KF 520 uPVC Windows: Opening Sashes That Look Like Fixed Glass
From outside, you can’t tell which KF 520 windows open and which are fixed. Internorm hides the sash behind a slim 85 mm frame, builds in I-tec Secure as standard and makes the whole window in uPVC, inside and out.
One uniform elevation, with openers wherever you need them
On most windows, an opening sash gives itself away: a second, thicker profile sits inside the frame. On the KF 520 the sash is not visible from outside, so an operable window and a fixed light look exactly the same. A whole elevation reads as clean, evenly framed glass, and you still decide freely where to let in fresh air.
Internorm builds the KF 520 in a single design style, the flush studio, with an 85 mm sightline, the slimmest frame-and-sash figure in its uPVC range. Internorm’s catalogue notes that the window has won multiple awards for this design. Inside, frame and sash sit flush, so the room side is as calm and linear as the exterior.
This page covers the all-uPVC KF 520. Of the Internorm windows on this site, it is the one that gives you the flush studio look with a uPVC exterior: the KF 410 in studio style comes only with aluminum cladding. If you prefer a powder-coated colour outside, see the KF 520 uPVC/Aluminum.
More glass, less frame, and a frame you can hide
Internorm’s KF 520 brochure sums up the design as increased light, increased glass and increased safety. The narrow frame gives the glass a larger share of every opening, and the sash blends into the glazing so the eye reads a single thin border.
The frame can also be plastered in on three sides. Set into the wall that way it becomes almost invisible, leaving what looks like a pane of glass placed straight into the opening. That detail has to be planned with your builder before the walls are finished, so raise it at the drawing stage.
Handles and rosettes are straight and angular to match the linear sash, yet comfortable in the hand. Internorm pitches the KF 520 at both traditional and modern homes; it is at its best on contemporary houses with large, regular openings.
- 85 mm frame sightline, sash hidden from outside
- Flush interior: frame and sash on one plane
- Frame can be plastered in on three sides
- Angular handles and rosettes
KF 520 uPVC technical data
From InterHaus’s KF 520 product data and Internorm’s 2026/27 technical overview.
| Material | uPVC inside and outside |
|---|---|
| Design style | studio (flush) only |
| Thermal insulation, optimum | Uw as low as 0.63 W/m²K |
| Thermal insulation, standard coating | Uw 0.70 W/m²K |
| Sound reduction | 33–46 dB |
| Security | I-tec Secure standard; up to RC 3 |
| Locking | Fully integrated in the sash |
| Construction depth | 90 mm |
| Sightline, frame/sash | 85 mm |
| Sightline, two sashes | 96 mm flying mullion · 148 mm mullion |
| Gaskets | Three, circumferential |
| Glass bonding | All-round, glass glued to the sash |
| uPVC colours | White, Dark Grey |
| Expected service life | 40+ years |
Internorm’s KF 520 brochure lists security options of RC 1 N, RC 2 and RC 3. We recommend a class for each opening based on its exposure.
Why the KF 520 works so well in uPVC
Fixed-glazing appearance
The sash is concealed behind the frame, so operable and fixed units match. Ideal when a facade mixes both and should keep a single, consistent rhythm.
85 mm sightline
The slimmest frame-and-sash width of any Internorm uPVC window, giving more of each opening to glass and daylight.
Flush, calm interior
Frame and sash sit on one plane inside the room. In white uPVC, the window almost reads as part of the wall.
I-tec Secure built in
Flap locking is fitted to every KF 520 as standard, not sold as an upgrade, with no visible locking parts.
Plaster-in installation
Render the frame in on three sides for a near-frameless look. Plan it early with your builder.
Single-material simplicity
With uPVC on both faces there is no cladding to specify, and the non-porous surface does not change colour over time.
A slim frame that still keeps the cold out
Slim frames often come at a thermal price. The KF 520 avoids it. Internorm lists a Uw as low as 0.63 W/m²K, identical to the KF 510, and 0.70 W/m²K with the standard coating, the best standard-coating figure of any Internorm uPVC window. Part of the reason is geometry: with a narrower frame, more of each window is triple glazing, and well-specified glass insulates better than any frame.
The glass is glued to the sash around its full edge, which improves stability, thermal insulation and sound reduction, while three circumferential gaskets seal the sash against wind and draughts. On the coldest nights, our standard iPlus Low Carbon double Low-E coating keeps the inner pane warm enough that a desk or window seat beside the glass stays pleasant to use.
