# Low-Slope Roof Insulation

> Rigid board or sprayed insulation laid above the deck of a low-slope roof, chosen for long-term thermal value, strength, fire and moisture behaviour.

- URL: https://roof.wiki/roof-insulation/
- Topic: Low-Slope & Flat Roof Systems
- Also known as: roof insulation; rigid roof insulation; above-deck insulation
- Regions: Global
- First published: 2026-11-02 · Last reviewed: 2026-11-02
- Cite as: "Low-Slope Roof Insulation." roof.wiki. https://roof.wiki/roof-insulation/

## Key facts

- **Common boards:** Polyiso, EPS, XPS, mineral wool, cellular glass
- **Long-term rating (N. America):** LTTR for polyiso and spray foam
- **UK limit at any point (BS 6229:2025):** U = 0.35 W/m2K
- **Inverted roof board:** Usually XPS

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.

## Sources

1. Polyisocyanurate — Wikipedia. https://en.wikipedia.org/wiki/Polyisocyanurate
2. Polystyrene — Wikipedia. https://en.wikipedia.org/wiki/Polystyrene
3. Mineral wool — Wikipedia. https://en.wikipedia.org/wiki/Mineral_wool
4. Foam glass — Wikipedia. https://en.wikipedia.org/wiki/Foam_glass
5. Spray foam — Wikipedia. https://en.wikipedia.org/wiki/Spray_foam
6. BS 6229:2025 - The key changes and implications for flat roof design — Bauder. https://www.bauder.co.uk/technical-centre/standards-and-knowledge/bs-6229-2025-key-changes