Wall Insulation Calculator UK

Standard 2.4m x 1.2m insulation boards cover 2.88m² each. For typical wall projects, allow 1.1 boards per linear metre of wall (at a 2.4m height) to account for cuts and waste. This tool works for PIR boards (Celotex/Kingspan), mineral wool, or insulated plasterboard.

Wall Details

Includes specialized 25mm insulation for internal window edges.

Large touch target for easier mobile use.

Results

Net Wall Area: 0 m²
Items Needed: 0 Boards
Area includes deductions for 0 doors and 0 windows.
Estimated Materials Cost: £0
(Based on average UK pricing)
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Internal Wall Insulation (IWI) Guide

Insulating your walls is one of the best ways to keep your home warm and reduce energy consumption. Internal wall insulation is often chosen for solid wall properties (pre-1920s) where cavity wall insulation isn't possible.

Insulation Choices for Walls

Breathable "Natural" Alternatives (Pre-1920s Properties)

For older solid-wall properties (built before 1920), using non-permeable PIR boards can trap moisture and cause structural damp. "Breathable" insulation allows moisture to pass through the wall naturally:

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UK Building Regulations (Part L)

Under current Part L Building Regulations, if you are insulating more than 50% of an external wall's surface area, you must meet modern thermal standards:

Project Type Target U-Value Typical PIR Thickness
Standard Renovation (Part L) 0.30 W/m²K 60mm - 90mm
Future Homes Standard (2026) 0.18 W/m²K 120mm - 150mm

*Note: Achieving a lower U-value improves your EPC rating and reduces energy bills, but requires thicker insulation which impacts room size.*

View official Part L guidance →

Interstitial Condensation & BS 5250

One of the highest risks in internal wall insulation is interstitial condensation—where moisture from the room traps between the new insulation and the original cold wall.

To comply with BS 5250:2021 (Code of practice for control of condensation), you must:

  • Install a continuous Vapor Control Layer (VCL) on the warm side of the insulation.
  • Ensure all board joints and service penetrations (sockets) are taped and sealed.
  • Consider a "breathable" build-up if your property has solid stone or brick walls with no cavity.

How much insulated plasterboard for 0.30 U-value UK?

To achieve a U-value of 0.30 W/m²K on a standard UK solid brick wall, you typically require at least 62.5mm of high-performance insulated plasterboard (PIR-backed). Below is a guide for different wall types:

Existing Wall Type Insulation Required (PIR) Resulting U-Value
Solid Brick (225mm) 62.5mm (PIR + Board) ~0.30
Uninsulated Cavity 50.0mm (PIR + Board) ~0.28
Part-Filled Cavity 37.5mm (PIR + Board) ~0.25
2026 New Build 100mm+ (PIR + Board) ~0.18

*Calculations assume standard masonry and PIR thermal conductivity (λ) of 0.022 W/mK.*

Technical Mathematical Foundation

1. Net Wall Area:

$$Area_{net} = (Length \times Height) - \sum(Doors_{Area} + Windows_{Area})$$

2. Board Requirement:

$$Boards = \text{ceil} \left( \frac{Area_{net}}{\text{Board}_{Area}} \times (1 + \frac{Waste \%}{100}) \right)$$

3. Thermal Resistance (R):

$$R = \frac{Thickness(m)}{\lambda}$$

(Where $\lambda$ is thermal conductivity, e.g., 0.022 W/mK for PIR insulation).

📜 Regulatory Compliance & Technical Specification

This estimator is aligned with UK Part L (2025/2026) Building Regulations. All material quantities and U-value estimates are derived from standardized technical datasheets for PIR (λ 0.022) and mineral wool (λ 0.034 - 0.044) insulation types.

⚠️ Critical Notice: While this tool provides high-authority technical estimates suitable for material ordering, it is a simulation for guidance only. It does not replace a formal SAP/RdSAP assessment or professional architectural specification. Estimates assume standard masonry performance and do not account for structural damp, thermal bridging, or interstitial condensation if vapor control layers (VCL) are omitted.

Frequently Asked Questions

Do I need to remove my skirting boards?

Yes, for the best results, you should remove skirting boards and coving. This allows you to run the insulation board from the floor to the ceiling, creating a continuous thermal envelope and preventing cold bridges.

What is a cold bridge?

A cold bridge (or thermal bridge) is an area where heat can easily escape through the structure, such as where an internal wall meets an external wall. When insulating, it is common practice to 'return' the insulation 300mm-400mm along the internal wall to minimize this risk.

Is mineral wool better than PIR boards for walls?

PIR boards offer much better thermal performance per mm of thickness, meaning you can achieve your target U-value with a thinner wall buildup. However, mineral wool is non-combustible and offers superior soundproofing.

How do I fix insulated plasterboard?

There are two main methods: 'Dot and Dab' (using adhesive dabs) or fixing to timber/metal studs with drywall screws. Dot and dab is faster but requires a solid, dry wall. Stud fixing is preferred for uneven walls or where a service cavity for electrics is needed.