Window Types: Casement, Sash, Tilt & Turn — Materials, U-Values & Building Regs

Quick Answer: Replacement windows in England require a minimum U-value of 1.4 W/m²K (whole window) under Building Regulations Part L (2022). New-build windows must achieve 1.2 W/m²K. All replacement window installations must be notified under a Competent Person Scheme (FENSA, CERTASS) or via building control.

Summary

Window selection involves balancing thermal performance, aesthetics, security, ventilation, and cost. The regulatory landscape has tightened significantly with the 2021 revision to Part L, which introduced stricter U-value requirements for replacement windows — a change that has all but eliminated single-pane and early double-glazed units from compliant replacement work.

Understanding window types is essential for quoting and fitting work. Casement windows — the most common in modern UK housing — have a different measurement and fitting methodology to sash windows, which are standard in Victorian and Edwardian properties and require sympathetic replacement to satisfy planning conservation requirements. Tilt-and-turn windows, common in continental European construction, are increasingly specified in UK new build for their easy-clean operation.

A critical compliance point that many tradespeople miss: replacement windows are notifiable works. You cannot simply fit replacement windows and walk away — either you must be registered with a Competent Person Scheme (FENSA or CERTASS) and self-certify the work, or the homeowner must obtain building control approval separately. Failure to comply can prevent a property sale.

Key Facts

• Replacement window U-value (Part L 2022) — 1.4 W/m²K maximum (whole window); was 1.6 in previous edition
• New build window U-value — 1.2 W/m²K maximum
• FENSA/CERTASS registration — mandatory for replacement glazing unless homeowner applies for building control
• Safety glazing — required in critical locations per Part K/Part N; toughened (BS EN 12600 Class 1) or laminated glass
• Minimum ventilation — equivalent area 5,000mm² of trickle vent or 8,000mm² in new builds per Part F; habitable rooms require ventilation
• Conservation areas — replacement windows visible from public highway may require planning permission; UPVC often not acceptable
• Listed buildings — replacement windows always require Listed Building Consent; period-appropriate materials required
• Minimum opening size — escape windows in habitable rooms on upper floors: minimum 450mm high × 450mm wide; minimum 0.33m² clear opening; cill maximum 1,100mm from floor
• Window energy ratings — BFRC Energy Rating Scheme rates A++ to G; minimum C rating generally achieves Part L compliance
• Security — PAS 24:2016 is the standard for enhanced security windows; required for Part Q (security in new dwellings)
• Frame depth — minimum 55–70mm typical for thermal performance; wider frames in some triple-glazed systems

Quick Reference Table

Detailed Guidance

Casement Windows: Measurement and Fitting

Casement windows are the most common type in UK housing built from the 1970s onwards. They have one or more opening sashes hinged at the side (side-hung casement) or top (top-hung casement). The side-hung casement is the most common.

Measuring for replacement casements:

1. Measure the existing aperture (masonry opening) width at top, middle, and bottom — use the smallest measurement
2. Measure height at left, centre, and right — use the smallest measurement
3. Deduct 10mm from width and 10mm from height to give the frame size — allows for shimming and adjustment during installation
4. For boxed-out reveals, measure the reveal depth to ensure the new frame sits correctly within or flush with the reveal

Fitting sequence:

1. Remove old frame — score silicone/mastic first; protect the reveals
2. Clear any mortar debris from reveals; make good if crumbling
3. Offer new frame; check level and plumb; shim as required (plastic packers)
4. Fix through frame into masonry — minimum 4 fixings per side, 2 top, 2 bottom; minimum 50mm penetration into masonry
5. Insulate around frame with low-expansion foam or mineral wool; leave minimum 5mm for movement
6. Apply external mastic to perimeter; apply internal cover beading or make good plaster returns
7. Fit and adjust openers; check for correct operation and weathertightness

Sash Windows: Special Considerations

Vertical sliding sash windows are the standard window type in Victorian (1837–1901) and Edwardian (1901–1910) properties. They are aesthetically important and, in conservation areas, listed buildings, or properties requiring planning permission for alterations, they must often be replaced like-for-like in appearance.

Traditional box sash vs. spiral balance: Traditional box sashes use weights in hollow jambs connected by cords over pulleys. Modern sash replacements typically use spiral balances (spring-loaded devices in the stiles) instead of weights, eliminating the need for a full box jamb and reducing frame width.

Key issues in sash replacement:

• Ensure the replacement sashes have the same sight lines as the originals (meeting rail position, glazing bar profile if present)
• Conservation area requirements may specify minimum sight-line widths and glazing bar profiles
• Draught-sealing: modern sash windows incorporate pile weatherstripping; traditional windows are notoriously draughty and secondary glazing is sometimes more appropriate for listed buildings than full replacement

Tilt & Turn Windows

Common in continental Europe and increasingly popular in UK new build, tilt-and-turn windows offer two modes of operation:

• Tilt mode (handle at 90°): top tilts inward by 15–20°; provides secure ventilation; cannot be accessed from outside
• Turn mode (handle at 180°): sash swings fully inward like a door; enables internal cleaning of the outer face; provides wide-open ventilation and escape route

Fitting requirements: because the sash swings fully inward, the window requires clear interior space — furniture cannot be placed in front of tilt-and-turn windows. The hinges are concealed within the sash perimeter (circumferential hardware) and must be correctly adjusted to ensure the tilt/turn mechanism operates smoothly.

