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R-value is a term predominantly used in the building industry to describe the insulation properties of certain building insulation materials. Its use is limited to situations where thermal insulation is achieved by retarding the flow of heat through the material itself rather than reflecting radiant heat away. The higher the R-value, the greater insulation. Although they offer a useful means of comparing the performance of different products, other factors need to be considered in maintaining thermal comfort (see building insulation).
R-value is a measure of apparent thermal conductivity, and thus describes the rate that heat energy is transferred through a material or assembly, regardless of its original source. (see below).
Units of measurement and relationship to other parameters
The imperial unit for R-value is ft²·°F·h/Btu. The conversion factor is 1 ft²·°F·h/Btu ≈ 0.1761 K·m²/W, or 1 K·m²/W ≈ 5.67446 ft²·°F·h/Btu.
Sometimes the nomenclature RSI is used to denote the SI form of the value. In contrast, the imperial unit is often written as R–## where the ## is the R-value. To complicate matters, some countries that employ the SI system (e.g. New Zealand) retain the R (in lieu of RSI) but incorporate a dash e.g. R–5.53. One tenth of an RSI is called a tog.
The relationship between U-factor or R-value and thickness is not always exactly linear and therefore its value cannot be precisely extrapolated for a material of different thickness, but assuming a linear relationship may be adequate.
R-value should also 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.
The U-factor (or U-value) describes how well a building material conducts heat. Methodologically, it measures the rate of heat transfer through a material of known thickness over a given area under standard conditions. The usual standard is at a temperature gradient of 24oC at 50% humidity in no wind conditions.
U is the inverse of R i.e. U = 1/R and the SI unit for U is W/(K·m²).
For example, if the interior of your home is at 20 °C, and the roof cavity is at 10 °C, the temperature difference is 10 K. Assuming a ceiling insulated to R–2, energy will be lost at a rate of 10 K / 2 K·m²/W = 5 watts for every square metre of ceiling.
It is reasonable to sum the R-values of bulk insulators e.g., R-value(brick) + R-value(fibreglass batt) + R-value(plasterboard) = R value(total).
R-value unit and relationship to thermal conductivity
The concept of thermal conductivity originally only referred to thermal conduction that occurs through a material. That is, for a layer of material of known area and thickness, the rate of energy transferred can be calculated based on the temperature differential on each side.
The definition of R-value based on apparent thermal conductivity can be found 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.
There are weaknesses to using a single laboratory model to simultaneously assess the properties of a material to resist conducted, radiated or convective heating. The rate of heat transfer through a material is dependent on different factors depending on the mode of transfer. For conductive transfer, the thermal conductance matters, and the result may be calculated by the formula above. Convective transfer may depend on the velocity of gas or fluid flow (amongst other variables), or, in the case of natural convection it may exhibit a nonlinear dependence on temperature difference. Radiative transfer depends on thermal emissivity and depends nonlinearly on temperatures. The extensive debate among representatives from different segments of the US insulation industry during revision of the US FTC's regulations about advertising R-values  illustrates the complexity of the issues.
Experimentally thermal conduction is measured by placing the material in contact between two conducting plates and measuring the energy fluxes required to maintain a certain temperature gradient. However, when calculating the response to radiant heat only the energy absorbed by the surface can be considered. This is dependent on the surface emissivity of the material. 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 and conduction even though the total energy contributions remains the same.
Similarly, as heat energy emerges from the opposite side, the amount of radiated energy (in addition to conducted energy) will also be dependent on the emissivity of the material based on its surface temperature.
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
For example - at one extreme are radiant heat barriers (e.g. foil). Whilst they are able to resist high levels of thermal radiation due to their high reflectivity (and poor emissivity) they are unable to resist a fraction of this energy if it is transmitted by conduction. Other insulation barriers also vary in the thermal energy they can transmit depending on whether it be from radiation or conduction.
