The R value or R-value is a measure of thermal resistance  used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat flow per unit area, ) through it or . The bigger the number, the better the building insulation's effectiveness. R-value is the reciprocal of U-value.
Around most of the world, R-values are given in SI units, typically square-metre kelvins per watt or m²·K/W (or equivalently to m²·°C/W). In the United States customary units, R-values are given in units of ft²·°F·h/Btu. It is particularly easy to confuse SI and US R-values, because R-values both in the US and elsewhere are often cited without their units, e.g. R-3.5. Usually, however, the correct units can be inferred from the context and from the magnitudes of the values.
Heat transfer through an insulating layer is analogous to electrical resistance. The heat flows can be worked out by thinking of resistance in series with a fixed potential, except the resistances are thermal resistances and the potential is the difference in temperature from one side of the material to the other. The resistance of each material to heat transfer depends on the specific thermal resistance [R-value]/[unit thickness], which is a property of the material (see table below) and the thickness of that layer. A thermal barrier that is composed of several layers will have several thermal resistors in the analogous circuit, each in series. Like resistance in electrical circuits, increasing the physical length of a resistive element (graphite, for example) increases the resistance linearly; double the thickness of a layer means half the heat flow and double the R-value; quadruple, quarters; etc. In practice, this linear relationship may be only approximate some materials.
Increasing the thickness of an insulating layer increases the thermal resistance. For example, doubling the thickness of fibreglass batting will double its R-value, perhaps from 2.0 m²K/W for 110 mm of thickness, up to 4.0 m²K/W for 220 mm of thickness. Heat transfer through an insulating layer is analogous to adding resistance to a series circuit with a fixed voltage. However, this only holds approximately because the effective thermal conductivity of some insulating materials depends on thickness. The addition of materials to enclose the insulation such as sheetrock and siding provides additional but typically much smaller R-value.
There are many factors that come into play when using R-values to compute heat loss for a particular wall. Manufacturer R values apply only to properly installed insulation. Squashing two layers of batting into the thickness intended for one layer will increase but not double the R-value. Another important factor to consider is that studs and windows provide a parallel heat conduction path that is unaffected by the insulation's R-value. The practical implication of this is that one could double the R value used to insulate a home and realize substantially less than a 50% reduction in heat loss. Even perfect wall insulation only eliminates conduction through the insulation but leaves unaffected the conductive heat loss through such materials as glass windows and studs as well as heat losses from air exchange.
The R-value is a measure of insulation's heat loss retardation under specified test conditions. The primary mode of heat transfer impeded by insulation is convection but unavoidably it also impedes heat loss by all three heat transfer modes: conduction, convection, and radiation. The primary means of heat loss across an uninsulated air-filled space is natural convection, which occurs because of changes in air density with temperature. Insulation greatly retards natural convection. Most insulations trap air so that significant convective heat loss is eliminated leaving only conduction and radiation transfer. The primary role of such insulation is to make the thermal conductivity of the insulation that of trapped, stagnant air. However this cannot be realized fully because the glass wool or foam is needed to prevent convection and increases the heat conduction compared to still air. Radiative heat transfer is minimised by having many surfaces interrupting a "clear view" between the inner and outer surfaces of the insulation. Such multiple surfaces are abundant in batting and porous foam. Radiation is also minimized by low emissivity (highly reflective) surfaces. Lower thermal conductivity and, therefore, high R-values can be achieved by replacing air with argon when practical such as between sealed double-glazed windows and within special closed-pore foam insulation.
The conversion between SI and US units of R-value is 1 h·ft²·°F/Btu = 0.176110 K·m²/W, or 1 K·m²/W = 5.678263 h·ft²·°F/Btu.
To disambiguate between the two, some authors use the abbreviation "RSI" for the SI definition.
Example (SI units) Edit
To find the heat loss per square metre, simply divide the temperature difference by the R value.
If the interior of your home is at 20 °C, and the roof cavity is at 10 °C, the temperature difference is 10 °C (= 10 K). Assuming a ceiling insulated to R–2 (R = 2.0 m²K/W), energy will be lost at a rate of 10 K / 2 K·m²/W = 5 watts for every square metre of ceiling.
The U-value (or U-factor), more correctly called the overall heat transfer coefficient, describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions. The usual standard is at a temperature gradient of 24 oC, at 50% humidity with no wind (a smaller U-value is better).
U is the inverse of R with SI units of W/(m²K) and US units of BTU/(h °F ft²)
R-value should not be confused with the intrinsic property of thermal resistivity and its inverse, thermal conductivity. The SI unit of thermal resistivity is K·m/W. Thermal conductivity assumes that the heat transfer of the material is linearly related to its thickness.
In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added:
- R-value(outside air film) + R-value(brick) + R-value(sheathing) + R-value(insulation) + R-value(plasterboard) + R-value(inside air film) = R-value(total).
To account for other components in a wall such as framing, an area-weighted average R-value of the whole wall may be calculated.
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Thermal conductivity versus apparent thermal conductivity Edit
Thermal conductivity is conventionally defined as the rate of thermal conduction through a material per unit area per unit thickness per unit temperature differential (delta-T). The inverse of conductivity is resistivity (or R per unit thickness). Thermal conductance is the rate of heat flux through a unit area at the installed thickness and any given delta-T.
