The short answer
Pane count alone does not decide the right window. Compare the whole assembly, the existing home, orientation, ventilation, installation details and the aims of the wider upgrade. A supplier should support a proposal with product documentation rather than a generic saving or payback claim.
| Question | Double glazing | Triple glazing |
|---|---|---|
| Evidence | Ask for the whole-window declaration or certificate for the quoted frame-and-glazing assembly | |
| Thermal comparison | Compare whole-window U-values on the same basis; a lower U-value indicates better thermal performance | |
| Solar and daylight | Ask how glazing choice and orientation affect solar gain and daylight for each elevation | |
| Installation | Compare opening sizes, reveals, sills, airtightness, ventilation, making-good and waste removal | |
| Operation | Confirm hardware, safety, access, restrictors and maintenance for each window type | |
What to ask for in writing
- The manufacturer, product or assembly name and whole-window performance evidence.
- The exact frame, glazing, spacer, hardware and finish included.
- How the installation handles airtightness, weathering, sills, reveals and thermal continuity.
- How required background ventilation will be maintained or upgraded.
- Warranty providers, durations, exclusions and the process for defects.
- An itemised price separating windows, doors, enabling work and making-good.
Use independent performance evidence
NSAI's Window Energy Performance scheme says its rating applies to a specific window frame and glazing assembly from a named manufacturer. It also explains that the published whole-window U-value combines glazing and frame performance. That makes a complete assembly certificate more useful than a centre-pane number quoted in isolation.
Check the 2026 SEAI route before ordering
SEAI now provides an individual windows and doors grant subject to its current property, insulation, product, contractor and post-works requirements. The live page says applicants should obtain the grant offer before commencing work and use registered contractors. Eligibility cannot be inferred from pane count alone.
- Check the current SEAI windows and doors grant page.
- Check the home's insulation evidence and any required BER assessment.
- Verify the contractor on the current windows and doors list.
- Confirm that the quoted assembly meets the scheme's current performance evidence requirements.
Use the current SEAI windows and doors page, NSAI's Window Energy Performance guidance, and our grant-route checklist.