R-value (insulation) - AbsoluteAstronomy.com

The R-value is a measure of thermal resistance used in the building and construction

Construction

In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...

industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux

Heat flux

Heat flux or thermal flux is the rate of heat energy transfer through a given surface. The SI derived unit of heat rate is joule per second, or watt. Heat flux is the heat rate per unit area. In SI units, heat flux is measured in W/m2]. Heat rate is a scalar quantity, while heat flux is a vectorial...

(heat transfer per unit area,

) through it or

.The R-value being discussed is the unit thermal resistance. This is used for a unit value of any particular material. It is expressed as the thickness of the material divided by the thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

. For the thermal resistance of an entire section of material, instead of the unit resistance, divide the unit thermal resistance by the area of the material. For example, if you have the unit thermal resistance of a wall, divide by the cross-sectional area of the depth of the wall to compute the thermal resistance. The unit thermal conductance of a material is denoted as C and is the reciprocal of the unit thermal resistance. This can also be called the unit surface conductance and denoted by h. The bigger the number, the better the building insulation

Building insulation

building insulation refers broadly to any object in a building used as insulation for any purpose. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation...

's effectiveness. R-value is the reciprocal

Multiplicative inverse

In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...

of U-value.

Around most of the world, R-values are given in SI

Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

units, typically square-metre kelvins per watt or m²·K/W (or equivalently to m²·°C/W). In the United States customary units

United States customary units

United States customary units are a system of measurements commonly used in the United States. Many U.S. units are virtually identical to their imperial counterparts, but the U.S. customary system developed from English units used in the British Empire before the system of imperial units was...

, R-values are given in units of ft²·°F

Fahrenheit

Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...

·h

Hour

The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

/Btu

British thermal unit

The British thermal unit is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F...

. 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. United States R-values are approximately six times SI R-values http://google.com/search?q=(1+feet)^2+*+0.555555555+Kelvin+*+1+hour+%2F+(1+BTU)+in+m^2+Kelvin+per+Watt.

Heat transfer through an insulating layer is analogous to electrical resistance. The heat transfers 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 transfer and double the R-value; quadruple, quarters; etc. In practice, this linear relationship may be only approximate for some materials .

The US Department of Energy has recommended R-values for given areas of the USA based on the general local energy costs for heating and cooling, as well as the climate of an area. There are four types of insulation: Rolls and batts, Loose-fill, Rigid foam, and Foam-in-place. Rolls and batts are typically flexible insulators that come in fibers, like fiberglass. Loose-fill insulation comes in loose fibers or pellets and should be blown into a space. Rigid foam is more expensive than fiber, but generally has a higher R-value per unit of thickness. Foam-in-place can be blown into small areas to control air leaks, like those around windows.

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 conduction 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

Natural convection

Natural convection is a mechanism, or type of heat transport, in which the fluid motion is not generated by any external source but only by density differences in the fluid occurring due to temperature gradients. In natural convection, fluid surrounding a heat source receives heat, becomes less...

, 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.

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–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.

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

Humidity

Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

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);

where k is the material's thermal conductivity and L is its thickness.

See also: tog (unit)

Tog (unit)

The tog is a measure of thermal resistance of a unit area, also known as thermal insulance, commonly used in the textile industry, and often seen quoted on, for example, duvets and carpet underlay....

or Thermal Overall Grade (where 1 tog = 0.1 m² K / W), used for duvet rating.

Thickness

R-value should not be confused with the intrinsic property of thermal resistivity and its inverse, thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

. 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:

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.

Thermal conductivity versus apparent thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

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.

For the most part, testing the R-value of insulation is done at a steady temperature, usually about 70°F with no surrounding air movement. Since these are ideal conditions, the listed R-value for insulation could be higher than it really is, because most situations with insulation are under different conditions. The general rule for the general effectiveness of insulation is that one inch of insulation roughly equals 30 inches of concrete.

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

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

Emissivity

The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...

of the material. Light, reflective, or metallic surfaces that are 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

Sol-air temperature

Sol-air temperature is a variable used to calculate cooling load of a building and determine the total heat gain through exterior surfaces...

(and hence the 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

Thermal comfort

Thermal comfort is a term used by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, an international body. It is defined as the state of mind in humans that expresses satisfaction with the surrounding environment...

