What Changed and What the New Terms Mean
The thermal performance requirements for roofs and wall cladding under NCC 2025 Part J have evolved significantly.
Under previous NCC versions, roof colour compliance was largely based on a single figure:
- Solar Absorptance
However, NCC 2025 now references a broader range of thermal performance properties including:
- Solar Absorptance (SA)
- Total Solar Reflectance (TSR)
- Thermal Emittance
- Solar Reflectance Index (SRI)
This change better reflects how roofing materials actually behave thermally in real-world conditions.
For builders, designers and clients, these additional figures can seem confusing at first. This article explains what they mean in simple practical terms.
Why Roof Colour Matters
Roof colours have a major impact on how much heat enters a building.
- Dark roofs absorb more solar heat
- Light roofs reflect more solar heat
- Hotter roofs increase cooling demand
- Cooler roofs improve energy efficiency and occupant comfort
Because of this, roof thermal properties directly affect:
- Part J compliance
- HVAC sizing
- Energy consumption
- Internal temperatures
- Building performance solutions
What Changed in NCC 2025?
Previous NCC Approach
Historically, compliance assessments commonly focused on:
Solar Absorptance
This was used as the primary indicator of roof heat gain.
For example:
- Light roofs = low absorptance
- Dark roofs = high absorptance
While useful, absorptance alone does not fully describe how hot a roof becomes or how it behaves thermally.
NCC 2025 Approach
NCC 2025 now recognises additional thermal properties including:
- Reflectance
- Emittance
- Solar Reflectance Index (SRI)
This provides a more complete understanding of:
- Heat absorption
- Heat reflection
- Heat release
- Overall roof surface temperature
In simple terms:
The NCC is moving from a “single-number” approach to a more complete thermal performance assessment.
1. Solar Absorptance (SA)
What is it?
Solar absorptance measures:
How much solar heat the surface absorbs.
It is measured between:
- 0 = absorbs no heat
- 1 = absorbs all heat
Simple Explanation
Think of standing outside wearing:
- A black shirt
- A white shirt
The black shirt becomes hotter because it absorbs more solar energy.
Roofs behave the same way.
Examples
| Colour | Approx. Absorptance |
|---|---|
| Surfmist | 0.33 |
| Shale Grey | 0.44 |
| Monument | 0.73 |
| Night Sky | 0.95 |
Simple Rule
Higher absorptance = hotter roof.
2. Total Solar Reflectance (TSR)
What is it?
TSR measures:
How much sunlight is reflected away from the surface.
It is essentially the opposite of absorptance.
Simple Explanation
A roof that reflects sunlight away stays cooler than one that absorbs it.
Light colours generally have higher TSR values.
Examples
| Colour | Approx. TSR |
|---|---|
| Surfmist | 0.67 |
| Monument | 0.27 |
| Night Sky | 0.05 |
Simple Rule
Higher TSR = cooler roof.
3. Thermal Emittance (Emittance)
What is it?
Thermal emittance measures:
How effectively a material releases heat after it becomes hot.
Most painted metal roofs have emittance values around:
- 0.85 to 0.90
Simple Explanation
Roofs not only absorb heat, they also release heat back into the environment.
Higher emittance helps the roof cool itself down more effectively.
Simple Rule
Higher emittance = heat escapes more easily.
4. Solar Reflectance Index (SRI)
What is it?
SRI is an overall measure of:
How hot the roof surface gets in the sun.
It combines:
- Solar reflectance
- Thermal emittance
Into one overall performance rating.
Simple Explanation
SRI is the easiest way to compare the overall thermal performance of different roof colours.
Higher SRI values mean cooler roof surfaces.
Typical SRI Values
| Roof Colour | Typical SRI |
|---|---|
| White roofs | 80–100+ |
| Mid-grey roofs | 30–50 |
| Dark roofs | 0–30 |
Simple Rule
Higher SRI = cooler roof.
Why This Matters for Builders
Under NCC 2025, roof colour selection can directly affect:
- Required insulation levels
- DTS compliance
- HVAC sizing
- Energy modelling outcomes
- Performance solutions
Darker roof colours may now require:
- Additional insulation
- Alternative construction measures
- Performance-based justification
To offset increased heat gain.
Quick Summary Table
| Term | Simple Meaning | Higher Number Means |
|---|---|---|
| Absorptance | Heat absorbed | Hotter roof |
| TSR | Heat reflected away | Cooler roof |
| Emittance | Ability to release heat | Cools faster |
| SRI | Overall roof temperature rating | Cooler roof |
Summary
The key change in NCC 2025 is that roof thermal performance is no longer viewed through absorptance alone.
The NCC now considers:
- How much heat is absorbed
- How much heat is reflected
- How effectively heat is released
- The overall roof surface temperature
This creates a more realistic assessment of roof thermal behaviour and building energy performance.
In practical terms:
Light, reflective roofs generally improve thermal performance and reduce cooling loads, while darker roofs may require additional compensation measures under Part J.