Insulation materials guide
Choosing the right insulation material depends on where you are installing it, your budget, and whether you have specific requirements for breathability or sustainability.
Common insulation types
1. Mineral Wool (Glass or Rock)
Made from molten glass or stone spun into fibres.
- Best for: Lofts, suspended floors, and timber stud walls.
- Pros: Cheap, fire-resistant, excellent sound absorption, and easy to DIY.
- Cons: Bulky (requires more thickness for the same performance as foam boards), can be itchy to handle.
2. Rigid Foam Boards (PIR/PUR)
Closed-cell plastic foam boards (like Celotex or Kingspan).
- Best for: Solid floors, solid walls (internal), and roofs where space is limited.
- Pros: Highest thermal performance per millimetre, easy to cut, moisture-resistant.
- Cons: More expensive, not breathable, requires precise cutting to avoid gaps.
3. Natural Fibres (Sheep’s Wool, Wood Fibre, Hemp)
Sustainably sourced materials that are increasingly popular in eco-retrofits.
- Best for: Period properties with solid walls or lofts where breathability is essential.
- Pros: Sustainable, breathable (manages moisture), safe to handle, good for the environment.
- Cons: Usually more expensive than mineral wool or foam boards.
Key technical terms
U-value
Measures the heat loss through a specific element of a building (like a wall or window). The lower the U-value, the better the insulation. Use our U-value calculator to see how different materials perform.
Thermal Conductivity (K-value)
Measures how well a specific material conducts heat. Lower K-values mean the material is a better insulator.
Vapor Permeability (Breathability)
The ability of a material to allow moisture to pass through it. This is critical for solid wall properties to prevent rot and damp.
Sustainability Tip: When choosing insulation, look for the embodied carbon—the energy used to manufacture and transport the material. Natural fibres usually have much lower embodied carbon than plastic-based foam boards.
Comparing materials (Typical K-values)
| Material | Conductivity (W/mK) | Typical Use |
|---|---|---|
| PIR Foam Board | 0.022 | Floors, Walls (IWI) |
| Glass Wool | 0.044 | Lofts, Joists |
| Sheep’s Wool | 0.038 | Lofts, Stud walls |
| Wood Fibre | 0.040 | Solid walls (EWI/IWI) |
Which should you choose?
- For a standard loft: Mineral wool is almost always the best value.
- For internal solid walls: Wood fibre or breathable PIR boards are recommended.
- For solid floors: High-density PIR foam boards are the standard choice.