When retrofitting homes to meet modern UK Building Regulations, achieving the right U-values for your thermal envelope is critical. U-values measure a material’s effectiveness as an insulator, with lower values indicating better insulation. Mineral wool is popular for external walls, cavities, roofs, and floors due to its excellent thermal performance, fire resistance, and sustainability. But what insulation thickness do you need for each part of your building? Let’s break it down based on the retrofit U-value targets set out by UK Building Regulations.

What are the U-values required in a retrofit?

U-value measures how well a building element conducts heat. The lower the U-value, the better the material is at insulating. The UK Building Regulations set maximum U-values for different building elements in retrofit situations to improve energy efficiency and reduce carbon emissions.

According to the Approved Document L1B (Conservation of fuel and power in existing dwellings), the target U-values for retrofit projects are:

• External Walls: 0.30 W/m²K
• Roofs: 0.16 W/m²K
• Floors: 0.22 W/m²K
• Cavity Walls: Varies depending on the existing structure

Mineral wool is a popular insulation material due to its excellent thermal properties, fire resistance, and sound insulation capabilities. It typically has a thermal conductivity (λ-value) ranging from 0.032 to 0.040 W/mK. For our calculations, we’ll use an average λ-value of 0.035 W/mK.

To determine the insulation thickness needed, we’ll use the formula:

Thickness (m) = Required R-value (m²\cdotpK/W) × λ

Where:

• The required R-value is the thermal resistance needed from the insulation layer alone.
• λ (lambda) is the thermal conductivity of the insulation material.

External Wall insulation thickness

Target U-value: 0.30 W/m²K (or lower for some upgrades)

For external walls, mineral wool thickness depends on the type of wall construction and whether it is part of an External Wall Insulation (EWI) system:

• Solid Walls (no cavity): To meet 0.30 W/m²K, you’ll typically need 100–150mm of mineral wool insulation when using a high-performance material with a thermal conductivity of around 0.032 W/mK.
• Cavity Walls: If the cavity is being filled or enhanced, 50–100mm of mineral wool batts may be sufficient, depending on the width of the cavity and the existing insulation.

For retrofits, thicker insulation may be required to improve thermal performance without replacing wall structures entirely.

Calculation

• Target U-value: 0.30 W/m²·K
• Required R-value (thermal resistance): R = 1 / U
  • R = 1 / 0.30 = 3.33 m²·K/W

• Adjustments for existing wall performance: Subtract R-values for the internal surface (0.13), brick wall (0.18), and external surface (0.04).
  • Required R-value for insulation: 3.33 – (0.13 + 0.18 + 0.04) = 2.98 m²·K/W

• Mineral Wool Thickness: Thickness = R × λ
  • Thickness = 2.98 × 0.035 = 0.104 m (104 mm).

Roofs insulation thickness

Target U-value: 0.16 W/m²K (pitched roofs) or 0.18 W/m²K (flat roofs)

Insulating a roof is one of the most effective ways to reduce heat loss. The required thickness for mineral wool varies depending on the roof type:

• Pitched Roofs (between rafters): For loft spaces, achieving the 0.16 W/m²K target usually requires 270–300mm of mineral wool in two layers, with one layer between joists and another cross-laid over them.
• Flat Roofs: Insulating flat roofs to 0.18 W/m²K typically requires 150–200mm of rigid mineral wool insulation boards.

Calculation

• Target U-value: 0.16 W/m²·K
• Required R-value: R = 1 / U
  • R = 1 / 0.16 = 6.25 m²·K/W
• Adjustments for existing roof performance: Subtract surface resistances for internal (0.10) and external (0.04).
  • Required R-value for insulation: 6.25 – (0.10 + 0.04) = 6.11 m²·K/W
• Mineral Wool Thickness: Thickness = R × λ
  • Thickness = 6.11 × 0.035 = 0.214 m (214 mm).

Floors insulation thickness

Target U-value: 0.22 W/m²K

For floors, the insulation thickness depends on whether you’re retrofitting a suspended timber floor or insulating a solid concrete slab:

• Suspended Timber Floors: Insulating between the joists with 100–150mm of mineral wool will generally meet the target. Adding a membrane for airtightness can further enhance performance.
• Solid Floors: 50–100mm of mineral wool insulation boards can be installed above or below the concrete slab for retrofitted concrete floors. High-density materials work well in these cases.

Calculation

• Target U-value: 0.22 W/m²·K
• Required R-value: R = 1 / U
  • R = 1 / 0.22 = 4.55 m²·K/W
• Adjustments for existing floor performance: Subtract internal surface resistance (0.17).
  • Required R-value for insulation: 4.55 – 0.17 = 4.38 m²·K/W
• Mineral Wool Thickness: Thickness = R × λ
  • Thickness = 4.38 × 0.035 = 0.153 m (153 mm).

Cavity wall insulation thickness

Target U-value: 0.55 W/m²K (filled cavity retrofits)

Filling existing cavities with mineral wool can significantly improve thermal performance in retrofits. The depth of the cavity usually dictates the achievable U-value:

• For standard cavity wall depths of 50–100mm, mineral wool cavity batts can be installed to achieve U-values close to 0.55 W/m²K. For greater thermal efficiency, consider hybrid systems combining cavity fill with external insulation.