Polyiso (Polyisocyanurate) Roof Insulation

Polyisocyanurate, called polyiso, PIR or ISO, is a rigid thermoset foam closely related to polyurethane but made with a higher proportion of MDI and a polyester polyol. The MDI molecules link into isocyanurate rings, giving a stiffer and more thermally stable structure than polyurethane. For roofing it is made as boards laminated between facers of coated glass-fibre mat, paper or foil.

Thermal performance

Polyiso has one of the lowest thermal conductivities of common roof insulations, about 0.023 W/(m.K) for typical products. Its North American trade association notes that R-value per inch increases with board thickness. Because it is a gas-blown foam, its thermal resistance changes over time and with temperature, so North American codes use long-term thermal resistance (LTTR) values for it.

Uses

• primary insulation under single-ply, modified bitumen and BUR membranes
• tapered boards to create slope
• high-density (HD) polyiso cover boards over softer insulation
• insulated metal sandwich panels for roofs and walls

Fire

Isocyanurate bonds break down at higher temperatures than urethane bonds, and polyiso performs well in North American foam-plastic fire tests such as ASTM E84 and NFPA 285 when used in listed assemblies. Some researchers caution that small-scale tests do not represent all fire hazards, and studies have found PIR combustion produces significant toxic gases including hydrogen cyanide. In the UK the material was examined in the Grenfell Tower inquiry for its use in façade systems.

Considerations

Polyiso should be kept dry; wet boards lose insulating value and facers can delaminate, reducing wind resistance of adhered membranes. Boards are typically mechanically fastened or adhered and staggered in multiple layers to limit joint heat loss.