Key takeaways
- Scottish Building Regulations require replacement windows to achieve a U-value of 1.4W/m2k – the equivalent of an A energy rating.
- CR Smith Lorimer double glazed windows achieve 1.4w/m2l across the full casement, tilt and turn, sliding sash, and reversible range.
- Window Energy Rates (WER) combine heat loss (U-vale) and solar gain (G-value) into a single A-G score, developed by the British Fenestration Rating Council.
- Triple glazing can achieve lower U values, but comes with extra weight, reduced solar gain, and significantly higher cost – with minimal practical benefit for most Scottish homes.
- In our experience, the quality of the sealed unit matters as much as the number of panes. Argon gas, low-emissivity glass, and a warm edge spacer bar are what make a double glazed window genuinely energy efficient.
The short answer
High-quality double glazed windows – filled with argon gas, fitted with low-emissivity glass and a warm edge spacer bar – can achieve a U-value of 1.4W/m2k and an A Window Energy Rating. This meets Scottish Building Regulations in full. For most Scottish homes, triple glazing delivers little measurable benefit over a well-specified double glazed unit.
However, the actual window installation is more crucial than the number of panes, because ensuring it is properly fitted and sealed will have the biggest impact on the thermal performance of your home.
What is a window energy rating?
Window Energy Ratings were developed by the British Fenestration Rating Council (BFRC) to give homeowners a straightforward way to compare window performance. The system works on an A to G scale – the same format used on fridges, washing machines, and other white goods. The higher the rating, the better the window performs.
A WER is not simply a measure of heat loss. It is an aggregate of two separate values: the U-value and the G-value. The U-value measures how much heat escapes through the window. The G-value – sometimes called solar gain – measures how much heat from sunlight the window allows into the room. A window that keeps heat in and lets solar warmth through will score well on both counts.
This is worth understanding because a window with a very low U value may not necessarily carry the highest WER. If the same window also blocks a significant amount of solar gain, the overall rating may be lower than expected. The BFRC system accounts for the while picture, not just one element of it.
What does a U value actually mean?
The U value of a window tells you how quickly heat passes through it. The measurement is expressed in watts per square metre per degree of temperature difference (W/m²K). The lower the number, the better the window is at retaining heat.
In Scotland, the standard is clear. Under Building Standards (Scotland) Standard 6.2.1, all replacement windows must achieve a U value of 1.4 W/m²K as a minimum. This is a more demanding requirement than the rest of the UK, reflecting the fact that Scottish winters are colder and homes need to work harder to stay warm.
A U value of 1.4 W/m²K is equivalent to an A energy rating. It is the benchmark CR Smith designs all Lorimer windows to meet and is a guaranteed standard across every product in the range.
Double glazing vs triple glazing — Does the extra pane make a difference?
Triple glazed windows add a third pane of glass and a second gas-filled cavity, which allows them to achieve U-values as low as 0.8 W/m²K. On paper, that sounds like a significant improvement. In practice, the picture is more complicated.
The question Scottish homeowners should ask is not ‘what is the best U-value technically achievable?’ but ‘what U-value does my home actually need, and what is the most cost-effective way to reach it?’
A-rated double glazing at 1.4 W/m²K already meets Scottish Building Regulations in full. According to research carried out by A Marshall et Al (‘Domestic building fabric performance’), the difference in real-world heat loss between a 1.4 and a 0.8 U-value window is measurable in a laboratory but rarely noticeable in a home. Triple glazed units are heavier, which places greater strain on frames, hinges, and opening mechanisms. They also tend to reduce solar gain — meaning less free warmth from the sun enters the room. In a Scottish climate where daylight hours in winter are already limited, that trade-off matters.
Triple glazing can make sense in specific circumstances — high-altitude locations, unusually exposed properties, or Passivhaus builds where near-zero heat loss is a design requirement. For the vast majority of Scottish homes, a well-engineered double glazed unit will perform just as well in everyday use and prove considerably better value.
Double glazing vs Triple glazing – At a glance
| Feature | A-Rated Double Glazing | Triple Glazing |
| U-value | 1.4W/m2k | 0.8 – 1.0W/m2k |
| Meets Scottish Building Regulations | Yes | Yes |
| Argon gas filled | Yes | Yes |
| Low-E glass | Yes | Yes |
| Weight (based on a 1m x 1m insulated unit) | Approximately 20kg | 50% heavier at approximately 30kg |
| Solar gain (G-value) | 0.45 | Circs 35% less than double glazing – meaning less free heat from the sun |
How CR Smith Lorimer windows achieve an A rating
All Lorimer windows from CR Smith are engineered to achieve a U value of 1.4W/m2K and an ‘A’ Window Energy Rating as standard. That performance comes from the specification of the window frame and the sealed unit itself, not from adding extra panes of glass.