That coating is also lighter on the planet. The 4 mm iPlus pane has a footprint of 5.8 kg CO₂-eq/m², more than 6 kg less than the ECLAZ Low-E glass it replaced, and it lets through 74% of visible light. With more than 400 glass combinations and adjustable Ug and SHGC values we can tune each elevation, and our article on thermal bridge reduction explains how the installation detail completes the picture.
- Uw as low as 0.63 W/m²K; 0.70 with standard coating
- All-round glass bonding
- Three circumferential gaskets
- 74% light transmission with iPlus Low Carbon
Security you can’t see, as standard
Hiding the sash would mean little if the hardware still showed. On the KF 520 the locking is fully integrated into the sash: I-tec Secure flaps press into the frame on all sides as you close the handle, and nothing on the outside hints at how the window locks.
Internorm describes the result as virtually impossible to lever, and the system comes on every KF 520 rather than as an option. The brochure lists RC 1 N, RC 2 and RC 3 configurations. Lockable handles and the Secustik security handle add protection at the handle, and laminated VSG glass holds together in the frame if it is ever broken.
There is a practical bonus. With only a few small parts protruding from the sash and almost no hardware on the frame, the rebate is a smooth uPVC surface that wipes clean easily.
Opening types
Combine tilt & turn, turn-only, tilt-only and fixed lights in any elevation. Because every KF 520 sash looks like fixed glass from outside, you can decide where windows open purely on function, ventilation or cleaning access, without affecting the look of the facade.
KF 520 uPVC beside its closest alternatives
The same KF 520 is available with an aluminum exterior, while the KF 510 and KF 410 offer different looks in uPVC.
| KF 520 uPVC | KF 520 uPVC/Alu | KF 510 uPVC | KF 410 uPVC | |
|---|---|---|---|---|
| Design style | studio | studio | home pure | home pure, home soft, ambiente |
| Looks like fixed glazing outside | Yes | Yes | No | No |
| Exterior finish | uPVC | Powder-coated aluminum | uPVC | uPVC |
| Uw, optimum (W/m²K) | 0.63 | 0.63 | 0.63 | 0.62 |
| Sound reduction (dB) | 33–46 | 33–46 | 34–46 | 34–47 |
| Security, up to | RC 3 | RC 3 | RC 3 | RC 2 |
| Construction depth (mm) | 90 | 93 | 90 | 90 |
| Sightline, frame/sash (mm) | 85 | 85 | 95 | 113 |
Values from Internorm’s 2026/27 technical overview.
“This was a VIP experience from start to finish. Fast, friendly and effective service. It really helped that Vik is an architect, she was able to guide us through the entire process and coordinated directly with our designer. The windows are top-notch, but the service was better than the product.”
KF 520 uPVC: frequently asked questions
The sash sits behind the frame, so from outside you see only the 85 mm frame around the glass, whether or not there is an opening sash inside it. Internorm describes it as a glass sash; it is what sets the KF 520 apart.
Yes, with the KF 520, whose exterior can be uPVC or aluminum. The KF 410 is also offered in studio style, but only as the KF 410 uPVC/Aluminum, so for a flush window with a uPVC exterior the KF 520 is the one to choose.
No. The KF 520 reaches a Uw as low as 0.63 W/m²K, matching the KF 510 and within 0.01 of the KF 410, and has the best standard-coating figure in the uPVC range at 0.70. Its 90 mm construction depth is the same as theirs.
No, it is standard on every KF 520. You still choose the security class, RC 1 N, RC 2 or RC 3, and can add lockable or Secustik handles.
White and dark grey. Internorm notes that classic white looks bright and elegant, while dark grey is in demand for how well it complements timber floors and concrete walls. White uPVC can be specified for both faces of the window.
On three sides, yes: plastering the frame in leaves it almost invisible. It has to be detailed before installation, so share your wall section with us. We coordinate with your builder and provide installation guidance and supervision where needed.
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Dimensions, profile sections, glazing build-ups and performance values — ready for your architect or builder.
Design a facade where every window matches
Send us your elevations and we’ll mark which KF 520 units should open, specify glazing and security for each, and quote supply and installation across Canada. Architects and builders get technical support throughout.