Thermal Insulation

Three families of thermal insulation dominate Iranian construction. Expanded polystyrene (EPS) is cheap and light, with a conductivity of 0.033 to 0.040 W/m.K, used on external walls and floors. Extruded polystyrene (XPS) is denser, absorbs almost no water and suits under-slab work, soil-retaining walls and inverted roofs. Mineral wool, in rock and glass forms, adds good acoustic performance and, crucially, does not burn, which makes it essential in escape routes, plant rooms and fire-rated separations.

Part 19 of the National Building Regulations classifies buildings by occupancy, floor area and climate zone, and sets a minimum thermal resistance for each element of the envelope. In Tehran, the Part 19 compliance calculation and certificate form part of the mandatory permit documents, so insulation thickness must come out of that calculation rather than from the installer's habit.

The most damaging error is loss of continuity at beams, columns, slab edges and around openings. These thermal bridges can cancel up to 30 per cent of the insulation's benefit and, in winter, cause surface condensation and mould. The second is allowing insulation to get wet; damp EPS and mineral wool lose almost all their thermal value and need vapour and moisture protection. The third is specifying ordinary EPS instead of the self-extinguishing grade where fire spread is a risk.

In refurbishment, insulating from outside the wall, for instance with an insulated composite facade, outperforms internal insulation because it also covers the structural thermal bridge and does not consume interior space.