thumb|Installed faced [[Building insulation materials#Fiberglass batts and blankets (glass wool)|fiberglass batt insulation with its R-value visible (R-21)]]

The R-value is a measure of thermal resistance, specifically how well a two-dimensional barrier, such as a layer of insulation, a window, or a complete wall or ceiling, resists the conductive flow of heat, in the context of construction. The higher the R-value, the more insulating the material is. Higher R-values can reduce heating bills in cold weather and cooling bills in hot weather.

R-value can be expressed with in both metric and United States customary units. When expressed in metric, the term RSI-value is often used.

<math display=block>\frac{R_\text{val{A} = R,</math>

where (using SI units):

  • <math>R_\text{val}</math> is the R-value (m<sup>2</sup>⋅K⋅W<sup>−1</sup>)
  • <math>A</math> is the barrier's exposed surface area (m<sup>2</sup>)
  • <math>R</math> is the absolute thermal resistance (K⋅W<sup>−1</sup>)

Absolute thermal resistance, <math>R</math>, quantifies the temperature difference per unit of heat flow rate needed to sustain one unit of heat flow rate. Confusion sometimes arises because some publications use the term thermal resistance for the temperature difference per unit of heat flux, but other publications use the term thermal resistance for the temperature difference per unit of heat flow rate. Further confusion arises because some publications use the character R to denote the temperature difference per unit of heat flux, but other publications use the character R to denote the temperature difference per unit of heat flow rate. This article uses the term absolute thermal resistance for the temperature difference per unit of heat flow rate and uses the term R-value for the temperature difference per unit of heat flux.

The greater the R-value, the greater the resistance, and so the better the thermal insulating properties of the barrier. R-values are used in describing the effectiveness of insulating material and in analysis of heat flow across assemblies (such as walls, roofs, and windows) under steady-state conditions. The temperature difference divided by the R-value and then multiplied by the exposed surface area of the barrier gives the total rate of heat flow through the barrier, as measured in watts or in BTUs per hour.

<math display=block>\phi = \frac{\Delta T \sdot A}{R_\text{val,</math>

where (using SI units):

  • <math>R_\text{val}</math> is the R-value (K⋅m<sup>2</sup>/W),
  • <math>\Delta T</math> is the temperature difference (K) between the warmer surface and colder surface of the barrier,
  • <math>A</math> is the exposed surface area (m<sup>2</sup>) of the barrier,
  • <math>\phi</math> is the heat flow rate (W) through the barrier.

As long as the materials involved are dense solids in direct mutual contact, R-values are additive; for example, the total R-value of a barrier composed of several layers of material is the sum of the R-values of the individual

For example, in winter it might be 2&nbsp;°C outside and 20&nbsp;°C inside, making a temperature difference of 18&nbsp;°C or 18&nbsp;K. If the material has an R-value of 4, it will lose 0.25&nbsp;W/(°C⋅m<sup>2</sup>). With an area of 100&nbsp;m<sup>2</sup>, the heat energy being lost is There will be other losses through the floor, windows, ventilation slots, etc. But for that material alone, 450&nbsp;W is going out, and can be replaced with a 450&nbsp;W heater inside, to maintain the inside temperature.

Terminology and units

Note that the R-value is the building industry term It is sometimes denoted RSI-value if the SI (metric) units are used.

An R-value can be given for a material (e.g., for polyethylene foam), or for an assembly of materials (e.g., a wall or a window). In the case of materials, it is often expressed in terms of R-value per unit length (e.g., per inch of thickness). The latter can be misleading in the case of low-density building thermal insulations, for which R-values are not additive: their R-value per inch is not constant as the material gets thicker, but rather usually decreases. is about 5.68 times larger than when expressed in SI units, so that, for example, a window that is R-2 in I-P units has an RSI of 0.35 (since 2/5.68 = 0.35). For R-values there is no difference between US customary units and imperial units.

All of the following mean the same thing: "this is an R-2 window"; "this is an R2 .</math>

where <math>U_\text{val}</math> is the U-value (W⋅m<sup>−2</sup>⋅K<sup>−1</sup>).

The derived R-value and U-value may be accurate to the extent that the heat flux through the heat flux sensor equals the heat flux through the building element. Recording all of the available data allows one to study the dependence of the R-value and U-value on factors like the inside temperature, outside temperature, or position of the heat flux sensor. To the extent that all heat transfer processes (conduction, convection, and radiation) contribute to the measurements, the derived R-value represents an apparent R-value.

