In by Jackie Dunn

A measure of air-to-heat transmission (loss or gain) through a window due to the thermal conductance and the difference in indoor and outdoor temperatures. As the U-value decreases, so does the amount of heat that is transferred through the glazing material. The lower the U-value, the more restrictive the fenestration product is to heat transfer. Reciprocal of R-value.

Merriam-Webster Online Dictionary
U-value (noun)
a measure of the heat transmission through a building part (as a wall or window) or a given thickness of a material (as insulation) with lower numbers indicating better insulating properties - compare r-value
U-value (Wikipedia)
Aerogel is an extremely good thermal insulator, which at a pressure of one-tenth of an atmosphere has an R-value of R-20 per inch of thickness, compared to R-3.5/inch for a fiberglass blanket.
Installed faced fiberglass batt insulation with its R-value visible (R-21)

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. ) between two sides of the object, and the R-value quantifies how effectively the object resists this drive: 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 example, 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 . 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 may not account for radiative or convective processes at the material's surface, which may be an important factor for some applications.[citation needed]

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.

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