User:Reuqr/R-value (insulation)





In building and construction, the R-value is a measure of how well an object, per unit of its exposed area, resists conductive flow of heat: the greater the R-value, the greater the resistance, and so the better the thermal insulating properties of the object. R-values are used in describing effectiveness of insulation and in analysis of heat flow across assemblies (such as walls, roofs, and windows) under steady-state conditions. Heat flow through an object is driven by temperature difference (e.g. $$T_{2}-T_{1}$$) between two sides of the object, and the R-value quantifies how effectively the object resists this drive: $$T_{2}-T_{1}$$ divided by the R-value and then multiplied by the surface area of the object's side gives the total rate of heat flow through the object (as measured in Watts or in BTUs per hour). Moreover, as long as the materials involved are dense solids in direct mutual contact, R-values are additive; for examle, the total R-value of an object composed of several layers of material is the sum of the R-values of the individual layers. Note that the R-value is the building industry term for what is in other contexts called ″thermal resistance per unit area.″ It is sometimes denoted RSI-value if the SI (metric) units are used.

An R-value can be given for a material (e.g. for polyethylene foam), or for an assembly of materials (e.g. a wall or a window). In the case of materials, it is often expressed in terms of R-value per unit length (e.g. per inch of thickness). The latter can be misleading in the case of low-density building thermal insulations, for which R-values are not additive: their R-value per inch is not constant as the material gets thicker, but rather usually decreases.

The units of an R-value (see below) are usually not explicitly stated, and so it is important to decide from context which units are being used: an R-value expressed in I-P (inch-pound) units is about 5.68 times larger than when expressed in SI units, so that, for example, a window that is R-2 in I-P units has an RSI of 0.35 (since 2/5.68=0.35). For R-values there is no difference between US customary units and imperial units. As far as how R-values are reported, all of the following mean the same thing: ″this is an R-2 window″; ″this is an R2 window″; ″this window has an R-value of 2″; ″this is a window with R=2″ (and similarly with RSI-values, which also include the possibility ″this window provides RSI 0.35 of resistance to heat flow″ ).

The more a material is intrinsically able to conduct heat, as given by its thermal conductivity, the lower its R-value. On the other hand, the thicker the material, the higher its R-value. Sometimes heat transfer processes other than conduction (namely, convection and radiation) significantly contribute to heat transfer within the material. In such cases, it is useful to introduce an ″apparent thermal conductivity″, which captures the effects of all three kinds of processes, and to define the R-value in general as $$R=\frac{\scriptstyle\text{thickness of the specimen}}{\scriptstyle\text{apparent thermal conductivity}}$$. This comes at a price, however: R-values that include non-conductive processes may no longer be additive and may have significant temperature dependence. In particular, for a loose or porous material, the R-value per inch generally depends on the thickness, almost always so that it decreases with increasing thickness (polyisocyanurate (″polyso″) being an exception; its R-value/inch increases with thickness ). For similar reasons, the R-value per inch also depends on the temperature of the material, usually increasing with decreasing temperature (polyso again being an exception); a nominally R-13 fiberglass batt may be R-14 at -12° C (10° F) and R-12 at +43° C (110° F). Nevertheless, in construction it is common to treat R-values as independent of temperature. Note that an R-value never accounts for radiative or convective processes at the material's surface, which may be an important factor for some applications.

The R-value is the reciprocal of the thermal transmittance (U-factor) of a material or assembly. The U.S. construction industry preferes to use R-values, however, because they are additive and because bigger values mean better insulation, neither of which is true for U-factors.

U-factor/U-value
The U-factor or U-value is the overall heat transfer coefficient that describes how well a building element conducts heat or the rate of transfer of heat (in watts) through one square metre of a structure divided by the difference in temperature across the structure. The elements are commonly assemblies of many layers of components such as those that make up walls/floors/roofs etc. It measures the rate of heat transfer through a building element over a given area under standardised conditions. The usual standard is at a temperature gradient of 24 °C (75.2 °F), at 50% humidity with no wind (a smaller U-factor is better at reducing heat transfer). It is expressed in watts per meter squared kelvin (W/m²K). This means that the higher the U-value the worse the thermal performance of the building envelope. A low U-value usually indicates high levels of insulation. They are useful as it is a way of predicting the composite behavior of an entire building element rather than relying on the properties of individual materials.

In most countries the properties of specific materials (such as insulation) are indicated by the thermal conductivity, sometimes called a k-value or lambda-value (lowercase λ). The thermal conductivity (k-value) is the ability of a material to conduct heat; hence, the lower the k-value, the better the material is for insulation. Expanded polystyrene (EPS) has a k-value of around 0.033 W/mK. For comparison, phenolic foam insulation has a k-value of around 0.018 W/mK, while wood varies anywhere from 0.15 to 0.75 W/mK, and steel has a k-value of approximately 50.0 W/mK. These figures vary from product to product, so the UK and EU have established a 90/90 standard which means that 90% of the product will conform to the stated k-value with a 90% confidence level so long as the figure quoted is stated as the 90/90 lambda-value.

U is the inverse of R with SI units of W/(m2K) and U.S. units of BTU/(hr °F ft2);


 * $$U=\frac{1}{R}=\frac{\dot Q_A}{\Delta T}=\frac{k}{L}$$

where $$\dot Q_A$$ is the heat flux, $$\Delta T$$ is the tempreture difference across the material, k is the material's coefficient of thermal conductivity and L is its thickness. In some contexts, U is referred to as unit surface conductance.

See also: tog (unit) or Thermal Overall Grade (where 1 tog = 0.1 m2&middot;K/W), used for duvet rating.

Note that the term "U-factor" (which redirects here) is usually used in the U.S. and Canada to express the heat flow through entire assemblies (such as roofs, walls, and windows ). For example, energy codes such as ASHRAE 90.1 and the IECC prescribe U-values. However, R-value is widely used in practice to describe the thermal resistance of insulation products, layers, and most other parts of the building enclosure (walls, floors, roofs). Other areas of the world more commonly use U-value/U-factor for elements of the entire building enclosure including windows, doors, walls, roof, and ground slabs.

Units: metric (SI) vs. inch-pound (I-P)
The SI (metric) unit of R-value is
 * square-metre kelvin per watt (m2·K/W or, equally, m2·°C/W),

whereas the I-P (inch-pound) unit is
 * square-foot·degree Fahrenheit·hour/British thermal unit (ft2·°F·h/BTU).

For R-values there is no difference between US customary units and imperial units, so the same I-P unit is used in both.

Some sources use ″RSI″ when referring to R-values in SI units.

R-values expressed in I-P units are approximately 5.68 times as large as R-values expressed in SI units. For example, a window that is R-2 in the I-P system is about RSI 0.35, since 2/5.68 &#8776; 0.35.

In countries where the SI system is generally in use, the R-values will also normally be given in SI units. This includes the U.K., Australia, and New Zealand.

I-P values are commonly given in the U.S. and Canada, though in Canada normally both I-P and RSI values are listed.

Because the units are usually not explicitly stated, one must decide from context which units are being used. In this regard, it helps to keep in mind that I-P R-values are 5.68 times larger than the corresponding SI R-values.

More percisely,


 * R-value (in I-P) = RSI-value (in SI) &times; 5.678263337
 * RSI-value (in SI) = R-value (in I-P) &times; 0.1761101838