Choosing the Right Insulation
Insulation choice affects energy performance, build depth, acoustic performance, cost, and compliance with Building Regulations. There is no single correct material — the right choice depends on where it goes, how much space is available, and the target U-value.
Lambda Value: The Key Comparison Metric
Lambda (λ), or thermal conductivity, is measured in W/mK. It tells you how well a material conducts heat: the lower the value, the better the insulation per millimetre of thickness.
Typical lambda values by material type:
- PIR rigid boards (Celotex, Kingspan, Recticel): λ0.022–0.023
- EPS (expanded polystyrene): λ0.031–0.038
- XPS (extruded polystyrene): λ0.029–0.036
- Mineral wool (glass): λ0.032–0.044
- Mineral wool (rock): λ0.033–0.040
To achieve the same U-value, PIR requires roughly half the thickness of mineral wool. This makes it critical in tight build-ups where every millimetre counts — between rafters, in wall cavities, or under a screed.
R-value (thermal resistance) is a useful secondary specification — it tells you the thermal resistance of a specific thickness of a specific product. Always confirm R-value using the product’s actual lambda, not a generic material value.
Building Regulations U-Value Targets
Part L of the Building Regulations (England) sets maximum U-values for building elements in new build and major renovation. As a working guide:
- Pitched roof (insulation between and under rafters): 0.16 W/m²K
- Flat roof: 0.18 W/m²K
- External walls: 0.28 W/m²K
- Ground floors: 0.18 W/m²K
Retrofit projects may have different targets, and conservation areas or listed buildings introduce additional constraints. Always calculate the full build-up using a U-value calculator, accounting for thermal bridging at structural elements.
PIR Rigid Boards (Celotex, Kingspan)
Polyisocyanurate (PIR) rigid boards are the highest-performing mainstream insulation material by lambda value. Foil-faced boards (the most common specification) have low emissivity surfaces that provide an additional radiation resistance when adjacent to an air gap.
Best for:
- Pitched roofs (between and under rafters)
- Flat roofs (above deck, warm roof build-up)
- Timber frame walls (between studs or as external insulation layer)
- Underfloor insulation (above and below screed)
- Anywhere space is limited
The Celotex GA4000 insulation board is a widely used general-purpose PIR board. Available in thicknesses from 25mm to 150mm.
Limitations: PIR is combustible (Euroclass F without facing). In some applications (e.g., above certain commercial flat roofs, as external wall insulation on buildings above 11m) non-combustible alternatives are required. Check fire classification requirements before specifying.
Mineral Wool (Glass and Rock)
Mineral wool is available as glass wool (rolls and batts) and rock wool (slabs and rolls). Both are non-combustible (Euroclass A1 or A2), making them essential in applications where fire performance is required by regulation.
Best for:
- Between joists and rafters in pitched roofs (between and at ceiling level)
- Acoustic separation between floors and rooms
- Cavity wall partial fill
- Fire-stopping applications
- Anywhere non-combustible insulation is required
Limitations: Mineral wool requires more thickness than PIR to achieve the same U-value. It can absorb moisture if not properly managed with breather membranes and vapour control layers — important in warm-side-up build-ups.
EPS and XPS
EPS (expanded polystyrene) is the white bead board familiar from packaging and insulated concrete formwork. Inexpensive, moisture-resistant, and stable for below-ground and floor applications.
XPS (extruded polystyrene) — recognisable by its blue, green, or pink colour — has a closed-cell structure giving it very low water absorption. It is the material of choice for:
- Below-ground applications (underfloor below a slab, below-ground external wall insulation)
- Inverted flat roof build-ups (above the waterproofing membrane)
- Cold-store applications
Both EPS and XPS are combustible and not suitable where non-combustible insulation is required.
Vapour Control and Breather Membranes
Insulation does not work in isolation. The position of the vapour control layer (VCL) and breather membrane within the build-up determines whether interstitial condensation occurs.
Vapour control layers (VCLs) restrict moisture movement from the warm interior into the insulation zone. They sit on the warm side (interior) of the insulation.
Breather membranes (such as the Tyvek Supro breather membrane) are vapour-permeable but wind- and water-resistant. They sit on the cold side of the insulation, allowing moisture that penetrates the build-up to escape outward while keeping wind and rain out.
Omitting or mislocating these layers is a leading cause of condensation damage in roofs and walls. For any build-up with more than minimal insulation, a condensation risk analysis (Glaser method) should be carried out.
Thermal Bridging
Thermal bridging occurs wherever a structural element (rafter, stud, joist) penetrates the insulation layer, creating a path of higher conductivity through the build-up. This significantly reduces the actual thermal performance compared to the theoretical U-value calculated through the insulation alone.
Mitigation strategies:
- Over-rafter insulation (rigid boards laid across rafters, not between them) eliminates bridging at rafter positions
- Service zone / counter batten inside the structural layer provides space for services without cutting into insulation and adds a thermal break
- Continuous external insulation on wall build-ups bypasses bridging at studs or masonry ties
- Staggered or overlapping layers of rigid board reduce the effect of joints and bridging points
Thermal bridging calculations (psi values) are required for SAP calculations in new build and are increasingly expected in major retrofit projects.
Spray Foam: A Caution
Spray polyurethane foam (SPF) applied to pitched roof rafters from inside the loft has been widely marketed as a retrofit insulation solution. It delivers reasonable thermal performance but has created significant problems for homeowners:
- Many mortgage lenders now decline to lend on properties with spray foam applied to roof rafters, due to concerns about hidden structural damage and the cost and difficulty of removal
- The foam prevents inspection of the roof structure for defects
- Removal before sale can be expensive and may damage the rafters
Spray foam has legitimate applications in certain commercial and agricultural settings, but for domestic pitched roofs it is not recommended. Always check with a structural engineer before proceeding.
