Insulation Materials, Compared

Insulation materials differ in how well they resist heat flow, how they handle moisture, how they behave in fire, how thick they need to be for a given performance, and how they suit different constructions. No single material is best for every job: a material ideal for a loft may be unsuitable for a damp solid wall, and a material ideal for a timber frame may be wrong for a flat roof. Choosing a material starts with the construction and the moisture and fire context, not with a league table.

Thermal performance is described by thermal conductivity, with lower meaning better insulation per unit thickness, and by U-value for a complete build-up. Two materials with similar conductivity can still perform differently in a real wall, because gaps, compression, moisture and detailing affect real performance more than laboratory conductivity alone. Material comparison should therefore include installation behaviour, not just a single number.

Mineral wool, including glass and stone wool, is widely used in lofts, cavities and stud walls. It is flexible, fills irregular spaces, does not burn readily, and allows moisture vapour to pass, but it loses performance if compressed or wet, and it needs protection from wind washing in ventilated lofts. Cellulose, made from recycled paper, is blown into lofts and timber frames, fills gaps well, and has good moisture buffering, but it needs correct density and moisture control to avoid settling.

Wood fibre boards suit breathable, traditional construction, offering moisture buffering and good summer heat storage, but they are thicker for a given thermal performance and cost more than mineral wool. Rigid foam boards, such as PIR, offer high performance per thickness, useful where space is limited, but they need careful joint sealing, as gaps between boards become thermal bridges, and their fire behaviour and vapour-closed nature suit some build-ups and not others. Polystyrene beads suit cavity filling, flowing into irregular cavities, but they need suitability checks for exposure and damp.

Check thermal conductivity and required thickness for the target U-value in the actual build-up, including timber bridging, gaps and finishes, not just the material alone. A material with excellent conductivity, fitted with gaps, can perform worse than a modest material fitted continuously. Ask for the build-up U-value calculation, not just a material data sheet.

Check moisture and fire behaviour for the specific location. A vapour-open material suits a breathable traditional wall, while a vapour-closed board suits a different build-up, and swapping them without design can cause condensation. Check fire classification and any fire stopping, cavity barrier or cladding requirements for the location, especially in flats, high-rise contexts, and around escape routes, where fire rules are strict.

In the UK, material choice is shaped by building regulations, fire safety rules, and traditional building guidance, with particular attention to fire performance in external wall systems and in flats following recent regulatory changes. Installers should be able to state fire classification and system compliance for the proposed location, and householders should not accept a material choice that ignores fire context.

In EU member states, material markets and standards vary, with wood fibre and cellulose more common in some countries, mineral wool dominant in others, and foam boards widely used where space is tight. National thermal and fire standards, labelling, and environmental product declarations differ, so a material specification from one country should be checked against the standards and fire rules of the country where it will be installed.

One material mistake is choosing by conductivity alone. A thin, high-performance board that leaves gaps at junctions, or a loose-fill material that settles, can underperform a thicker, more forgiving material fitted continuously. Installation quality and detailing often matter more than a small conductivity advantage, and a material should be judged as installed, not as sold.

Another mistake is ignoring moisture compatibility. A vapour-closed foam on a breathable lime wall, or a moisture-absorbent material in a wall exposed to driving rain without protection, can each cause damp problems that outweigh thermal benefit. Material and wall must be compatible in moisture terms, and that compatibility should be stated in the design, not assumed from thermal data alone.

Keep material data sheets, build-up U-value calculations, fire classifications, moisture design notes, and photographs of installation, especially joint sealing and continuity. That record allows a future professional to understand what materials were used, where, and why, without opening up finished work.

Ask a designer or installer why a particular material suits this construction, location and moisture context, what alternatives were considered, and what detailing ensures continuity. Ask how the material will be protected from moisture, compression and wind washing in service. Those answers show a material choice made for the home, not from a catalogue preference.