What are ASHRAE 90.1 minimum insulation requirements for roofs?

ASHRAE 90.1-2022 roof insulation minimums (above-deck rigid insulation for non-residential buildings): Climate Zone 1A–2A: R-20; Zone 3A–3C: R-20; Zone 4 (except Marine): R-25; Zone 5A–5C: R-30; Zone 6A–6B: R-35; Zone 7: R-35; Zone 8: R-35. These are continuous insulation (ci) R-values for metal building roofs with continuous insulation. Steel stud and wood frame roofs have different requirements. Source: ASHRAE 90.1-2022 Table 5.5-4.

What climate zones does GB 50189 use for insulation?

GB 50189-2015 (Design Standard for Energy Efficiency of Public Buildings) uses 5 climate zones based on heating degree days (HDD) and cooling degree days (CDD): Zone I (严寒, Severe Cold): HDD>5000, e.g. Harbin — strictest insulation; Zone II (寒冷, Cold): 3000 50, e.g. Guangzhou, Shenzhen; Zone V (温和, Mild): e.g. Kunming, Guiyang.

Building Insulation: ASHRAE 90.1 vs EN ISO 6946 vs GB 50189 — R-Value, U-Value & Climate Zones

Q: How do I convert R-value (US) to U-value (SI)?

R-value (US) is in ft²·°F·h/BTU. To convert to RSI (metric R-value, m²·K/W): RSI = R-value (US) × 0.1761. To convert to U-value (W/m²·K): U = 1/RSI = 1/(R-value × 0.1761). Examples: R-20 = RSI 3.52 = U 0.28 W/m²·K; R-30 = RSI 5.28 = U 0.19 W/m²·K; R-38 = RSI 6.70 = U 0.15 W/m²·K. European building codes (EU EPBD, EN standards) use U-value in W/m²·K — lower U-value means better insulation.

Q: How does EN ISO 6946 calculate thermal resistance?

EN ISO 6946:2017 calculates the total thermal resistance of a building element as: RT = Rsi + R1 + R2 + ... + Rn + Rse, where Rsi is internal surface resistance (0.13 m²·K/W for walls, 0.10 for upward heat flow in roofs), Rse is external surface resistance (0.04 m²·K/W), and R1...Rn are individual layer resistances = thickness (m) / thermal conductivity λ (W/m·K). The U-value is then U = 1/RT (W/m²·K). This is the same physics as ASHRAE, but with different surface resistance conventions and metric units.

Calculate Heat Loss Free →

Building envelope insulation requirements are specified differently in the United States (ASHRAE 90.1), Europe (EN ISO 6946 + national energy codes), and China (GB 50189). Engineers must navigate three different unit systems — US R-value, metric RSI, and EU U-value — plus three different climate zone classifications. This guide provides direct comparisons and conversion tables.

1. Unit Systems: R-Value vs U-Value vs RSI

Unit	Standard	Meaning	Good Insulation	Poor Insulation
R-value (US)	ASHRAE, IECC	Thermal resistance (ft²·°F·h/BTU)	R-38 (high)	R-5 (low)
RSI (metric R)	ISO, EN	Thermal resistance (m²·K/W)	RSI 6.7 (high)	RSI 0.88 (low)
U-value	EU EPBD, EN, GB	Thermal transmittance (W/m²·K)	0.15 (low = good)	1.13 (high = poor)

Conversion formulas:

RSI = R-value (US) × 0.1761
R-value (US) = RSI × 5.678
U-value (W/m²·K) = 1 / RSI

2. R-Value to U-Value Conversion Table

R-value (US)	RSI (m²·K/W)	U-value (W/m²·K)	Typical Application
R-5	0.88	1.14	Single-pane window (rough equiv.)
R-10	1.76	0.57	Below-grade wall minimum (ASHRAE CZ 1-2)
R-13	2.29	0.44	Standard cavity insulation (3.5" batt)
R-15	2.64	0.38	3.5" high-density batt
R-19	3.35	0.30	5.5" batt (2×6 wall)
R-20	3.52	0.28	ASHRAE 90.1 roof min (CZ 1–3)
R-25	4.40	0.23	ASHRAE 90.1 roof (CZ 4)
R-30	5.28	0.19	ASHRAE 90.1 roof (CZ 5) / EU PassivHaus wall
R-38	6.70	0.15	ASHRAE 90.1 attic (CZ 6–8)
R-49	8.63	0.116	Passivhaus roof standard

