R-value (insulation)

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Aerogel is an extremely good man-made insulator and has a very high R-value.

The R value or R-value is a measure of thermal resistance [1] used in the building and construction industry. The bigger the number, the better the building insulation's effectiveness[2]. R value is the reciprocal of U-value.

Increasing the thickness of an insulating layer increases the R value. For example, each inch of glass wool batting thickness adds approximately 3.5 (ft²·°F·h/Btu) to its R value. 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. Packing two layers of R-19 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 heat loss through such materials as glass windows and studs not to mention 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 retards heat loss by all three heat transfer modes: conduction, convection, and radiation. The primary means of heat loss across an uninsulated space is by 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. Radiation heat transfer is minimized 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 special closed-pore foam insulation.


[edit] Units

The world-wide definition of R-value is kelvin square meters per watt (K·m²/W), using the SI system.

American customary units, used in the United States, measure R-value in degrees Fahrenheit, square feet hours per Btu, (ft²·°F·h/Btu). This is commonly written in the form R–## (eg. R–19). The conversion is 1 ft²·°F·h/Btu ≈ 0.1761 K·m²/W, or 1 K·m²/W ≈ 5.678 ft²·°F·h/Btu.[citation needed]

To disambiguate between the two, some authors use the abbreviation "RSI" for the SI definition[1].

[edit] Relationships

[edit] U-value

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 °C, at 50% humidity with no wind[3] (a smaller U-value is better).

U is the inverse of R with SI units of W/(m²K).


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 [4] e.g., R-value(brick) + R-value(fibreglass batt) + R-value(plasterboard) = R value(total).

[edit] Thickness

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.

[edit] Controversy

[edit] Thermal conductivity versus apparent thermal conductivity

Thermal conductivity is conventionally defined as the rate of thermal conduction that occurs through a material. That is, for a layer of material of known area and thickness, the rate of thermal energy transferred can be calculated based on the surface temperature differential between sides. It is not specifically related to the difference in air temperature or heating energy.

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.

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 [5] illustrating the complexity of the issues.

[edit] Surface temperature in relationship to mode of heat transfer

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 sides.

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.

[edit] The limitations of R-values in evaluating radiant barriers

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 [6] are the appropriate metric for radiant barriers. Their effectiveness when employed to resist solar radiation is established[7], even though R-value do not adequately describe them.

[edit] Deterioration

[edit] Insulation aging

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, 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.

[edit] 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 a water-tight and air-tight seal directly against the substrate to reduce this effect.

[edit] Example values

Note that these examples use the non-SI definition and/or given for a 1 inch (25.4 mm) thick sample.

Vacuum insulated panel has the highest R-value of (approximately ~45 in American customary 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, typical straw bale houses 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.

[edit] Typical R-values thickness

All values are approximations, based on the average of available results.

[edit] List of examples

Values per inch. The R-values are given in imperial units (ft²·°F·h/Btu).; the numbers in parentheses are their SI equivalents.

