What uPVC is, and how a frame is built.

uPVC is rigid PVC. The "u" stands for unplasticized: nothing has been added to make it soft. It is the same family of plastic as a drain pipe, not a garden hose. A uPVC window frame is a profile of this material, divided into chambers, stiffened with steel, welded at the corners and sealed with gaskets. The plastic insulates and keeps the weather out. The steel carries the load.

The material.

PVC is made from salt and from ethylene, which comes from oil or gas. On its own the resin is not ready for a window. It is blended with stabilizers, impact modifiers, pigment and UV protection, then pushed hot through a die to make a long profile with a constant cross-section. This is extrusion, the same process that shapes aluminum sections.

Flexible PVC, used for cable sheathing and flooring, has plasticizers added. Window profiles do not. That is the whole meaning of the "u". In North America the same rigid material is called vinyl. What separates a High Performance uPVC window from a residential vinyl window is the design, not the polymer. That is covered in uPVC and vinyl.

Frames made this way were developed in Europe and have been standard there for decades. They came to North American mid-rise work later.

How a frame is built.

Chambers. Inside, the profile is divided by thin internal walls into separate chambers. Still air in small cells is a good insulator, so each chamber slows the heat passing from the room side to the weather side. Chambers also have jobs. One holds the steel. One collects and drains water. Others take the screws for hardware. The Vistaza Vi5 profile is 70 mm (2-3/4″) deep with five chambers. The Vi6 is 82 mm (3-1/4″) with six.

Steel reinforcement. uPVC is rigid, but it is not as stiff as metal. A steel section slides into the main chamber and is screwed in place. It resists wind load, carries the weight of the glass through the hinges, and gives the hardware something solid to fix to. Every Vistaza frame and sash carries a steel core.

Welded corners. The profiles are cut to a miter, the cut faces are heated against a hot plate, and the two pieces are pressed together until they fuse. The corner becomes one piece of material. There is no mechanical joint to loosen and no sealant line to fail at the miter.

Gaskets. Flexible seals run around the sash and around the glass. When the handle turns, locking points pull the sash against the gaskets and compress them. That compression is what keeps air, water and noise out.

Drainage. Some water always gets past the outer seal in a storm. The frame is designed for it. Slots in the glazing rebate lead down to a drainage chamber, and weep slots let the water out at the face.

Glass and beads. The glass unit sits on setting blocks in the rebate and is held by beads that clip in from the room side.

How to judge these parts on a sample is covered in welds and seals.

What it is good at.

• Insulating. The frame is part of the rated area of a window, and a uPVC frame does not conduct much heat. The Vistaza Vi5 casement is NFRC-certified at U 0.30 with plain insulated double-pane glass and an aluminum spacer. The Vi6 casement reaches U 0.17 with a krypton-filled triple-pane and a warm-edge spacer.
• Staying tight. Welded corners and compression gaskets leave few paths for air. The Vi5 casement measured under 0.01 cfm/ft² in its air test, report L4554.01-250-44.
• Carrying heavy glass. A deep glazing pocket takes triple and laminated units. The Vi6 casement reaches STC 47 and OITC 40 with a laminated acoustic triple-pane, report J4759.
• Standing up to water. uPVC does not absorb moisture, rot or corrode.
• Low upkeep. There is no paint to renew. Frames are washed with mild soap and water. Hardware wants occasional lubrication and adjustment, as on any window.
• Reaching commercial classes. With steel inside, the frame is tested to the same NAFS standard as any other material. The Vi6 casement is rated AW-PG100. See what a window's class really tells you.

What it is not good at.

• The slimmest sightlines. A uPVC frame is wider than an aluminum one. If the design depends on the thinnest possible frame, aluminum does that better.
• Very long spans. Steel inside the profile goes a long way, but curtain wall, window wall and storefront are aluminum work.
• Any color at all. White is the base. Every other color is a laminated foil from a set range. See color and foil. Repainting later is not practical.
• Staying still. uPVC expands and contracts with temperature more than metal or wood. Dark frames in full sun get hot. The installation detail has to leave room for the movement.
• Fire-rated openings. uPVC is a plastic. It is not a non-combustible material, and a standard window is not a fire-rated assembly. A rated opening needs a rated product. What has been tested, and how a uPVC window is approved on a non-combustible building, is set out in fire, smoke and uPVC windows.
• Historic work. Where a landmark review applies, a uPVC frame is often not accepted.

Made to order, and how big.

A commercial uPVC window is not picked from a rack. Each one is made to the opening, from approved shop drawings, in the type, color, glass and hardware on the schedule. That is why replacement work into existing openings is as practical as new construction. Vistaza's current lead times are 1–2 weeks for shop drawings and 8–10 weeks for delivery.

Size is a fair question with no single answer. The limit for a given window is set by four things: the weight of the glass, what the hinges and locks are rated to carry, how far the steel can span under the project's wind load, and the size that was tested. A NAFS rating only applies up to the tested size. A fixed lite can go much larger than an opening sash. Large openings are usually built from several frames joined with reinforced mullions.

Ask what was tested, and at what size. Then design inside it.

The way to settle it for a real project is on the shop drawings. To start, send your plans.