3. Climate Zone Comparison

ASHRAE CZ	Representative US Cities	EN/EU Climate Type	GB 50189 Zone	Representative CN Cities
Zone 1A (Hot-Humid)	Miami, Houston	Mediterranean/Tropical	Zone IV (夏热冬暖)	Guangzhou, Shenzhen, Haikou
Zone 2A (Hot-Humid)	Atlanta, Dallas	Warm Temperate	Zone III (夏热冬冷)	Shanghai, Chengdu, Wuhan
Zone 3A (Warm-Humid)	Memphis, Nashville	Warm Temperate	Zone III (夏热冬冷)	Hangzhou, Nanjing
Zone 4A (Mixed-Humid)	New York, Chicago	Central European	Zone II (寒冷)	Beijing, Tianjin, Xi'an
Zone 5A (Cool-Humid)	Minneapolis, Denver	Northern European	Zone II (寒冷)	Shenyang, Changchun
Zone 6A (Cold-Humid)	Burlington VT, Duluth MN	Scandinavian/Baltic	Zone I (严寒)	Harbin, Hohhot
Zone 7 (Very Cold)	Fairbanks AK	Subarctic	Zone I (严寒)	Mohe, Heihe

4. Minimum Insulation Requirements: Roofs

Climate	ASHRAE 90.1-2022 (R-value ci)	EU Typical (U-value, W/m²·K)	GB 50189-2015 (U-value, W/m²·K)
Hot / Tropical (CZ 1–2 / Zone IV)	R-20 (U 0.28)	0.25–0.35 (Mediterranean)	≤0.50 (Zone IV)
Warm Temperate (CZ 3 / Zone III)	R-20 (U 0.28)	0.20–0.30	≤0.45 (Zone III)
Mixed / Cool (CZ 4–5 / Zone II)	R-25 to R-30 (U 0.19–0.23)	0.15–0.20 (Central Europe)	≤0.45 (Zone II)
Cold (CZ 6–7 / Zone I)	R-35 (U 0.16)	0.10–0.15 (Scandinavia)	≤0.35 (Zone I)

Sources: ASHRAE 90.1-2022 Table 5.5-4 (continuous insulation for non-residential roofs); EU U-values are typical national building code requirements per the Energy Performance of Buildings Directive (EPBD); GB 50189-2015 Table 3.3.1.

5. Minimum Insulation Requirements: Walls

Climate	ASHRAE 90.1-2022 (Wall, R-value)	EU Typical (U-value)	GB 50189-2015 (U-value)
Hot / Tropical (CZ 1–2)	R-13 ci (mass walls: none)	0.35–0.50	≤0.80 (Zone IV)
Warm (CZ 3)	R-13+R-3.8 ci or R-20 ci	0.25–0.40	≤0.70 (Zone III)
Mixed (CZ 4)	R-13+R-7.5 ci or R-20 ci	0.20–0.30	≤0.50 (Zone II)
Cold (CZ 5–6)	R-13+R-12.5 ci or R-20+R-5 ci	0.15–0.20	≤0.45 (Zone I)
Very Cold (CZ 7–8)	R-13+R-15.6 ci or R-25+R-10 ci	0.10–0.15 (Nordic)	≤0.35 (Zone I, severe cold)

ASHRAE notation: "R-13+R-7.5 ci" = R-13 cavity insulation plus R-7.5 continuous insulation. "ci" = continuous insulation (no thermal bridges).

6. EN ISO 6946: U-Value Calculation Method

EN ISO 6946:2017 calculates wall/roof thermal transmittance as:

U = 1 / RT = 1 / (Rsi + R₁ + R₂ + ... + Rn + Rse)

Where:

Rsi (internal surface resistance): 0.13 m²·K/W (walls), 0.10 (upward heat flow, roofs)
Rse (external surface resistance): 0.04 m²·K/W (all orientations)
R = d / λ per layer: thickness d (m) divided by thermal conductivity λ (W/m·K)

Common λ values: mineral wool 0.035–0.045 W/m·K; EPS foam 0.036–0.042; XPS foam 0.030–0.038; concrete 1.65; brick 0.6–1.0; glass wool 0.035–0.050.

7. Frequently Asked Questions

How do I convert R-value (US) to U-value (SI)?

RSI = R-value × 0.1761. U-value (W/m²·K) = 1 / RSI. Example: R-30 → RSI 5.28 → U 0.19 W/m²·K. For quick mental math: R-value × 0.176 ≈ RSI; U ≈ 5.68 / R-value.

What are ASHRAE 90.1 minimum roof insulation requirements?

ASHRAE 90.1-2022 non-residential roof insulation (continuous): CZ 1–3: R-20; CZ 4: R-25; CZ 5: R-30; CZ 6: R-35; CZ 7–8: R-35. These values apply to above-deck continuous insulation. Attics and metal buildings have separate tables.

How does EN ISO 6946 calculate thermal resistance?

RT = Rsi (0.13) + sum of d/λ for each layer + Rse (0.04). Then U = 1/RT. Insulation layer example: 100 mm mineral wool (λ = 0.04): R = 0.10 / 0.04 = 2.5 m²·K/W.

What climate zones does GB 50189 use?

Five zones by heating degree days: Zone I 严寒 (Harbin, HDD>5000), Zone II 寒冷 (Beijing, Shenyang), Zone III 夏热冬冷 (Shanghai, Chengdu), Zone IV 夏热冬暖 (Guangzhou, Shenzhen), Zone V 温和 (Kunming). GB 50189-2015 gives U-value limits per zone for roofs, walls, and glazing.