Westgate, Leeds: a technically improved glazed façade for Q Three Residence
How replacement windows, curtain walling and low-emissivity glazing improved thermal performance, daylight and solar control across 1,529 m² of façade.
Project overview
In May 2022, Colbre Projects appointed Dortech to replace the windows, curtain walling and entrance doors at Westgate in Leeds. The work formed part of the conversion of an existing office block into apartments and studios, now known as Q Three Residence.
The existing façade dated from the 1980s. Its grey body-tinted double-glazed units and purple powder-coated aluminium frames were in an advanced state of ageing and no longer provided the appearance, daylight or thermal performance expected from a modern residential development.
| Client | Colbre Projects |
|---|---|
| Project | Westgate, Leeds — now Q Three Residence |
| Appointment | May 2022 |
| Total glazed area | 1,529 m² |
| Scope | Replacement windows, curtain walling and thermally broken entrance doors |
The challenge: an ageing 1980s glazed envelope
Even when new, the original air-filled units would not have achieved a centre-pane U-value better than approximately 2.8 W/m²K. That performance represented a significant limitation for an office-to-residential conversion, where longer and more varied occupancy makes perimeter comfort and seasonal heat loss especially important.
The original glazing had a visible light transmission of approximately 0.39 and a g-value of approximately 0.47. In practical terms, around 39% of visible daylight and 47% of incident solar energy passed through the glass.
Dortech’s façade replacement solution
Dortech replaced the existing frames and glazed units with thermally improved systems in a contemporary grey PPC finish. The installed façade systems comprised:
- Curtain walling: TECHNAL MX52
- Windows: TECHNAL FY65 tilt-and-turn vents
- Entrance doors: SAPA STII thermally broken entrance doors
The replacement insulating glass units used a clear heat-soak-tested outer pane, an argon-filled cavity and an inner pane incorporating Saint-Gobain PLANITHERM ULTRA N low-emissivity coating. For this calculation, the replacement centre-pane Ug-value is taken as 1.1 W/m²K.
Before-and-after glazing performance
| Performance measure | Original glazing | Replacement glazing | Change |
|---|---|---|---|
| Visible light transmission | 0.39 / 39% | 0.66 / 66% | +27 percentage points; approximately 69% relative increase |
| g-value / solar factor | 0.47 / 47% | 0.44 / 44% | 6.4% relative reduction in total solar-energy transmission |
| Centre-pane Ug-value | 2.8 W/m²K | 1.1 W/m²K | 1.7 W/m²K reduction; approximately 60.7% lower |
More daylight with slightly less solar gain
Raising light transmission from 0.39 to 0.66 provides a substantial improvement in natural daylight. At the same time, the lower g-value means the replacement glass transmits slightly less total solar energy. This is a useful combination for residential use: brighter interiors without a corresponding increase in solar gain through the glass.
A step-change in insulation
The centre-pane Ug-value reduced from 2.8 to 1.1 W/m²K. At a representative 20°C indoor-to-outdoor temperature difference, calculated centre-pane heat flow across the full 1,529 m² falls from approximately 85.6 kW to 33.6 kW — a reduction of about 52.0 kW at that condition.
Indicative heating-energy saving
The calculation follows the Dortech Commercial Glazing Energy-Saving Model approach, using the glazed area, improvement in centre-pane U-value and an assumed 2,000 heating degree days per year.
| Heating result | Indicative annual value |
|---|---|
| Useful heating demand avoided | 124,766 kWh/year |
| Purchased gas input avoided at 85% efficiency | 146,784 kWh/year |
| Heating-cost saving at 5.16 p/kWh | Approximately £7,574/year |
Solar-gain and cooling scenario
The reduction in g-value from 0.47 to 0.44 lowers theoretical annual solar heat transmission by approximately 25,320 kWh across the glazed area, using 552 kWh/m²/year of incident annual irradiation.
Where apartments or common areas are mechanically cooled, a scenario allowing for the proportion of gain that coincides with cooling operation and for cooling-system efficiency indicates a further 2,110–5,697 kWh/year of avoided electricity use. At 22.19 p/kWh, this equates to approximately £468–£1,264/year. Where there is no active cooling, this element should be treated as a comfort and overheating benefit rather than a direct bill saving.
Project outcome
The completed façade gives the converted building a cleaner, contemporary appearance while retaining its distinctive red-brick character. Technically, the most significant improvement is the approximately 60.7% reduction in centre-pane Ug-value. The project also provides around 69% more visible light relative to the original glass while slightly reducing total solar-energy transmission.
On the assumptions used by the model, the glazing upgrade produces an indicative annual heating-cost saving of approximately £7,574. If active cooling is installed and the cooling scenario applies, the combined indicative energy-cost saving becomes approximately £8,042–£8,838 per year.
Technical limitations
This model uses centre-pane Ug-values. It does not calculate whole-window Uw-values or curtain-wall Ucw-values and excludes frame heat transfer, spacer and edge effects, thermal bridging, interfaces, opening-area proportions and air leakage. Because the Westgate works replaced frames as well as glass, the completed façade may deliver further benefits, but these cannot be quantified without the project-specific whole-element performance data.
Operational energy use will also depend on occupancy, set points, ventilation, shading, internal gains, heating and cooling-system efficiency, tariffs and weather. The results should therefore support option comparison and early-stage decision-making, not be read as a guaranteed prediction of bills, consumption or carbon savings.
