View source for R-value (insulation)

[[Image:Aerogel matches.jpg|right|thumb|[[Aerogel]] is an extremely efficient man-made insulator and has a very high R-value.]]
The '''R-value''' is a measure of [[thermal resistance]] <ref>[[Oak Ridge National Laboratory]], [http://www.ornl.gov/sci/roofs+walls/insulation/ins_02.html Which Kind Of Insulation Is Best?], retrieved 2008-07-13.</ref> 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, <math>\dot Q_A</math>) through it or <math> R = \Delta T/\dot Q_A</math>. The bigger the number, the better the [[building insulation]]'s effectiveness<ref>[[US Department of Energy]], [http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11340 The R-Value of Insulation], retrieved 2008-07-13.</ref>. R-value is the [[Multiplicative inverse|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 [[foot (length)|ft]]²·[[Fahrenheit|°F]]·[[hour|h]]/[[British thermal unit|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 {{Citation needed|reason=Needs a specific example or reference.  I can imagine how a very thin layer of fiberglass bats might not have the expected insulation value.  Seems like a detail that could be put elsewhere in the article.|date=January 2010}}.

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&nbsp;mm of thickness, up to 4.0 m²K/W for 220&nbsp;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.

== Units==

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.<ref>[http://knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=2554&VerticalID=0 2009 ASHRAE Handbook - Fundamentals (I-P Edition)]. (pp: 38.1).  American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc</ref>

To disambiguate between the two, some authors use the abbreviation "'''RSI'''" for the SI definition[http://books.google.com/books?id=0abt7CegTXYC&pg=PA81&lpg=PA81&dq=RSI+thermal+resistance&source=web&ots=_TBPR0kYja&sig=uKxOcQR7j9HVy5V4GyMMZb44jUg&hl=en&sa=X&oi=book_result&resnum=7&ct=result#PPP1,M1].

==Example (SI units) ==

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&ndash;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.

==Relationships==
===U-value===
The ''U-value'' (or ''U-factor''), more correctly called the [[Heat_transfer_coefficient#Overall_heat_transfer_coefficient|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<ref> [P2000 Insulation System [http://www.p2000insulation.com], [http://www.p2000insulation.ca/testing/pdf/R-value%20Testing.pdf R-value Testing]</ref> (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)

:: <math>U=\frac{1}{R}=\frac{\dot Q_A}{\Delta T}</math>

===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.

===Multiple layers===
In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added:<ref>http://www.ornl.gov/sci/roofs+walls/insulation/ins_02.html</ref>  

:R-value<sub>(outside air film)</sub> + R-value<sub>(brick)</sub> + R-value<sub>(sheathing)</sub> + R-value<sub>(insulation)</sub> + R-value<sub>(plasterboard)</sub> + R-value<sub>(inside air film)</sub> = R-value<sub>(total)</sub>. 

To account for other components in a wall such as framing, an area-weighted average R-value of the whole wall may be calculated.

== Controversy ==
{{Refimprove|section|date=July 2008}}
=== Thermal conductivity versus apparent thermal conductivity ===

[[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 <ref>[http://www.ftc.gov/bcp/rulemaking/rvalue/review.shtm R-Value Rule Review<!-- Bot generated title -->]</ref> illustrating the complexity of the issues.

=== 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 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 [[sol-air temperature|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.  As radiative heat transfer is related to the cube power of the absolute temperature, such transfer is then at its most significant when the objective is to cool (i.e. when solar radiation has produced very warm surfaces). On the other hand, the conductive and convective heat loss modes play a more significant role during the cooler months. At such lower ambient temperatures the traditional fibrous, plastic and cellulose insulations play by far the major role: the radiative heat transfer component is of far less importance and the main contribution of the radiation barrier is in its superior air-tightness contribution.
In summary - claims for radiant barrier insulation are justifiable at high temperatures such as minimizing summer heat transfer, but are not in traditional winter keeping warm conditions.

===The limitations of R-values in evaluating radiant barriers===

{{See|Radiant barrier}}
{{See|Cool roofs}}

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 [[Building insulation materials|bulk insulation]] materials.  All of the products quoted at the end are examples of these. 

Calculating the performance of [[radiant barrier]]s 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 <ref>http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm#Metals%20and%20Conversion%20Coatings</ref> are the appropriate metric for radiant barriers.  Their effectiveness when employed to resist solar radiation is established<ref>[http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1231-01-es/ FSEC-CR-1231-01-ES<!-- Bot generated title -->]</ref>,
even though R-value does not adequately describe them.

==Deterioration==
===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 [[Haloalkane|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;{{Citation needed|date=September 2009}} critics argue that a more fair assessment would be its settled value.{{Citation needed|date=September 2009}} The foam industry in '''year?''' adopted the '''LTTR''' (Long-Term Thermal Resistance) method,<ref>[http://www.professionalroofing.net/archives/past/feb02/polyisofeature.asp  "Thermal resistance and polyiso insulation"] by John Clinton, ''Professional Roofing'' magazine, February 2002</ref> 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.

===Infiltration===
Correct attention to [[weatherization]] and construction of [[vapour barrier]]s 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 (mechanical)|seal]] directly against the substrate to reduce this effect.

