INSULATION / THERMAL VALUES
U-values explained.
Know what the numbers mean.
Sometimes in life, it’s better to lower your values.
Understand how U-value, R-value and thermal conductivity describe heat moving through your home — and why the finished construction matters as much as the insulation product.
01
The three values
R, U and lambda describe different parts of the heat-loss story.
02
What regulations say
Figures depend on the work and the building element.
03
Insulation choices
Thickness, conductivity and existing construction work together.
04
Try the calculator
Explore a simple layered-wall example.
THE STARTING POINT
One home. Three ways to describe heat flow.
Thermal values are useful when you compare insulation, check a design or read a survey report. They describe related things, but they are not interchangeable.
Higher R-value means more resistance. Lower U-value means less heat passes through a complete element. Lower lambda means a material conducts heat less readily.
The quick rule
R-value: higher is better. U-value: lower is better. Lambda: lower is better.
01 / THE THREE VALUES
Know which number you are looking at.
R-value · m²K/W
The thermal resistance of a material or layer. The thicker the same material, the higher its R-value.
U-value · W/m²K
The rate of heat transfer through the complete roof, wall, floor, window or door. It accounts for the whole assembly.
Lambda λ · W/mK
The thermal conductivity of an individual material. A lower lambda gives more resistance at the same thickness.
For a uniform layer, R = thickness in metres ÷ lambda. In a simple one-dimensional construction, U = 1 ÷ total R, including relevant surface resistances. Actual elements can need more detailed methods.
HOW MATERIALS COMPARE
Same thickness. Different resistance.
At 100 mm thickness, a mineral wool product with a declared conductivity of 0.040 W/mK has R ≈ 2.50 m²K/W. A PIR board with 0.023 W/mK has R ≈ 4.35 m²K/W.
These are product-layer examples, not the U-value of an installed roof or floor. Use the actual product declaration and the complete construction to make a project decision.
The calculation
Mineral wool: 0.10 ÷ 0.040 = 2.50 m²K/W. PIR: 0.10 ÷ 0.023 ≈ 4.35 m²K/W.
02 / WHAT REGULATIONS SAY
First identify the kind of work.
The figures below come from England’s Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments). Different tables apply when you upgrade an existing element and when you install a new or replacement element. These are examples of limiting or improved values, not universal targets for every project.
Upgrading existing fabric
Table 4.3 improved values: roof 0.16; wall with internal or external insulation 0.30; floor 0.25 W/m²K. Cavity wall insulation has a separate 0.55 figure.
New or replacement fabric
Table 4.2 limits: roof 0.15; wall 0.18; floor 0.18 W/m²K. Windows: 1.4 W/m²K or Window Energy Rating Band B; most doors: 1.4 W/m²K or the relevant doorset rating.
The precise requirement can depend on the work, exceptions and which regulatory standards apply. The 2026 edition has been published for the standards due to take effect in 2027, subject to transition rules. See the applicable Approved Document L guidance.
03 / APPLY IT TO THE HOME
A low number needs good detailing.
A calculated U-value is a useful design measure. Gaps, compressed insulation, repeating timber joists, poorly treated junctions and air leakage can weaken what is achieved in practice.
Moisture, ventilation, fire safety, access and the nature of the existing construction must inform the insulation choice.
What to check
Available depth, thermal bridges, ventilation routes, moisture behaviour, product suitability and installation quality.
PRACTICAL EXAMPLE
More insulation usually lowers the U-value.
The earlier article used a suspended timber-floor example with 0.25 W/m²K as an upgrade value. These results are illustrative only: the joists, floor finish, air spaces, surface conditions and product data must be included in an actual floor calculation.
100 mm loft roll
Illustrative floor result: 0.44 W/m²K.
150 mm loft roll
Illustrative floor result: 0.29 W/m²K.
200 mm loft roll
Illustrative floor result: 0.22 W/m²K.
A 100 mm PIR example in the earlier article was also 0.22 W/m²K. The calculation depends on its declared conductivity and the complete floor construction; do not assume another floor will have the same result.
04 / EXPLORE THE NUMBERS
Try a simple U-value calculator.
Add layers to see how material conductivity and thickness affect an illustrative solid wall. Change the conductivity to the declared value for the product you want to explore.
Indicative solid-wall layer calculation
| Material | Thickness (mm) | λ (W/mK) | R (m²K/W) |
|---|
Uses internal surface resistance 0.13 and external surface resistance 0.04 m²K/W for a simple vertical wall. Conductivities are illustrative: replace them with declared product values for real work. This omits air spaces, repeating thermal bridges, fixings and moisture effects; it cannot establish regulatory compliance.
CHECK THE FULL DESIGN
The number matters. The building matters more.
For a real roof, wall or floor, the calculation and specification need to reflect the entire construction, product declarations, junctions and workmanship. Ventilation and moisture should be considered alongside heat loss.
Manufacturer calculators can help with specific systems: Kingspan, Knauf Insulation, Ecotherm and Superglass. Use the applicable guidance and professional calculations for final compliance.