FixMeasure
Basics
R-value and U-value explained: insulation basics
R-values and U-values are how insulation performance is measured and how building rules set requirements. This guide explains both, shows how to calculate them and how they relate to the thickness you need.
1. How heat leaves a building
Heat always flows from warm to cold: through walls, roofs, floors and windows (conduction), through gaps (air leakage) and by radiation. A thermal camera image shows the weak spots clearly: warm colors on the outside of a building mark where heat is escaping. Insulation slows conduction; airtightness stops leakage.
2. R-value: resistance of a layer
The R-value (thermal resistance) says how strongly a layer resists heat flow. For a uniform material it is simply thickness divided by thermal conductivity:
R = d / λ (d in meters, λ in W/m·K, R in m²·K/W)
For example, 12 cm of mineral wool with λ = 0.035 gives R = 0.12 / 0.035 ≈ 3.4 m²·K/W. Doubling the thickness doubles the R-value. The Insulation calculator works this out in both directions.
3. U-value: the whole construction
A wall has several layers, and each one adds its own R-value. Add them up, together with small surface resistances on the inside and outside (typically about 0.13 and 0.04 m²·K/W for a wall), and take the inverse:
U = 1 / (Rsi + R1 + R2 + … + Rse) in W/m²·K
The lower the U-value, the less heat is lost. A U-value of 0.25 W/m²·K means 0.25 watts flow through each square meter for every degree of temperature difference.
4. Metric and US units
In North America, R-values are usually given in imperial units (ft²·°F·h/BTU), often per inch of material. To convert: R(US) ≈ R(SI) × 5.68. So R-13 in a US wall is about R 2.3 in SI units. Always check which system a product label uses before comparing.
5. Typical values
| Element | Typical U-value (W/m²·K) |
|---|---|
| Uninsulated solid brick wall | About 1.5–2.0 |
| Well-insulated new wall | About 0.15–0.30 |
| Single glazing | About 5.0–5.8 |
| Modern double glazing | About 1.1–1.6 |
| Triple glazing | About 0.5–0.9 |
These are rough figures for comparison. Required values depend on your country and climate; check the current local building energy rules.
6. What reduces real performance
Calculated values assume perfect installation. In practice, thermal bridges such as concrete beams, balconies and window reveals, gaps between boards, compressed or wet insulation, and air leaks all raise heat loss. A continuous, tightly fitted layer and an airtight inner surface matter as much as the material. See Insulation materials compared: wool, EPS, XPS, PIR to compare materials.
Common mistakes
- Comparing R-values in US and SI units as if they were the same.
- Looking only at the insulation layer and forgetting the U-value of the whole wall.
- Ignoring thermal bridges at beams, balconies and window edges.
- Assuming double the thickness halves the heat loss of the whole house; windows and leaks remain.
- Using a product's lambda from memory instead of the declared value on the label.
Frequently asked questions
What is the difference between R-value and U-value?
R-value is the resistance of a layer (higher is better); U-value is the heat loss through the whole construction (lower is better). U is roughly the inverse of the total R.
How do I calculate R-value?
Divide the thickness in meters by the material's lambda. For example, 0.10 m ÷ 0.035 W/m·K ≈ 2.9 m²·K/W.
What is a good U-value for a wall?
Modern requirements are typically between about 0.15 and 0.35 W/m²·K, depending on country and climate.
How do I convert US R-value to metric?
Divide the US value by about 5.68. R-19 (US) is about 3.3 m²·K/W.
How thick should my insulation be?
Multiply the target R-value by the material's lambda to get the thickness in meters, or use the insulation calculator.