==Example values==
{{Globalize|date=January 2010}}
:''Note that these examples use the '''non-SI''' definition and/or given for a 1 inch (25.4 mm) thick sample.''
[[Vacuum insulated panel]]s have the highest R-value (approximately R&ndash;45 per inch in American customary units); [[aerogel]] has the next highest R-value (about R&ndash;10-30 per inch), followed by isocyanurate and phenolic foam insulations with, R&ndash;8.3 and R&ndash;7 per inch, respectively. They are followed closely by [[polyurethane]] and [[polystyrene]] insulation at roughly R&ndash;6 and R&ndash;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&ndash;-2.5 to R&ndash;-4 per inch. Straw bales perform at about R&ndash;1.5. However, typical [[straw bale]] houses have very thick walls and thus are well insulated.  Snow is roughly R&ndash;1.
Brick has a very bad insulative ability at a mere R&ndash;0.2, however it does have a good [[Thermal mass]].

=== Typical per-inch R-values for material ===
{{:Insulation (list of insulation material)}}

=== Typical R-values for a specified unit (not per inch) ===

==== Non-reflective surface R-values for air films<ref>[http://knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=2554&VerticalID=0 2009 ASHRAE Handbook - Fundamentals (I-P Edition & SI Edition)]. (pp: 26.1).  American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc</ref> ====
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="font-size: 85%; text-align: center; width: auto;"
|-
! style="width:20em" | Surface position
! Direction of heat flow
! R<sub>US</sub> (hr·ft²·°F/(Btu·in))
! R<sub>SI</sub> (m²·K/(W·in))
|-
! {{rh}} | Horizontal (e.g.: a flat ceiling)
| Upward (e.g.: winter)
| 0.61
| 0.11
|-
! {{rh}} | Horizontal (e.g.: a flat ceiling)
| Downward (e.g.: summer)
| 0.92
| 0.16
|-
! {{rh}} | Vertical (e.g.: a wall)
| Horizontal 
| 0.68
| 0.12
|-
! {{rh}} | Outdoor surface, any position, moving air 6.7 m/s (winter)
| Any direction 
| 0.17
| 0.030
|-
! {{rh}} | Outdoor surface, any position, moving air 3.4 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="font-size: 85%; text-align: center; width: auto;"
|-
! style="width:12em" | Material
! Value not per inch (Min)
! Value not per inch (Max)
! Reference
|-
! {{rh}} | [[Radiant barrier|Reflective insulation]]
| R-1 <ref>[http://www.rimainternational.org/pdf/ftc-letter.pdf FTC Letter], Regarding reflective insulation used under slab where no air space is present</ref> (For assembly without adjacent air space.) 
| R-10.7 (heat flow down), R-6.7 (heat flow horizontal), R-5 (heat flow up)  
Ask for the R-value tests from the manufacturer for your specific assembly.
| <ref name="DOE">DOE Handbook.[http://buildingsdatabook.eren.doe.gov/TableView.aspx?table=5.1.3 Link text]</ref><ref>[http://www.icc-es.org/reports/pdf_files/ICC-ES/ESR-1236.pdf ICC ES Report], ICC ES Report ESR-1236 Thermal and Moisture Protection - ICC Evaluation Services, Inc.</ref>
|}

===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.  "The Commission issued the R-Value Rule<ref>[http://www.ftc.gov/bcp/rulemaking/rvalue/16cfr460.shtm Labeling and Advertising of Home Insulation (R-Value Rule) 16 CFR 460 (Federal Trade Commission, USA)]</ref> 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.

====Thickness====
The R-value Rule specifies:<ref>http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=cb66a61ae5c0b2a136f438291a8f6cd3&rgn=div5&view=text&node=16:1.0.1.4.58&idno=16 <!-- See also 44 Fed Reg. at 50,224, 27 August 1979 --></ref> 

{| 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:
: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.
|}

==See also==
* [[Building insulation]]
* [[Building insulation materials]]
* [[Cool roofs]]
* [[Installing building insulation]]
* [[Superinsulation]]
* [[Thermal bridge]]
* [[Condensation]]
* [[Passive solar design]]
* [[Sol-air temperature]]
* [[Heat transfer]]
* [[Thermal mass]]
* [[Thermal conductivity]]
* [[Thermal comfort]]

==References==
{{Reflist}}

==External links==
*[http://www.plastics.org.nz/page.asp?id=468 Information on the calculations, meanings, and inter-relationships of related heat transfer and resistance terms]
*[http://www.monolithic.com/stories/foam-chapter-04 "R" Fairy Tale: The Myth of Insulation Values]
*[http://www.coloradoenergy.org/procorner/stuff/r-values.htm American building material R-value table]
;Tables of R-values:
*[http://www.e-star.com/ecalcs/table_rvalues.html R-Values of Materials], E-Star: Energy Saving Calculations
*[http://coloradoenergy.org/procorner/stuff/r-values.htm R-Value Table], ColoradoENERGY.org
*[http://www.allwallsystem.com/design/RValueTable.html R-Value Table], All Wall Systems

[[Category:Building engineering]]
[[Category:Insulators]]
[[Category:Thermal protection]]
[[Category:Heat transfer]]

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