Retail Company, Sleaford Case Study
Full window replacement to improve comfort, reduce heat loss and control summer overheating at a retail company premises in Sleaford.
Better winter comfort
Replacing draughty single glazing and non-thermally broken frames supports warmer, more stable internal conditions.
Reduced summer overheating
COOL-LITE XTREME 70/33 II helps cut solar heat gain on the elevations most affected by excessive summer sun.
Lower energy-cost exposure
Improved thermal and solar performance can reduce both heating demand and cooling demand where solar-control glazing is applied.
Project overview
Dortech Maintenance worked with TSK Group on a retail company premises in Sleaford to address a combined winter and summer performance problem through a full replacement window strategy.
The building had draughty single-glazed aluminium vertical sliding sash windows that were in a poor state of repair. The replacement strategy focused on improving winter heat retention, reducing draught-related discomfort and controlling overheating on the southerly facing elevations.
The challenge: draughty windows, winter cold and summer overheating
TSK Group reported that the existing glazing at the Sleaford retail premises was draughty and in a poor state of repair. The client had also complained that the building was cold during winter and experienced overheating on the south-facing side during summer.
This created a combined building-performance issue. In winter, the existing windows were allowing excessive heat loss and draughts, making internal spaces harder to heat and less comfortable for building users. In summer, high solar gain through the glazing, particularly on the southerly facing elevations, contributed to overheating.
For any occupied commercial building, these issues can affect staff comfort, productivity, energy consumption and the overall usability of the internal working environment.
Site review and existing window assessment
Dortech visited site and identified approximately 217m² of single-glazed aluminium vertical sliding sash windows.
The existing frames were non-thermally broken, meaning there was no effective thermal separation between the external and internal aluminium sections. This type of construction can allow heat to transfer more readily through the frame, increasing heat loss in winter and contributing to poor internal comfort.
Dortech estimated that the existing windows would have had an approximate U-value of 6.0 W/m²K. The existing single glazing was also estimated to have a light transmission of 0.90, or 90%, and a g-value of 0.85, or 85%.
| Performance measure | Existing single glazing |
|---|---|
| U-value | Approx. 6.0 W/m²K |
| Light transmission | 0.90 / 90% |
| g-value / solar factor | 0.85 / 85% |
Understanding the technical issue
Three factors were especially important on this project: U-value, g-value and frame condition. The existing windows were underperforming in both winter and summer. They were not simply old or visually tired; they were actively affecting the internal environment and the building’s energy performance.
A high U-value meant the building was losing heat quickly through the windows in winter. A high g-value meant too much solar heat was entering the building in summer. The draughty, non-thermally broken aluminium frames further reduced comfort and increased heating demand.
The Dortech replacement window strategy
Dortech replaced the existing windows with new thermally broken aluminium windows. The new window system provides a significantly improved U-value of 1.4 W/m²K.
On the southerly facing elevations, Dortech introduced double-glazed units incorporating Saint-Gobain COOL-LITE XTREME 70/33 II with argon gas-filled cavities.
This means the new windows deliver both a substantial thermal improvement and a significant reduction in solar heat gain on the elevations most affected by overheating.
| Performance measure | Existing windows | Replacement windows | Improvement |
|---|---|---|---|
| U-value | Approx. 6.0 W/m²K | 1.4 W/m²K | Major reduction in heat loss |
| Light transmission | 0.90 / 90% | 0.70 / 70% | High daylight retained |
| g-value / solar factor | 0.85 / 85% | 0.33 / 33% | Major reduction in solar gain |
Major improvement in winter thermal performance
The U-value improvement is particularly significant. The existing single-glazed, non-thermally broken aluminium windows were estimated at 6.0 W/m²K. The replacement thermally broken aluminium windows achieve 1.4 W/m²K.
This gives a U-value improvement of 4.6 W/m²K, which is approximately a 77% reduction in conductive heat loss through the window area, based on the project U-value comparison.
