Insulation: expanded polystyrene, foams and mineral wool

How insulation works

The CSIRO Building Technology Resource “Thermal insulation” (2021) splits insulation into two kinds. Bulk insulation is a low-density material full of trapped air: the still air insulates, and the thin material around it adds little heat flow of its own. Reflective insulation uses a shiny aluminium surface that reflects heat across an airspace; it works only with that airspace next to it.

Performance is given as the R value, in m²·K/W: the thickness divided by the material’s thermal conductivity. A higher R value means more resistance to heat flow, and for a bulk material, more thickness means a higher R value.

Insulation types compared

Insulation types compared

Material	Density	R value for 100 mm
Glass fibre batt, low density	8 kg/m³	2.0
Glass fibre batt, medium density	16 kg/m³	2.6
Rockwool batt	40 kg/m³	2.8
Expanded polystyrene (EPS)	16 kg/m³	2.8
Extruded polystyrene (XPS)	30 kg/m³	3.3
PUR, PIR and phenolic foam, new	30 kg/m³	4.0
PUR, PIR and phenolic foam, aged	30 kg/m³	3.7
Vertical reflective airspace	—	0.6

Source: CSIRO, Table 1, typical values.

The table also answers the question of thickness. To reach R 2.8, you need 100 mm of EPS at 16 kg/m³, but only 70 mm of new PIR foam (our calculation: 2.8 ÷ 4.0 × 100 mm).

Expanded polystyrene (EPS) and XPS boards

Expanded polystyrene is made from moulded beads that are expanded with steam and fused into a shape, CSIRO explains; extruded polystyrene is pushed out of a die as a uniform sheet. EPS boards and EPS panels are sold by density, and CSIRO’s R 2.8 applies to EPS at 16 kg/m³, so check the density printed on the board.

XPS costs more than EPS but insulates better, and CSIRO names it for insulation under concrete slabs because it is tough and absorbs very little water. Two cautions apply to both: polystyrene softens in contact with organic solvents, and it is combustible, although flame-retardant grades are made.

Polyurethane spray foam (PU foam)

PIR and PUR foams have the highest R values in the CSIRO table and, CSIRO notes, the foam can be sprayed into place on site. Their R value falls with age, from 4.0 to 3.7 for 100 mm in the table, because the gases in the cells, which insulate better than air, slowly diffuse out.

Glass wool and rock wool

Glass wool and rockwool, also called mineral wool or mineral fibre, are made of fine fibres spun from molten glass, or from slag and rock such as basalt, and held together with a binder, according to CSIRO. They come as batts and blankets. CSIRO notes that rockwool is generally denser, and that protective equipment should be worn when installing mineral fibre insulation.

Fibreglass insulation (spelled fiberglass in the US) is the same material as glass wool; CSIRO lists it as glass fibre batts.

Other insulation materials

Other insulation materials

Material	What it is, per CSIRO	R value for 100 mm
Cellulose fibre, loose fill (35 kg/m³)	pulverised paper with fire-retardant chemicals, blown into ceilings; may settle, and is dusty to install	2.5
Polyester fibre batt (8–16 kg/m³)	polyester fibre, often blended with sheep’s wool, in batts and blankets	1.6–2.2
Wool batt (8–16 kg/m³)	sheep’s wool, usually bonded with polyester “melt” fibre	1.7–2.2
Vermiculite and perlite	steam-expanded mica and porous volcanic rock, as loose fill or mixed into light cement and plaster; non-combustible	—
Vacuum insulation panel (250 kg/m³)	a porous core sealed in a gas-tight wrap with the air drawn out; thin, but slowly loses vacuum if air gets in	18.0

The US National Insulation Association sorts the same materials into three internal structures in its insulation materials guide: fibrous (glass and mineral wool), cellular (polystyrene, polyurethane, polyisocyanurate, elastomeric foam, cellular glass) and granular (calcium silicate, vermiculite, perlite).

Roof insulation

For roofs, both kinds are in use. CSIRO describes the most widely used reflective insulation as a laminate of aluminium foil, usually on both sides, supplied in rolls; it adds R value only where it faces an airspace. A bulk layer — glass wool, rockwool or foam board — adds resistance through its own thickness.

What to check when you buy insulation

1. R value and thickness together, in m²·K/W for the thickness you will install.
2. Density for wool and EPS: in the CSIRO table, glass fibre batts at 8 and 16 kg/m³ differ by R 0.6.
3. Flame-retardant grade for any polystyrene board.
4. Aged R value for PIR and PUR foam, not only the value when new.
5. Facing and airspace for foil products, as they need the airspace to work.

Insulation in Zanzibar

Kitovu sells from stock in Zanzibar, and insulation boards and wool are not among its products. For roof and wall boards, see fibre cement board and aluminium composite panel.