Choosing the right insulation for a UK building project is not just about meeting Building Regulations. The wrong product in the wrong location can cost more in materials, reduce performance over time or create problems with moisture and condensation. This guide covers the main insulation types available to UK contractors and builders, with practical guidance on where each product works best.
The Main Insulation Types Available in the UK
Mineral Wool (Glass Wool and Rock Wool)
Mineral wool is the most widely used insulation in UK construction. It comes in rolls and batts and is available in multiple thicknesses and densities. Glass wool is lighter and used mainly in loft and partition applications. Rock wool (stone wool) is denser and has better fire resistance and acoustic performance, making it suitable for party walls and floor cassettes.
Typical thermal conductivity (lambda): 0.032 to 0.044 W/mK depending on density and product.
Best for: Loft insulation between joists, stud wall cavities, timber floor cassettes, between rafters in warm roof builds.
Rigid Foam Board (PIR, EPS, XPS)
Rigid insulation boards offer higher thermal performance per millimetre than mineral wool, which makes them the preferred choice where space is limited. There are three main types:
- PIR (polyisocyanurate) - such as Kingspan or Recticel products - offers the best lambda values, typically 0.022 to 0.026 W/mK. Used in flat roofs, cavity closers, floor insulation and walls where thin sections are needed.
- EPS (expanded polystyrene) - lower cost than PIR, lambda typically 0.031 to 0.038 W/mK. Used in below-slab floor insulation, external wall insulation (EWI) systems and as infill in cavity walls.
- XPS (extruded polystyrene) - higher compressive strength than EPS and moisture resistant. Lambda typically 0.033 to 0.037 W/mK. Used under floor slabs where groundwater pressure is a factor and in inverted flat roofs.
Spray Foam
Spray polyurethane foam is applied as a liquid that expands and sets in place, filling gaps and creating an airtight layer. It is used for insulating between rafters from inside, under floors and in awkward voids.
Important note: Open-cell spray foam in roof spaces has caused significant problems with mortgage valuations in recent years. Many lenders now refuse to approve mortgages on properties with spray foam in roof spaces because of concerns about moisture trapping and structural inspection obstruction. If you are specifying spray foam in a roof, make sure your client is aware of the implications before proceeding.
Natural and Recycled Insulation
Products including sheep wool, wood fibre, hemp, cellulose and recycled denim are available from specialist suppliers. These materials offer good thermal performance, are vapour-permeable (which helps manage moisture in traditional buildings) and have lower embodied carbon than foam products.
- Sheep wool - lambda typically 0.035 to 0.040 W/mK, excellent for timber frame and loft applications
- Wood fibre board - lambda typically 0.038 to 0.042 W/mK, good for external insulation on traditional buildings and sarking board under roof tiles
- Cellulose (blown) - lambda typically 0.035 to 0.040 W/mK, can be blown into existing cavities
Natural insulation products cost more per square metre than mineral wool but may be the right choice for Listed buildings, solid wall retrofit projects or clients with sustainability requirements.
Insulation by Application
Loft Insulation
The most cost-effective insulation measure in most UK homes. The standard recommendation is 270mm of mineral wool between and across the joists in a cold loft (insulation at ceiling level, not at rafter level). For warm roofs (where the insulation is at rafter level and the loft space is used), rigid PIR board between and below the rafters is the usual choice.
Key consideration: ventilate cold lofts properly and do not block eaves ventilation with insulation.
Wall Insulation
The correct product depends on wall construction:
- Cavity walls (built after 1920s) - EPS bead or mineral wool blown into the cavity is the standard retrofit approach
- Solid walls (pre-1920s masonry) - either external wall insulation (mineral wool or EPS render board system) or internal wall insulation (PIR boards or stud wall with mineral wool) are both used
- New timber frame walls - mineral wool or rigid PIR between and across studs, combined with a breather membrane and airtightness layer
- New masonry cavity walls - full-fill EPS or partial-fill mineral wool or PIR batt in the cavity, depending on exposure zone
Floor Insulation
For ground floors, the main choice is between:
- Under-slab insulation - XPS or EPS boards laid before the slab is poured, typical thickness 100 to 150mm for new builds
- Between timber floor joists - mineral wool friction-fit batts supported on netting or timber noggins, used in retrofit and new timber ground floors
- Floating floor over existing slab - PIR boards with a chipboard or screed topping, used in renovation projects where floor build-up can be accommodated
U-Value Requirements Under Part L
Building Regulations Part L sets minimum U-value requirements for new builds and extensions. Current indicative values for 2026:
- External walls - 0.18 W/m2K for new builds, 0.28 W/m2K for extensions
- Ground floors - 0.13 W/m2K for new builds, 0.18 W/m2K for extensions
- Roofs - 0.11 W/m2K for new builds, 0.15 W/m2K for extensions
Always confirm current targets with your Building Control officer and structural engineer at design stage. Targets tightened with the 2022 Part L uplift and may tighten further.
Buying Insulation for Trade
Most builders merchants stock mineral wool and standard rigid PIR board. For natural insulation products and specialist systems, you will usually need a specialist distributor or a direct account with a manufacturer. Trade account holders get meaningful discounts on insulation, which is a high-volume product on most projects, so it is worth negotiating your pricing on these lines specifically when you open or review your account.