From the above discussion, it can be seen the actual performance of the product needs to be understood in the context of the experimental conditions in which it was derived. For instance, a bulk insulator exposed to air-infiltration would appear to under-perform, if was originally tested in a situation of no air-flow. Similarly, it would be inappropriate to evaluate the effectiveness of RFL in the same way as fibre-glass batts.
The limitations of R-values in evaluating radiant barriers
see Radiant barrier see Cool roofs
Radiant barriers operate by reflecting radiant energy away rather than retarding the flow of heat. In fact, materials such as reflective foil have a high thermal conductivity and would function poorly as a conductive insulator.
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 for which the apparent thermal conductivity is a sufficient model. 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 properly applied is established, even though a single R-value does not adequately describe them.
Surface temperature is not synonymous with thermal comfort
R-values are derived from the temperature differential between the surfaces of an insulating material. However, the perception of thermal comfort is a complex interplay of the net heat transfer between a human body and his surroundings. Both radiant sources and ambient air temperature contribute to this process. The heat energy that emanates from an object is a combination of the degree of radiant heat that is being emitted and the amount of conducted heat into the air. Even during warm weather, being in close proximity to a stainless-steel teapot (with high surface temperature but low emissivity) may feel comfortable. But lying on a warm mattress (with modest surface temperature and low conductivity) during summer may not.
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 a polyurethane and polyisocyanurate are blown with heavy gases such as chlorofluorocarbons or hydrochlorofluorocarbons (CFCs). However, over time 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 isocyanate). 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, while a more fair assessment would be its settled value. The foam industry has now adopted the LTTR method which rates the R-value based on a 15 year weighted average. While more realistic, the LTTR effectively provides only 8 year aged R-value, short in the scale of a building which may have a lifespan of 50-100 years.
Insulation and air-infiltration
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 an air-seal against the substrate to reduce this effect.
Note that these examples use the non-SI definition and are per inch. Vacuum insulated panel has the highest R-value of (approximately 45 in English units) for flat, aerogel has the next highest R-value 10, followed by isocyanurate and phenolic foam insulations with, 8.3 and 7, respectively. They are followed closely by polyurethane and polystyrene insulation at roughly R–6 and R–5. Loose cellulose, fiberglass both blown and in batts, and rock wool both blown and in batts all possess an R-value of roughly 3. Straw bales perform at about R–3. However, a typical straw bale house have walls 18 inches thick providing an effective R–54. Snow is roughly R–1.
Absolutely still air has an R-value of about 5 but this has little practical use: Spaces of one centimeter or greater will allow air to circulate, convecting heat and greatly reducing the insulating value to roughly R–1.
Typical R-values per inch of thickness
The Federal Trade Commission (FTC)'s R-value Rule generally prohibits calculating R-value per inch of thickness. (16 C.F.R. 460.20.) The FTC explained the reason for this prohibition: Since the record demonstrates that R-values are not linear, advertisements, labels, and other promotional materials that express a product's thermal resistance in terms of R-value per inch deceive customers. The FTC further explained that references to the R-value for a one-inch thickness of the material will encourage consumers to think that it is appropriate to multiply this figure by the desired number of inches, as though R-value per inch were constant. (44 Fed Reg. at 50,224 (27 August 1979).)
All values are approximations, based on the average of the values listed on dozens of websites. If I saw wildly different values, then I took the lowest and highest values and expressed the R-value here as a range somewhere between them. For a more-official list, refer to one of these websites with duplicate R-value tables:   
Furthermore, comparisons per inch of thickness are mostly relevant for conductive and convective heat transfer -- not radiant heat transfer -- but some of the materials listed below are designed to prevent radiant heat transfer.
List of examples
Values per inch
Values for a specified unit (not per inch)
Materials such as natural rock, dirt, sod, adobe, and concrete have poor thermal resistance (R-value typically less than 1), but work well for thermal mass applications because of their high specific heat.
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "R-value_(insulation)". A list of authors is available in Wikipedia.|