For a building that was reported to be cold in winter, this is a major performance improvement. It should help reduce cold perimeter conditions, reduce draught-related discomfort and lower the demand placed on the heating system.
Major reduction in summer solar gain
On the southerly facing elevations, the replacement glazing reduced the g-value from 0.85 to 0.33. This gives a g-value reduction of 0.52, equivalent to approximately 61% less solar energy transmission through the glass.
This is the key summer-comfort improvement. Instead of allowing excessive solar heat to enter the building and then relying on ventilation or mechanical systems to manage overheating, the new solar-control glazing helps reduce unwanted solar gain at source.
The replacement glass also maintains a strong level of natural daylight, with light transmission of 0.70 / 70%, helping to avoid the space feeling dark while still reducing excessive heat gain.
Indicative energy-saving calculation
The following indicative calculation follows the same approach used in the Dortech Maintenance commercial glazing energy-saving model: commercial glazing energy-saving model.
Indicative heating-side saving
The heating-side calculation is based on reduced conductive heat loss through the upgraded windows.
| Indicative heating result | Value |
|---|---|
| Useful heat reduction | Approximately 47,914 kWh per year |
| Avoided gas input, allowing for 85% efficiency | Approximately 56,369 kWh per year |
| Indicative heating-side saving at 5.16p/kWh | Approximately £2,909 per year |
This heating-side calculation only considers the reduction in conductive heat loss. It does not include the additional comfort benefit of reducing draughts, improving frame performance or addressing air leakage from older window systems.
Indicative cooling-side benefit
The cooling-side benefit depends on the area of glazing treated with COOL-LITE XTREME 70/33 II on the southerly facing elevations. The total window replacement area was 217m², but the solar-control glazing was specifically introduced on the southerly facing elevations, so the maximum calculation below is illustrative.
| Indicative cooling result | Value |
|---|---|
| Theoretical annual reduction in solar heat entering the building | Approximately 62,288 kWh per year |
| Avoided cooling electricity demand | Approximately 5,191 to 14,015 kWh per year |
| Indicative cooling energy saving at 22.19p/kWh | Approximately £1,152 to £3,110 per year |
| Solar-control glazing area | Indicative avoided cooling cost |
|---|---|
| Per 1m² of south-facing solar-control glazing | Approx. £5.31-£14.33/year |
| 100m² | Approx. £531-£1,433/year |
| 150m² | Approx. £796-£2,150/year |
| 217m² illustrative maximum | Approx. £1,152-£3,110/year |
The cooling-side saving should be refined once the actual area of southerly facing COOL-LITE glazing is confirmed. The figures above are indicative only and should not be treated as a guaranteed financial return.
Indicative total annual energy saving
| Saving category | Indicative annual saving |
|---|---|
| Heating-side saving | Approx. £2,909/year |
| Cooling-side saving | Approx. £1,152-£3,110/year |
| Combined indicative saving | Approx. £4,061-£6,019/year |
Energy prices remain a key commercial driver for building-performance upgrades. This context supports the value of reducing unnecessary heating and cooling demand, while the savings in this case study remain indicative only.
Improving comfort for building users
This project was not just about energy performance. It was also about making the building more comfortable to use.
The existing windows were draughty, single glazed and in poor condition. This would have contributed to cold internal conditions in winter, particularly near the window line. Older aluminium non-thermally broken windows can also create cold surfaces and localised discomfort, even when the heating system is operating.
By installing new thermally broken aluminium windows, Dortech helped improve the building envelope, reduce heat loss and create a more stable internal environment.
On the south-facing elevations, the use of COOL-LITE XTREME 70/33 II also helped address summer overheating by reducing solar heat gain while retaining good natural light. Together, these improvements support a more comfortable year-round working environment.
A full replacement strategy rather than glass-only reglazing
Unlike some projects where the existing frames can be retained and only the glass units replaced, the Sleaford retail premises required a full replacement window strategy.
