Notes: R-value is expressed in ft2•h•°F/Btu. U-value is expressed in W/m-K per standard convention.
- Thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It appears primarily in Fourier’s Law
for heat conduction. Thermal conductivity is measured in watts per kelvin per metre (W•K−1•m−1). Multiplied by a temperature
difference (in kelvins, K) and an area (in square metres, m2), and divided by a thickness (in metres, m) the thermal conductivity
predicts the power loss (in watts, W) through a piece of material.
- Wall assembly R-values are computed and expressed in the U.S. in (ft2•h•°F/Btu). Thermablok U -value expressed using this
measurement is 0.0971 (BTU-in/ft2•h•°F).
- Metric U-factors are defined as Watts per square meter per degree Celsius. To convert inch-pound Imperial U-values to metric U-factors,
multiply by 5.678. To convert metric U-factors to Imperial inch-pound U-factors, divide by 5.678.To convert Imperial inch-pound
R-values to metric R-values, multiply by 0.1761. One inch = 2.54 cms. One (RSI) U-factor coefficient = 5.678; one R-value = .1761 RSI.
- USA R Value X 0.176 = Metric R Value
Basic Thermal Definitions
Thermal conductivity is an intensive property of a material that measures its capacity to sustain heat flow. The units of thermal conductivity are often provided in metric values of Watts/meter-degree Kelvin or W/m-K. The symbol used to denote thermal conductivity is k (or lambda, λ).
- There are many different unit conversions to Imperial and others:
- 1 BTU/ft hr F = 1.73 W/m-K or 1730 mW/m-K
- 12 BTU-in/ft2 hr F = 1 BTU/ft hr F = 1.73 W/m-K
- 1 BTU-in/ft2 hr F = 0.144 W/m-K or 144 mW/m-K
- The reciprocal of thermal conductivity is thermal resistivity; an intensive property of a material to resist one-dimensional heat flow.
Thermal resistivity has units of m-K/W.
Thermal Resistance (R value)
- R is thermal resistance, which is the capacity of a material to impede heat flow over a given area and at a specific temperature.
The greater the value at a given set of conditions, the better the material will perform as a thermal insulator.
- The units of thermal resistance are provided in m2-K/W (or ft2 hr °F/BTU).
- Thickness/k value = R value.
- Resistances of insulators in series can be added.
- R-value can be for whole thickness or normalized (e.g. R-value is 12 per in or 24 for 2 inches of R-12 per inch material).
- R/in = 144/k (in mW/m-K) -> 12 mW/m-K = 12 per inch R value.
- The reciprocal of thermal resistance is thermal conductance (also known as the heat transfer coefficient), with units of W/m2 K.
This value is frequently measured in systems as the overall heat transfer coefficient (OHTC).
- Thermal resistance values are often used in the building and construction markets as a relative measure of thermal insulation
performance for materials and systems.
Heat Transfer Coefficient (U Value)
- U values measure thermal conductance, a measure of a material or system to permit power flow per unit area and temperature change (W/m2 K).
U-value is heat transfer coefficient.
Low U values mean good insulation values (very similar to thermal conductivity k).
OHTC -> Overall heat transfer coefficient; frequently used to measure the thermal conductance of systems.
In the world of systems thermal engineering – U value and OHTC are often used synonymously.