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, typically when minimizing summer heat transfer; but these claims are not justifiable in traditional winter (keeping-warm) conditions.

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 transfer 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

Building insulation materials

Building insulation materials are thermal insulation used in the construction or retrofit of buildings.The materials are used to reduce heat transfer by conduction, radiation or convection and are employed in varying combinations to achieve the desired outcome .-Categories:Insulation may be...

materials. All of the products quoted at the end are examples of these.

Calculating the performance of radiant barrier

Radiant barrier

Radiant barriers or reflective barriers inhibit heat transfer by thermal radiation. Thermal energy may also be transferred via conduction or convection, however, and radiant barriers do not necessarily protect against heat transfer via conduction or convection....

s is more complex. With a good radiant barrier in place, most heat flow is by convection, which depends on many factors other than the radiant barrier itself. Although radiant barriers have high reflectivity

Reflectivity

In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...

(and low emissivity

Emissivity

) over a range of electromagnetic spectra (including visible and UV light), their thermal advantages are mainly related to their emissivity in the infra-red range. Emissivity values are the appropriate metric for radiant barriers. Their effectiveness when employed to resist heat gain in limited applications is established,
even though R-value does not adequately describe them.

Insulation aging

R-values of products may deteriorate over time. For instance the compaction of loose fill cellulose creates voids that reduce overall performance; this may be avoided by densely packing the initial installation. Some types of foam

Foam

-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...

insulation, such as polyurethane and polyisocyanurate are blown with heavy gases such as chlorofluorocarbons

Haloalkane

The haloalkanes are a group of chemical compounds derived from alkanes containing one or more halogens. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially and, consequently, are known under many chemical and...

(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 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.
There has been a test method conceived to test the flammability of thermal/acoustic insulation. This type of insulation usually contains a thin film of moisture barrier over a batting material, with the possibility of foam being a second barrier. The test also takes into account small detail parts of the insulation which might contribute to whether or not the insulation is flammable. Such details include thread, tape, and fasteners. The test consists of putting the insulation next to an ignition source, then observing whether or not it catches fire. Then, if the specimen has caught fire, the ignition source is removed and the insulation is observed to see if it continues to burn.

Infiltration

Correct attention to weatherization

Weatherization

Weatherization or weatherproofing is the practice of protecting a building and its interior from the elements, particularly from sunlight, precipitation, and wind, and of modifying a building to reduce energy consumption and optimize energy efficiency.Weatherization is distinct from building...

and construction of vapor barrier

Vapor barrier

A vapor barrier is often used to refer to any material for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through wall, ceiling and floor assemblies of buildings and of packaging...

s are important for the optimal function of bulk insulators. Air infiltration can allow convective heat transfer 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

Seal (mechanical)

A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage , containing pressure, or excluding contamination...

directly against the substrate to reduce this effect.

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

A vacuum insulated panel is a form of thermal insulation consisting of a nearly gas-tight enclosure surrounding a rigid core, from which the air has been evacuated...

s have the highest R-value (approximately R–45 per inch in American customary units); aerogel

Aerogel

Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...

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

Polyurethane

A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...

and polystyrene

Polystyrene

Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

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

Thermal mass

Thermal mass is a concept in building design which describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect...

Typical per-unit-thickness R-values for material

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.

Surface position	Direction of heat transfer	R_US (hr·ft²·°F/Btu)	R_SI (K·m²/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 m/s (winter)	Any direction	0.17	0.030
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

Insulated glazing

Insulated glazing also known as double glazing are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope....

for a comparison of R-values for windows, with some effective R-values that include an air cavity.

Radiant barriers

Material	Value (Min)	Value (Max)	Reference
Reflective insulation Radiant barrier Radiant barriers or reflective barriers inhibit heat transfer by thermal radiation. Thermal energy may also be transferred via conduction or convection, however, and radiant barriers do not necessarily protect against heat transfer via conduction or convection....	R-1 (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

Federal Trade Commission

The Federal Trade Commission is an independent agency of the United States government, established in 1914 by the Federal Trade Commission Act...

(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.

Thickness

The R-value Rule specifies:

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."

External links

Tables of R-values:
Calculations

R-Value Table, ColoradoENERGY.org (US Imperial units)

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.