Experimentally, thermal conduction is measured by placing the material in contact between two conducting plates and measuring the energy flux required to maintain a certain temperature gradient.
A definition of R-value based on apparent thermal conductivity has been proposed in document C168 published by the American Society for Testing and Materials. This describes heat being transferred by all three mechanisms—conduction, radiation, and convection.
Debate remains among representatives from different segments of the U.S. insulation industry during revision of the U.S. FTC's regulations about advertising R-values  illustrating the complexity of the issues.
Surface temperature in relationship to mode of heat transfer Edit
There are weaknesses to using a single laboratory model to simultaneously assess the properties of a material to resist conducted, radiated or convective heating. Surface temperature varies depending on the mode of heat transfer.
In the absence of radiation or convection, the surface temperature of the insulator should equal the air temperature on each side.
In response to thermal radiation, surface temperature depends on the thermal emissivity of the material. Light, reflective or metallic surfaces exposed to radiation tend to maintain lower temperatures than dark, non-metallic ones
Convection will alter the rate of heat transfer (and surface temperature) of an insulator depending on the flow characteristics of the gas or fluid in contact with it.
With multiple modes of heat transfer, the final surface temperature (and hence observed energy flux and calculated R-value) will be dependent on the relative contributions of radiation, conduction and convection even though the total energy contribution remains the same.
This is an important consideration in building construction because heat energy arrives in different forms and proportions. The contribution of radiative and conductive heat sources also varies throughout the year and both are important contributors to thermal comfort
In the hot season, solar radiation predominates as the source of heat gain. On the other hand, conductive and convective heat losses play a more significant role during the cooler months.
The limitations of R-values in evaluating radiant barriersEdit
Unlike bulk insulators, radiant barriers resist conducted heat poorly. Materials such as reflective foil have a high thermal conductivity and would function poorly as a conductive insulator. Radiant barriers retard heat flow by two means - by reflecting radiant energy away from its surface or by reducing the emission of radiation from its opposite side.
The question of how to quantify performance of other systems such as radiant barriers has resulted in controversy and confusion in the building industry with the use of R-values or 'equivalent R-values' for products which have entirely different systems of inhibiting heat transfer. According to current standards, R-values are most reliably stated for bulk insulation materials. All of the products quoted at the end are examples of these.
Calculating the performance of radiant barriers is more complex. The tests and procedures to evaluate bulk insulators are not applicable to radiant barriers. Although radiant barriers have high reflectivity (and low emissivity) over a range of electromagnetic spectra (including visible and UV light), its thermal advantages are mainly related to its emissivity in the infra-red range. Emissivity values  are the appropriate metric for radiant barriers. Their effectiveness when employed to resist solar radiation is established, even though R-value does not adequately describe them.
R-values of products may deteriorate over time. For instance the compaction of loose cellulose fill reduces the volume of air spaces and its insulation value. Some types of foam insulation, such as polyurethane and polyisocyanurate are blown with heavy gases such as chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HFCs). However, over time a small amount of these gases diffuse out of the foam and are replaced by air, thus reducing the effective R-value of the product. There are other foams which do not change significantly with aging because they are blown with water or are open-cell and contain no trapped CFCs or HFCs (e.g. half-pound low density foams). On certain brands, twenty-year tests have shown no shrinkage or reduction in insulating value.
This has led to controversy as how to rate the insulation of these products. Many manufacturers will rate the R-value at the time of manufacture; critics argue that a more fair assessment would be its settled value. The foam industry in year? adopted the LTTR (Long-Term Thermal Resistance) method, which rates the R-value based on a 15 year weighted average. However, the LTTR effectively provides only an eight-year aged R-value, short in the scale of a building that may have a lifespan of 50 to 100 years.
Correct attention to weatherization and construction of vapour barriers are important for the optimal function of bulk insulators. Air infiltration can allow convective flow or condensation formation - both of which degrade the performance of the material.
One of the primary values of spray-foam insulation is its ability to create a water-tight and air-tight seal directly against the substrate to reduce this effect.
Vacuum insulated panels have the highest R-value (approximately R–45 per inch in American customary units); aerogel has the next highest R-value (about R–10-30 per inch), followed by isocyanurate and phenolic foam insulations with, R–8.3 and R–7 per inch, respectively. They are followed closely by polyurethane and polystyrene insulation at roughly R–6 and R–5 per inch. Loose cellulose, fiberglass (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–1.5. However, typical straw bale houses have very thick walls and thus are well insulated. Snow is roughly R–1. Brick has a very bad insulative ability at a mere R–0.2, however it does have a good Thermal mass.
Typical per-inch R-values for material Edit
Typical R-values for a specified unit (not per inch) Edit
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.
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 Edit
R-Value Rule in the U.S.Edit
The Federal Trade Commission (FTC) governs claims about R-values to protect consumers against deceptive and misleading advertising claims. "The Commission issued the R-Value Rule to prohibit, on an industry-wide basis, specific unfair or deceptive acts or practices." (70 Fed. Reg. at 31,259 (May 31, 2005).)
The primary purpose of the Rule, therefore, is to correct the failure of the home insulation marketplace to provide this essential pre-purchase information to the consumer. The information will give 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.
The R-value Rule specifies: