At a glance
Insulation is one of the highest-return home improvement investments available to UK homeowners, but only when the right material is chosen for the specific application. The four main insulation material types used in UK residential construction and renovation – rigid PIR board, mineral wool, spray foam and multifoil – have significantly different thermal performances, installation requirements, costs and suitability for different parts of a building. Choosing PIR for a loft that would be better served by thick mineral wool roll, or installing spray foam in a cavity wall where it creates mortgage and resale problems, are expensive mistakes that are difficult or impossible to correct after the fact.
The thermal performance of an insulation material is measured by its lambda value – the thermal conductivity expressed in watts per metre kelvin (W/mK). The lower the lambda value, the better the material insulates per centimetre of thickness. This number is the most important specification to compare when choosing between materials, because the available depth for insulation in walls, roofs and floors varies significantly. A material with a lambda of 0.022 W/mK (PIR board) delivers the same thermal resistance as mineral wool at 0.035 W/mK in roughly half the thickness – a critical advantage where depth is constrained by a shallow rafter or a solid wall with limited internal space.
Understanding the difference between thermal conductivity (lambda) and thermal resistance (R-value) is important when comparing products. Lambda is a property of the material itself – it does not change with thickness. R-value is the actual thermal resistance achieved by a specific thickness of that material, and it is R-value that determines how well a given installation will perform. To calculate the R-value of an insulation layer, divide the thickness in metres by the lambda value. A 100mm (0.1m) layer of mineral wool with a lambda of 0.035 gives an R-value of 0.1 / 0.035 = 2.86. Building regulations specify minimum R-values for different elements, and the insulation specification for any project should be calculated to meet or exceed these minimums rather than simply choosing a standard thickness from a product catalogue.
Main Insulation Materials Compared
PIR rigid board (sold under the Celotex and Kingspan brands, among others) is the best all-round insulation material available in the UK for the majority of applications where depth is limited. Its closed-cell structure means it does not absorb moisture, so it retains its thermal performance over decades without degradation. It can be used in roofs, floors, walls and specialist applications including below-ground and below-slab insulation, and it is available in thicknesses from 25mm to 150mm. The main disadvantages are cost – it is roughly twice the price per square metre of mineral wool – and the fact that it must be cut accurately to avoid gaps, which require sealing with aluminium foil tape to prevent thermal bridging at the joints. Standard silver gaffer tape is not a substitute – it lacks the required airtightness and degrades over time.
Mineral wool – whether glass wool or rock wool – is the best choice for accessible loft insulation where depth is unlimited and there is no structural constraint on thickness. The recommended loft insulation depth is 270mm, which is most economically achieved with mineral wool laid in two layers at right angles to each other. Mineral wool is significantly cheaper than PIR, easy to handle and cut, and available from all major DIY retailers. Its limitations are in applications where it can get wet – it absorbs moisture readily and wet mineral wool loses most of its thermal value – so it must always be protected by a vapour barrier on the warm side in any application other than an open accessible loft where it can dry freely in the ventilated air. In a cold roof application between and over rafters, a polythene vapour control layer is mandatory on the warm interior face of the insulation.
Thermal Performance Figures
Which Material for Which Application
The table above represents the single most cost-effective insulation intervention available to UK homeowners in properties with an accessible loft: topping up existing insulation from 100mm (a common older standard) to 270mm. The material cost for a typical 50m2 loft is approximately £200-300 in mineral wool, and the installation is a DIY project for most people. No other single home improvement offers a comparable return on investment in terms of annual energy saving relative to upfront cost.
Floor insulation is the most commonly overlooked element of a whole-house insulation programme. Suspended timber ground floors – present in a large proportion of pre-1950 UK housing – can account for 15% of total heat loss and are relatively straightforward to insulate from below if there is sufficient crawl space access. The preferred method is cutting rigid PIR boards to fit between the floor joists and securing them with netting or noggins from below, then sealing the edges with expanding foam. For concrete slab ground floors, the only practical option is to insulate on top of the slab under a new floor covering – either PIR board with a floating floor over it, or insulated underfloor heating boards if underfloor heating is being installed simultaneously. The marginal cost of insulating at the same time as a new floor is small; the cost of doing it as a standalone retrofit is much higher because the floor covering must be lifted and replaced.
For walls, the choice of material depends on whether the construction is cavity or solid. Cavity walls built between approximately 1930 and 1990 are the best candidates for blown mineral wool cavity fill, which is installed by drilling small holes and blowing loose mineral fibre into the cavity – a professional installation that takes half a day and costs £400-800 for a typical house. Solid walls – typical of pre-war and Victorian construction – require either internal dry lining with PIR board or external wall insulation, both of which are more disruptive and expensive. Spray foam should never be used in cavity walls because it cannot be removed without damage to the wall and causes mortgage and resale difficulties with many UK lenders.
Installation Rules That Matter
Spray foam in cavity walls will likely affect your mortgage and make the property harder to sell. Most UK lenders will not lend on properties with spray foam cavity wall insulation because it prevents inspection of the cavity and is very difficult to remove. If you are considering cavity wall insulation, use blown mineral wool or EPS bead only – never spray foam in a cavity.
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