U-Values and the Part L 2022 Changes

The 2022 update to Part L tightened the replacement window standard from 1.6 W/m²K to 1.4 W/m²K (whole window). This is a significant change — many window specifications that previously complied no longer do.

Achieving 1.4 W/m²K:

• Double glazing with Low-E coating and argon gas fill: typically achieves 1.2–1.6 W/m²K (whole window)
• Triple glazing: typically achieves 0.6–1.0 W/m²K (whole window)
• Warm-edge spacer bars (instead of aluminium): improve whole-window U-value by 0.1–0.2 W/m²K
• Frame material matters: aluminium frames (even with thermal breaks) have poorer performance than timber or UPVC in typical scenarios

BFRC Window Energy Ratings: The British Fenestration Rating Council rates windows A++ to G using an energy index that considers U-value, solar gain factor (g-value), and air leakage. An A-rated window is net energy positive (gains more solar heat than it loses). For Part L compliance via energy rating route, minimum C rating is typically required, though the U-value route is more straightforward for specifying.

Trickle Ventilators

Part F (ventilation) requires trickle ventilators in replacement windows to provide background ventilation in habitable rooms. The minimum equivalent area is:

• 5,000mm² per habitable room (sitting room, dining room, bedroom)
• 8,000mm² where whole house mechanical ventilation is not provided in new builds

Trickle vents are fitted in the top rail or head of the window frame. Importantly, if the existing windows have trickle vents, replacement windows must also have them — even if the homeowner objects on aesthetic grounds.

Frequently Asked Questions

Do replacement windows need building control approval?

Yes, but in practice this is handled through the Competent Person Scheme. If the window fitter is FENSA or CERTASS registered, they self-certify compliance and issue a certificate. The homeowner should receive a FENSA certificate within 30 days of installation. If the fitter is not registered, the homeowner must apply for building control approval. Absence of a FENSA certificate shows up in conveyancing searches and can delay property sales.

Can I use UPVC windows in a conservation area?

Usually not without planning permission, and even with permission, approval is rarely granted for UPVC in conservation areas. Most conservation area policies require timber or aluminium windows with a profile sympathetic to the original character of the building. Some conservation areas distinguish between front and rear elevations — UPVC may be acceptable at the rear but not the front. Always check with the local planning authority before specifying.

What's the difference between Low-E and standard double glazing?

Standard double glazing has a cavity between two panes of float glass, typically 12–16mm. Low-E (low emissivity) glass has a microscopic metallic coating that reflects infra-red heat back into the room, dramatically reducing radiant heat loss. All modern compliant double glazing will have Low-E coating — if a product doesn't, it won't achieve 1.4 W/m²K and is non-compliant. Argon gas fill (instead of air) in the cavity provides additional improvement of 0.1–0.2 W/m²K.

When is safety glass required in windows?

Under Part K/N (protection from falling, collision, and impact), safety glass is required in critical locations:

• Glazing in doors and within 300mm either side of a door (in the same plane): full-pane safety glass
• Low-level glazing: any glass pane with a bottom edge within 800mm of floor level and over 900mm wide
• Glazing adjacent to stairs/landing: where a fall from height could occur through the glass Safety glass must meet BS EN 12600 Class 1 (either toughened safety glass or laminated glass).

Regulations & Standards

• Building Regulations Approved Document L (2022) — conservation of fuel and power; U-value requirements for replacement windows

• Building Regulations Approved Document F (2022) — ventilation; trickle vent requirements

• Building Regulations Approved Document K — protection from falling, collision, and impact; safety glazing

• Building Regulations Approved Document Q — security in dwellings; PAS 24 requirement for new builds

• Building Regulations Approved Document B — fire safety; escape window requirements

• PAS 24:2016 — enhanced security performance requirements for doorsets and windows

• BS EN 14351-1 — windows and doors; performance standard including thermal, air, water, and wind resistance testing

• BFRC Window Energy Rating Scheme — energy labelling for windows

• Approved Document L (2021) — MHCLG 2022 edition with updated U-value standards

• FENSA: Glazing Compliance — competent person scheme for replacement glazing

• BFRC Window Energy Ratings — energy rating scheme and product database

• Historic England: Windows in Historic Buildings — guidance for conservation area and listed building work

• glazing regs — safety glass requirements in critical locations

• fire doors — fire door glazing requirements

• thermal bridging — window reveal thermal bridging

• part p notifications — notifiable works context