This was appropriate because the existing windows were not only thermally poor but also draughty and in a poor state of repair. Simply changing the glass would not have addressed the performance limitations of the non-thermally broken aluminium frames or the reported draught issues.
By replacing the complete window system, Dortech was able to improve thermal performance, airtightness and comfort, summer solar control on the southerly elevations, winter heat retention, and the overall condition and reliability of the window installation.
Conclusion: improving the building envelope through targeted window replacement
The Retail Company, Sleaford project demonstrates how full window replacement can provide a practical solution where existing windows are draughty, thermally inefficient and in poor condition.
By replacing approximately 217m² of single-glazed, non-thermally broken aluminium vertical sliding sash windows with new thermally broken aluminium windows, Dortech significantly improved the thermal performance of the building envelope.
The U-value improvement from approximately 6.0 W/m²K to 1.4 W/m²K supports better winter comfort and reduced heat loss. On the southerly facing elevations, the introduction of COOL-LITE XTREME 70/33 II reduced the g-value from 0.85 to 0.33, helping to reduce summer overheating while retaining good natural daylight.
For building owners, occupiers and project teams dealing with older commercial buildings, this case study shows that targeted replacement window strategies can address multiple issues at once: draughts, poor winter comfort, summer overheating, energy performance and long-term building usability.
Project information
| Item | Detail |
|---|---|
| Project lead / client relationship | TSK Group |
| Site | Retail Company, Sleaford |
| Existing windows | Single-glazed aluminium non-thermally broken vertical sliding sashes |
| Existing window area | Approximately 217m² |
| Reported issues | Draughts, poor repair, cold in winter, overheating on the south side in summer |
| Replacement strategy | Full replacement thermally broken aluminium windows |
| Solar-control glazing | COOL-LITE XTREME 70/33 II to southerly facing elevations |
| Existing estimated U-value | Approx. 6.0 W/m²K |
| Replacement U-value | 1.4 W/m²K |
| Existing light transmission | 0.90 / 90% |
| Replacement light transmission | 0.70 / 70% |
| Existing g-value | 0.85 / 85% |
| Replacement g-value | 0.33 / 33% |
The three core benefits of changing glass
The TSK Group x Retail Company, Sleaford project demonstrates the three core benefits of targeted window and glazing replacement: improved aesthetics, a better working environment and reduced energy costs through improved thermal and solar performance.
To assess the potential energy-saving benefits of upgrading windows, doors and glazed screens, Dortech Maintenance has developed a commercial glazing energy-saving model: commercial glazing energy-saving model.
Calculation assumptions used in this case study
| Assumption | Value |
|---|---|
| Window area | 217m² |
| Existing window type | Single-glazed aluminium non-thermally broken vertical sliding sashes |
| Replacement window type | Thermally broken aluminium windows |
| Existing U-value | Approx. 6.0 W/m²K |
| Replacement U-value | 1.4 W/m²K |
| U-value improvement | 4.6 W/m²K |
| Existing light transmission | 0.90 / 90% |
| Replacement light transmission | 0.70 / 70% |
| Existing g-value | 0.85 / 85% |
| Replacement g-value | 0.33 / 33% |
| Heating degree days | 2,000 K·days/year |
| Assumed heating efficiency | 85% |
| Assumed gas rate | 5.16p/kWh |
| Assumed electricity rate | 22.19p/kWh |
Disclaimer: This case study is provided for general guidance and illustrates the potential benefits of upgrading commercial glazing and replacement window systems. Performance data and energy savings are indicative only and are based on the assumptions stated within this document together with recognised industry data. Actual results will vary depending on building design, occupancy, HVAC systems, operating conditions, weather, maintenance and energy costs.
References to improving building performance, energy efficiency, thermal comfort or supporting future energy-performance objectives do not constitute a guarantee of any specific EPC outcome or regulatory result. Where EPC performance is relevant, a qualified EPC assessor should be consulted for project-specific assessments.
This document should not be relied upon as a guarantee of performance, energy savings or financial return, and should be used as an early-stage decision-support tool only.