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Thermal Resistance & Building Science

Insulation R-Value

The Resistance Metric. Convert thermal resistance from North American R-Value units to International RSI (Metric) units to ensure energy code compliance in heating and cooling design.

North American (R-Value)
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Metric International (RSI)
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U-Value (W/m²·K)
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Standard Ranges

2x4 Stud Wall (US)R-13 to R-15
2x6 Stud Wall (US)R-19 to R-21
Attic Blown-In (US)R-38 to R-60

The R-to-U Ratio

The R-Value is the thermal resistance. The U-Value is the thermal transmittance ($U = 1/R$). While architects focus on the R-value of material layers, HVAC engineers focus on the whole-assembly U-value.

The Physical Foundation of Thermal Resistance

In the architectural and construction trades, "R-Value" is the primary shorthand for the energy efficiency of a building's envelope. While the public is often familiar with R-values for insulation, professional engineers use the **RSI** (International Metric Thermal Resistance) to calculate precisely how many Megajoules per hour will leak through a wall or ceiling system. Transposing these units between North American standards and international Metric specifications is a critical step in modern energy modeling.

Defining the 1.0 R-Value Standard

An R-value of **1.0 (US)** represents a thermal resistance of one hour, per square foot of surface area, per degree of Fahrenheit temperature difference across the material, for every BTU of heat flow. In Metric (SI) terms, this same performance would be expressed as approximately **0.176 m²·K/W**. Understanding this conversion factor ensures that architects using European software can accurately import North American product data.

Insulation Density and Conduction

It is important to remember that R-value is not just a function of material type; it is also a function of **Density and Thickness**. A dense spray foam insulation will provide more thermal resistance per inch than a loose-fill fiberglass material. Beyond the material itself, the R-value of an assembly is often reduced by "thermal bridging"—the unintended conductive path created by metal or wood studs that pass directly through the insulation layer.

Common Insulation R-Value Reference

Insulation Type R-Value (per inch) Metric RSI (per 25mm) Efficiency Level
Fiberglass Batts 3.1 - 3.4 0.55 - 0.60 Low (Standard)
Mineral Wool 3.8 - 4.2 0.67 - 0.74 Moderate (Fire Rated)
Polyisocyanurate (PIR) Board 5.6 - 7.0 0.98 - 1.23 High (Continuous)
Closed-Cell Spray Foam 6.0 - 7.5 1.05 - 1.32 Ultra High (Air Barrier)

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Frequently Asked Questions

What is an Insulation R-Value?

R-value is a measure of thermal resistance—how well building insulation material resists the conductive flow of heat. The higher the R-value, the greater the insulating power.

How many US R-Values is 1 RSI?

1 US R-value (hr·ft²·°F/Btu) is approximately equal to 0.1761 RSI (m²·K/W). Multiply RSI by 5.6782 to get the US R-value.

What is the R-value of standard fiberglass batts?

Typical 3.5" (90mm) fiberglass insulation provides approximately R-11 to R-15 (US), which is roughly RSI 1.9 to 2.6.