maintains strength – larger windows utilise reinforcing where needed to pass wind load requirements.

  • Thermal Break & Insulation: Utilising polyamide thermal break technology, the Alitherm 400 frames have a wide thermal gap (for instance, 34 mm polyamide strips) that prevent cold bridging. Some profiles also allow insertion of foam insulators to push performance further. The windows are Document L 2022 compliant by design, meaning they meet the low U-value targets set for new builds (e.g., <=1.2 W/m²K with appropriate glazing). Glazing depth capacity ranges typically up to 40–45 mm, so triple glazing or double glazing with thicker acoustic laminates can be used – giving flexibility to reach even better thermal or sound specs.

  • Hardware & Operation: High-quality stainless steel friction stays are used for casement openers, with egress easy-clean hinges available for side-hungs (to meet fire escape requirements and allow easy external cleaning). Multipoint locking espagnolette or shoot-bolt systems secure the sashes closed; these engage into robust keeps on the frame. We use key-locking window handles (various styles and finishes available) that operate the lock – locking the handle secures the mechanism as a child safety feature. For tilt & turn windows, specialised hinge and handle sets are used that allow the two modes (tilt for ventilation, turn for full opening). All hardware meets corrosion resistance standards (important for coastal installations – we can specify enhanced corrosion-resistant hardware for such locations). Trickle vents can be fitted either through the frame or overhead add-on profile to meet building ventilation requirements; Alitherm 400 even introduced a hidden trickle vent option that is more discreet than older solutions.

  • Integration & Couplings: The system includes a full suite of accessories – bay poles, corner posts, coupling adapters to join windows to doors or other windows seamlessly. This means you can have an integrated look for say a combination of fixed and opening lights in a row, or a window above a door, without mismatched pieces. A great feature for larger projects is that Alitherm 400 can be used also for making single and double doors with matching sightlines – so your patio doors or French doors will line up aesthetically with the windows (in fact, our Aluminium French Doors are often built with Alitherm profiles). There’s even an option for integrating with Smart’s Designer Door panels if a matching front door is desired. For commercial customers, note that these windows are suitable for medium-duty commercial use (e.g., apartment blocks, schools) as well – they are not just residential-light; their performance and build quality meet many specs for larger builds.

The Alitherm 400 windows can be finished in practically any colour you can imagine. We often supply them in popular standard colours like Anthracite Grey (RAL 7016), Jet Black (RAL 9005), or Pure White (RAL 9010) to match contemporary design trends. However, you have the freedom to choose from the full RAL or BS colour ranges. If you want a statement colour, maybe a strong blue or a soft sage green, we can do that with no issue. The system is also available in Smart’s Sensations textured colours which have a tactile finish and subtle metallic hues (for example, “Sensations Cotswold Green” gives a lovely texture in a heritage green tone). Dual colour is an option too: e.g., black outside and white inside is a common combo (gives you a bolder external aesthetic while keeping the interior bright). All coatings are applied in our advanced powder-coating facility, with strict quality control and oven curing for a long-lasting finish. The result is a fade-resistant, chalk-resistant surface even after years of sun and weather exposure. In terms of style, you can also choose gloss, satin, or matte sheen levels for many colours. The elegant lines of Alitherm 400 are enhanced by the colour – whether you go for a modern monochrome or a traditional white, the end product will look refined. We also ensure any ancillary components (like trickle vents, if external) are colour matched, and the window handles come in various finishes (white, black, chrome, etc.) to complement the frame colour.

Given the high performance nature of the Alitherm 400, we pair it with high-quality glazing. Standard glazing is usually a 28 mm double-glazed unit with a low-E coating on the inner pane and argon gas fill, plus a warm-edge spacer bar. This configuration typically achieves around the 1.2–1.4 W/m²K Ug value needed for an A-rated window. If needed, we can incorporate triple glazing (typically 36 or 40 mm units) which can push U-values even lower (into 0.8–1.0 territory), but often double glazing suffices due to diminishing returns. For sound reduction, we can use laminated acoustic glass configurations – the system can accommodate asymmetrical glass makeups (like 6.8mm laminate outer + 4mm inner) to dampen noise, which is great for busy road locations. The beauty of Alitherm 400 is it has co-extruded glazing gaskets that are pre-fitted, making the glazing pressure uniform and sealed, you get a consistent seal that won’t shrink (older push-in wedge gaskets sometimes shrank; co-extruded ones are fixed in place). For specialties: if you want decorative glass (leaded patterns, Georgian grids, etc.), we can incorporate those either inside the unit or as external astragal bars that align with internal duplex bars for an authentic look (commonly done for heritage styles). Also worth noting: the system design allows for hidden trickle vents in the frame head, these don’t go through the glass but above it, ensuring the glass area isn’t reduced. All glass we use is toughened or laminated as required by safety codes (e.g., any low windows or doors get safety glass). We also ensure the glass choices meet the required specifications for things like solar gain, e.g. in a project needing to meet a specific Solar Factor for overheating, we can provide different coating options or tints. Ultimately, we tailor the glazing to your needs: be it maximising thermal efficiency, enhancing acoustics, adding privacy, or preserving a certain look, and the Alitherm frames will snugly and securely hold that glass for decades to come.

For a deep dive into the Alitherm 400 range, you can download the Smart Systems Alitherm 400 brochure (often available via our website or Smart’s site) – it highlights the features and options and has nice visuals of the various styles. We also offer an Alitherm 400 Windows Specification Sheet which lists technical data like profile dimensions, available configurations, thermal values, and testing standards met. If you are an architect or engineer, the NBS Plus data and section details (DWG drawings) for Alitherm 400 can be provided, just let us know and we can supply those or you might find them on Smart’s specifier resources. Installers can refer to the Fabrication and Installation Manual (Smart provides detailed manuals – we can share relevant excerpts if needed for site installation guidance, though as a client of BM Bifolding Doors, we handle fabrication so you’d mostly follow our installation guidelines). In our downloads section, you’ll also find a Maintenance Guide for aluminium windows which covers simple steps to ensure longevity (like occasional lubrication of moving parts, cleaning tips, etc.). We encourage you to browse these documents to see the pedigree behind Alitherm 400, you’ll find it’s a very well-thought-out system, and our aim is to communicate that through both our service and these informative resources.