Material Value per inch (Min) Value per inch (Max) Reference
Still Air R-5 (0.88)
Still Air with convective currents R-1 (0.18) (or less) R-5 (0.88) (Still)
Wood chips and other loose-fill wood products R-1 (0.18)
Snow R-1 (0.18)
Straw bale R-1.45 (0.26) [2]
Wood panels, such as sheathing R-2.5 (0.44)
Vermiculite loose-fill R-2.13 (0.38) R-2.4 (0.42)
Perlite loose-fill R-2.7 (0.48)
Rock and slag wool loose-fill R-2.5 (0.44) R-3.7 (0.65) [8]
Rock and slag wool batts R-3 (0.52) R-3.85 (0.68)
Fiberglass loose-fill R-2.5 (0.44) R-3.7 (0.65) [8]
Fiberglass rigid panel R-2.5 (0.44)
Fiberglass batts R-3.1 (0.55) R-4.3 (0.76)
High-density fiberglass batts R-3.6 (0.63) R-5 (0.88)
Cementitious foam R-2 (0.35) R-3.9 (0.69)
Cellulose loose-fill R-3 (0.52) R-3.8 (0.67) [9]
Cellulose wet-spray R-3 (0.52) R-3.8 (0.67) [9]
Cotton batts (Blue Jean Insulation) R-3.7 (0.65) [10]
Icynene spray R-3.6 (0.63) [11]
Icynene loose-fill (pour fill) R-4 (0.70) [11]
Urea-formaldehyde foam R-4 (0.70) R-4.6 (0.81)
Urea-formaldehyde panels R-5 (0.88) R-6 (1.06)
Polyethylene foam R-3 (0.52)
Phenolic spray foam R-4.8 (0.85) R-7 (1.23)
Phenolic rigid panel R-4 (0.70) R-5 (0.88)
Molded expanded polystyrene (EPS) low-density R-3.7 (0.65)
Molded expanded polystyrene (EPS) high-density R-4 (0.70)
Extruded expanded polystyrene (XPS) low-density R-3.6 (0.63) R-4.7 (0.82)
Extruded expanded polystyrene (XPS) high-density R-5 (0.88) R-5.4 (0.95)
Open-cell polyurethane spray foam R-3.6 (0.63)
Closed-cell polyurethane spray foam R-5.5 (0.97) R-6.5 (1.14)
Polyurethane rigid panel (Pentane expanded) initial R-6.8 (1.20)
Polyurethane rigid panel (Pentane expanded) aged 5-10 years R-5.5 (0.97)
Polyurethane rigid panel (CFC/HCFC expanded) initial R-7 (1.23) R-8 (1.41)
Polyurethane rigid panel (CFC/HCFC expanded) aged 5-10 years R-6.25 (1.10)
Polyisocyanurate spray foam R-4.3 (0.76) R-8.3 (1.46)
Foil-faced polyisocyanurate rigid panel (Pentane expanded ) initial R-6.8 (1.20)
Foil-faced polyisocyanurate rigid panel (Pentane expanded) aged 5-10 years R-5.5 (0.97)
Silica aerogel R-10 (1.76)
Vacuum insulated panel R-30 (5.28) R-50 (8.80)
Cardboard R-3 (0.52) R-4 (0.70)
Thinsulate clothing insulation R-5.75 (1.01)

Values for a specified unit (not per inch)

Material Value not per inch (Min) Value not per inch (Max) Reference
Reflective insulation R-2 R-14 (dubious claim for a specific complete assembly including a radiant barrier for one heat flow direction) [8] [12]
Single pane glass window R-1 (0.18)
Double pane glass window R-2 (0.35)
Double pane glass window with low emissivity coating R-3 (0.52)
Triple pane glass window R-3 (0.52)

Materials such as natural rock, dirt, sod, adobe, and concrete have poor thermal resistance (R-value typically less than R-1 (0.17)), but work well for thermal mass applications because of their high specific heat.

[edit] U.S. regulation

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[13] 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.

[edit] Thickness

The U.S. Federal Trade Commission's R-value Rule specifies:[14]

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:
a. You can do this if you suggest using your product at a one-inch thickness.
b. You can do this if actual test results prove that the R-values per inch of your product does not drop as it gets thicker.

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.

[edit] See also

[edit] References

  1. ^ Oak Ridge National Laboratory, Which Kind Of Insulation Is Best?, retrieved 2008-07-13.
  2. ^ US Department of Energy, The R-Value of Insulation, retrieved 2008-07-13.
  3. ^ P2000 Insulation System, R-value Testing
  4. ^ http://www.ornl.gov/sci/roofs+walls/insulation/ins_02.html: "In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added."
  5. ^ R-Value Rule Review
  6. ^ http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm#Metals%20and%20Conversion%20Coatings
  7. ^ FSEC-CR-1231-01-ES
  8. ^ a b c DOE Handbook.Link text
  9. ^ a b ICC Legacy Report ER-2833 - Cocoon Thermal and Sound Insulation Products, ICC Evaluation Services, Inc., http://www.icc-es.org
  10. ^ Environmental Home Center Cotton Batt Information
  11. ^ a b Icynene product information
  12. ^ Link text, ICC ES Report ESR-1362 Thermal and Moisture Protection - ICC Evaluation Services, Inc.
  13. ^ Labeling and Advertising of Home Insulation (R-Value Rule) 16 CFR 460 (Federal Trade Commission, USA)
  14. ^ http://www.ftc.gov/bcp/rulemaking/rvalue/16cfr460.shtm#20.

[edit] External links

Tables of R-values
Personal tools