Low-Slope Roof Insulation

Insulation on low-slope roofs is usually placed above the structural deck as continuous rigid boards, so it insulates without thermal bridging through framing. In a conventional (warm) roof the insulation sits below the membrane; in a protected membrane (inverted) roof it sits above it. Spray polyurethane foam combines insulation and roof substrate in one layer.

Main materials

• Polyisocyanurate (polyiso, PIR): the most common North American roof board; high thermal resistance per thickness, foil or fibre-glass facers.
• Expanded polystyrene (EPS): economical, available in many densities and as tapered boards.
• Extruded polystyrene (XPS): low water absorption; the standard inverted-roof board.
• Mineral (stone) wool: non-combustible, dense boards used where fire performance or acoustics matter.
• Cellular glass: rigid, vapour-tight, high compressive strength; used under heavy loads and in buried roofs.
• Wood fibre, perlite and phenolic boards in some markets.

Thermal performance and LTTR

Insulation performance is stated as R-value (imperial) or thermal conductivity, lambda, in W/(m.K), and RSI or U-value in metric. Some foams lose thermal resistance as blowing gases diffuse out and air diffuses in, so North America uses long-term thermal resistance (LTTR), a time-weighted value intended to reflect performance over years of service, for polyiso and spray foam. Medium-density closed-cell spray foam has LTTR values of roughly R-5 to R-6 per inch.

Selection factors

• thermal value and its long-term stability
• compressive strength under traffic, equipment and ballast
• moisture tolerance (crucial for inverted and green roofs)
• fire classification of the assembly
• compatibility with the membrane and adhesives
• dimensional stability and ability to be tapered

Codes

Energy codes set minimum roof R-values or U-values; in the UK, BS 6229 references a maximum U-value of 0.35 W/m2K at any point on a heated roof, including low points of tapered schemes. Insulation also forms part of wind-uplift and fire-rated assemblies, so boards, fasteners and adhesives must match tested systems.