Sample values

Vacuum insulated panels have the highest R-value, approximately R-45 (in U.S. units) per inch; aerogel has the next highest R-value (about R-10 to R-30 per inch), followed by polyurethane (PUR) and phenolic foam insulations with R-7 per inch. They are followed closely by polyisocyanurate (PIR) at R-5.8, graphite impregnated expanded polystyrene at R-5, and expanded polystyrene (EPS) at R-4 per inch. Loose cellulose, fibreglass (both blown and in batts), and rock wool (both blown and in batts) all possess an R-value of roughly R-2.5 to R-4 per inch.

Straw bales perform at about R-2.38 to 2.68 per inch, depending on orientation of the bales. compared to R-3.5/m for a fiberglass blanket.]]

Typical R-values

Typical R-values for surfaces

Non-reflective surface R-values for air films

When determining the overall thermal resistance of a building assembly such as a wall or roof, the insulating effect of the surface air film is added to the thermal resistance of the other materials.

{| class="wikitable sortable" style="width:auto; font-size:85%; text-align:center;"

|-

! style="width:20em;" | Surface position

! Direction of heat transfer

! R<sub>U.S.</sub> (hr⋅ft<sup>2</sup>⋅°F/Btu)

! R<sub>SI</sub> (K⋅m<sup>2</sup>/W)

|-

! | Horizontal (e.g., a flat ceiling)

| Upward (e.g., winter)

| 0.61

| 0.11

|-

! | Horizontal (e.g., a flat ceiling)

| Downward (e.g., summer)

| 0.92

| 0.16

|-

! | Vertical (e.g., a wall)

| Horizontal

| 0.68

| 0.12

|-

! | Outdoor surface, any position, moving air 6.7&nbsp;m/s (winter)

| Any direction

| 0.17

| 0.030

|-

! | Outdoor surface, any position, moving air 3.4&nbsp;m/s (summer)

| Any direction

| 0.25

| 0.044

|}

In practice the above surface values are used for floors, ceilings, and walls in a building, but are not accurate for enclosed air cavities, such as between panes of glass. The effective thermal resistance of an enclosed air cavity is strongly influenced by radiative heat transfer and distance between the two surfaces. See insulated glazing for a comparison of R-values for windows, with some effective R-values that include an air cavity.

Radiant barriers

{| class="wikitable sortable" style="width:auto; font-size:85%; text-align:center;"

|-

! style="width:12em" | Material

! Apparent R-value (min.)

! Apparent R-value (max.)

! Reference

|-

! | Reflective insulation

| Zero (For assembly without adjacent air space.)

| R-10.7 (heat transfer down), R-6.7 (heat transfer horizontal), R-5 (heat transfer up)

Ask for the R-value tests from the manufacturer for your specific assembly.

|

|}

R-value rule in the U.S.

The Federal Trade Commission (FTC) governs claims about R-values to protect consumers against deceptive and misleading advertising claims. It issued the R-value rule.

The primary purpose of the rule is to ensure that the home insulation marketplace provides this essential pre-purchase information to the consumer. The information gives consumers an opportunity to compare relative insulating efficiencies, to select the product with the greatest efficiency and potential for energy savings, to make a cost-effective purchase and to consider the main variables limiting insulation effectiveness and realization of claimed energy savings.

The rule mandates that specific R-value information for home insulation products be disclosed in certain ads and at the point of sale. The purpose of the R-value disclosure requirement for advertising is to prevent consumers from being misled by certain claims which have a bearing on insulating value. At the point of transaction, some consumers will be able to get the requisite R-value information from the label on the insulation package. However, since the evidence shows that packages are often unavailable for inspection prior to purchase, no labeled information would be available to consumers in many instances. As a result, the Rule requires that a fact sheet be available to consumers for inspection before they make their purchase.

Thickness

The R-value Rule specifies:

{| class="wikitable"

|-

| In labels, fact sheets, ads, or other promotional materials, do not give the R-value for one inch or the "R-value per inch" of your product. There are two exceptions:

You can list a range of R-value per inch. If you do, you must say exactly how much the R-value drops with greater thickness. You must also add this statement: "The R-value per inch of this insulation varies with thickness. The thicker the insulation, the lower the R-value per inch."

|}

See also

  • Building insulation
  • Building insulation materials
  • Condensation
  • Cool roofs
  • Heat transfer
  • Passivhaus
  • Passive solar design
  • Sol-air temperature
  • Superinsulation
  • Thermal bridge
  • Thermal comfort
  • Thermal conductivity
  • Thermal mass
  • Thermal transmittance
  • Tog (unit)

References

  • Table of Insulation R-values at InspectApedia includes original source citations
  • Information on the calculations, meanings, and inter-relationships of related heat transfer and resistance terms
  • American building material R-value table
  • Working with R-values

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