Argon gas – flooded, not injected
The cavity between the two panes in a Lorimer double glazed unit is filled with argon gas. CR Smith flood its units with argon rather than injecting it – a distinction that matters. Flooding achieves a higher and more consistent fill level, which means that gas barrier between the panes is more effective at slowing heat transfer. Gas content is tested to BS EN 1279 standard, which require a minimum 90% argon concentration.
Low-emissivity glass
One pane in every Lorimer double-glazed unit is low-emissivity (Low-E) glass. A thing, almost invisible coating reflects long-wave heat radiation back into the room rather than allowing it to escape through the glass. This has a significant effect on U value performance without adding weight, thickness, or a third pane.
Warm edge spacer bar
The spacer bar separates the two panes of glass and keeps them at the correct distance apart. CR Smith uses a warm edge spacer bar – made from an insulating composite material rather than standard aluminium – which prevents heat from conducting along the edge of the sealed unit. Cold edges cause condensation and reduce overall thermal performance. The warm edge spacer bar eliminates both problems.
Multi-chambered uPVC frame
The frame itself contributes to the window’s energy performance. The Lorimer profile uses a multi-chamber design that traps air within the frame, adding a further layer of insulation. Co-extruded gaskets are permanently bonded to the frame – they will not shrink, move, or create gaps over time.
All Lorimer windows carry BFRC Window Energy Rating certificates. The full range — casement, tilt and turn, sliding sash, and reversible — achieve an A rating and a 1.4 W/m²K u-value. These are independently verified, not manufacturer claims.
What Scottish Building Regulations Say About Window Energy Performance
Under Building Standards (Scotland), Standard 6.2.1, any replacement window installed in a Scottish home must achieve a minimum U value of 1.4 W/m²K — equivalent to an ‘A’ Window Energy Rating. This standard has been in place since 2010, when it was updated from the previous minimum of 1.6 W/m²K.
It is the homeowner’s legal responsibility to ensure replacement windows comply with Building Standards (Scotland). CR Smith, however, offers a guarantee that every Lorimer window installation will meet, as a minimum, the exacting requirements of Building Standards (Scotland) at the time of installation — covering energy efficiency, safety glass, fire escape, ventilation, handle heights, and safe cleaning.
CR Smith holds ISO 9001:2015 Quality Management System certification, and all sealed units are manufactured and tested to BS EN 1279-2 and BS EN 1279-3. Casement, tilt and turn, and reversible windows carry PAS 24 enhanced security accreditation.
The Lorimer range comes with a 10-year product and installation guarantee — covering parts, products, and workmanship. Now in its fifth generation, CR Smith has been fully honouring our commitments under its terms since 1972. It really is a guarantee you can trust.
Find out more about CR Smith Lorimer windows
CR Smith is Scotland’s leading window and door manufacturer, designing and building the Lorimer range at its Fife-based state-of-the-art facility. Book a free design appointment and discover how the Lorimer range can improve the energy performance, comfort, and appearance of your home.
FAQ’s
A Window Energy Rating (WER) is a score developed by the British Fenestration Rating Council (BFRC) that measures overall window energy performance on an A to G scale — similar to the label system used on white goods. It combines a window’s U value (heat loss) and G value (solar gain) into a single, easy-to-understand rating. CR Smith Lorimer windows carry an A rating across the full range.
Scottish Building Regulations (Standard 6.2.1) require replacement windows to achieve a U value of 1.4 W/m²K or better — equivalent to an A energy rating. This is a more demanding standard than the rest of the UK. CR Smith Lorimer windows achieve 1.4 W/m²K across the full casement, tilt and turn, sliding sash, and reversible range.
Triple glazing adds weight, reduces solar gain, and typically costs more, with minimal real-world benefit in return. A well-specified double glazed window with argon gas, low-emissivity glass, and a warm edge spacer bar can achieve a U value of 1.4 W/m²K — fully meeting Scottish Building Regulations. CR Smith can manufacture triple glazed windows, but our recommendation for the domestic replacement window market is that double glazing is best.
Argon is a dense, non-toxic inert gas that conducts heat less readily than air. CR Smith floods its double glazed sealed units with argon gas rather than injecting it — a process that achieves a more consistent and higher fill level. This reduces heat transfer between the panes, improving the U value and contributing to the window’s A energy rating. Gas content is tested to BS EN 1279 standards, requiring a minimum 90% argon concentration.
A U value measures heat loss only — the lower the number, the less heat escapes. A Window Energy Rating (WER) combines both heat loss (U value) and solar gain (G value) into a single A to G score. A window with excellent heat retention but poor solar gain may have a better U value but a lower WER. CR Smith Lorimer windows carry both an A WER and a 1.4 W/m²K U value.
