Thinking about uPVC sash windows? Here is what changes on your house

Balance mechanisms: what replaced the sash cord

Traditional timber sashes hang on cords running over pulleys to counterweights hidden in the box frame. A uPVC sash window is not deep enough for that, so it uses spiral or gas pressure balances built into the side jambs, which hold the sash at any height without cords, weights or the associated wear.

The deep bottom rail and why it matters

Look at an original timber sash and the bottom rail is noticeably deeper than the top, a proportion that reads as correct on a period elevation. A well made uPVC sash copies that deep bottom rail rather than using equal top and bottom rails, which is one of the quickest ways to tell a considered installation from a generic one.

Run through horns

Horns are the small projections where the sash stiles extend past the joint with the top and bottom rails, originally a structural feature of jointed timber sashes. uPVC versions reproduce them, run through the corner of the frame, purely to keep the silhouette recognisable from the street, even though the uPVC frame does not need the extra strength.

Bar styles: astragal vs Georgian

Astragal bars sit on the surface of the glass and can follow a genuinely period pattern, including six over six or two over two layouts specific to a house's age. Georgian bars are the more generic grid pattern often used where a precise historical match is not required.

Both are usually supplied with a matching bar inside the sealed unit, so the visual bar lines up front and back and the window still performs as a single insulated pane, not as small individual panes.

• Astragal, surface applied, can follow a specific period pattern
• Georgian, evenly spaced grid, a common and cost effective default
• Bar inside the unit, keeps the sealed glass performance intact
• No bars at all, a clean look some conservation streets still allow

Woodgrain foils for a timber match

Sash windows are the style most often finished in a woodgrain foil, since the whole point is usually to keep a timber look. Golden oak and rosewood are the most requested foils on sash replacements, and both are heat fused rather than painted, so they do not need repainting the way timber does.

Tilt for cleaning, and other practical detail

Many uPVC sash windows include a tilt facility on the inner sash, letting it rotate inward at the top so the outer face of the glass can be cleaned from inside, which matters upstairs or wherever ladder access is difficult. Ventilation locks allow the sash to be held slightly open and still locked for security, useful overnight or with the house empty.

A trickle vent can also be built into the head of a uPVC sash frame to satisfy Part F background ventilation, though on some period style sash profiles it needs positioning carefully so it does not spoil the run through horn detail from the street.

Single hung vs double hung sliding sashes

A double hung sash has both the upper and lower sash able to slide, so either can be lowered from the top or raised from the bottom, useful for controlling ventilation without a draught blowing directly across a room. A single hung sash only allows the bottom sash to move, with the top fixed shut, which costs less but limits how you can angle the airflow through the opening.

Most Victorian and Edwardian originals were double hung, so matching that arrangement in uPVC keeps the operation as well as the look consistent with the house's period.

Security hardware on a sliding sash

Because a sash window does not have a handle driving a multi point mechanism in the same way as a casement, security relies on dedicated sash locks fitted into the meeting rail where the two sashes overlap, plus additional locks part way up the frame. A ventilation catch lets the bottom sash be locked in a slightly raised position, giving airflow overnight without leaving the window free to slide open fully.

What commonly goes wrong with old timber sashes

Sash cords rot or snap over decades, leaving a sash that will not stay up without being propped, and the box frame around the weights can suffer damp damage that is not visible until it is opened up. Paint layers build up over repeated repainting until the sashes stick in the frame or will not close square, and single glazed sashes rattle in wind once the beading and putty have shrunk with age.

None of this makes replacement automatic, plenty of these issues are fixable by a joiner, but it explains why sash windows are often the first thing a homeowner in an older Leicester property looks at when planning any window works.