What is a U-value?

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U-values measure how effective an insulator is. When it comes to thermal performance, we look in detail at terminology and core concepts.

Although the main focus of the environmental performance of buildings is now on the use of carbon, the thermal performance of the building fabric still needs to be considered as a contributing factor. Thermal performance is measured heat loss and is expressed as U-value or R-value in the construction industry. U-value calculations will be required when designing construction strategies. Many terms have similar meanings, and conflicting interpretations can be found on the Internet. Various terminology and how they relate to each other are explained in this article.

U-value or thermal transmittance (reciprocal R-value)

Thermal transmittance, also known as U-value, is the frequency of heat transfer through a structure (which may be a single material or a composite material) divided by the difference in temperature across that structure. The measurement units are W/m2K. The better-insulated design, the lower the U-value. Workmanship and construction quality may have a significant impact on thermal transmission. If the insulation is designed with holes and cold bridges, the thermal transmittance can be higher than expected. Thermal transmittance takes into account the loss of energy due to conduction, convection and radiation.

How is U-value Calculated?

The basic calculation of the U-value is simple. The U-value can be determined by finding the reciprocal amount of each material’s thermal resistance that makes up the building component concerned. Remember that the internal and external faces, as well as the surface resistances, also have resistances to incorporate. These are fixed values.
A variety of requirements cover thermal transmittance measurement methods.
Simple U-value calculations can be made by considering layer-by-layer construction of the building component. Though, this does not compensate for cold bridging (e.g. by wall ties), air gaps around insulation, or the various thermal properties of e.g. mortar joints.

How do you continue to measure U-value?

Although design calculations are hypothetical, post-construction measurements are also possible. These have the advantage that they can compensate for workmanship. Heat flux metre can be used to measure thermal transmittance for roofs or walls. It consists of a thermopile sensor attached to the test area to control heat flow from inside to outside. Thermal transmission is derived from dividing average heat flux (flow) by average temperature difference (between inside and outside) over a continuous span of about 2 weeks (or over a year in the case of a ground floor slab due to heat storage in the soil).

U-value calculators

Because calculating U-values can be time-consuming and complex (especially where cold bridging needs to be accounted for, for example), many online U-value calculators were published. Many of these, though, are only available on subscription, and those free appear to be too simplistic. Another alternative is to request a u-value calculation estimate from Briary Energy.
Building Regulations Approved Documents L1A, L2A, L1B and L2B in England and Wales all apply to the publication BR 443 U-value calculation conventions for approved calculation methodologies, while the companion document U-value conventions in operation.

R-value, or thermal insulance (reciprocal of U-value)

Thermal insulance is the opposite of thermal transmittance; in other words, a material’s ability to resist heat flow. R-values are more used in some parts of the world (e.g. Australasia), as opposed to UK U-value choice. Thermal transmittance measurement units are m2K/W and, again, a higher figure suggests better performance (unlike the lower U-value figure).

k-value, or thermal conductivity (also known as lambda or λ value; reciprocal of thermal resistivity)

Thermal conductivity is a material’s ability to conduct heat. So, high thermal conductivity means a higher level of heat transfer across a material; note that this is also temperature-dependent. Thermal conductivity devices are W/m2k. Like U-values and R-values, yet, k-values don’t depend on the material’s thickness.

Y-value, or thermal admittance, or heat transfer coefficient

A material’s ability to absorb and release heat from an internal space as temperature changes in that space is called thermal admittance (or heat transfer coefficient) and is described in BS EN ISO 13786:2007 Thermal performance of building components. This also provides the basis for the Simple dynamic model in CIBSE Guide A: Environmental design, used to measure cooling loads and summer space temperatures. The higher thermal admittance, the higher the thermal mass. Thermal admittance is like thermal transmittance (using the same measuring units). Still, it tests a material’s thermal storage efficiency, i.e. a material’s ability to keep and release heat over time, usually 24 hours. Together with thermal transmittance, measuring units are W/m2K.

Note that ‘ Y-value ‘ thermal admittance should not be confused with ‘ y-value ‘ thermal bridging factor described in SAP Calculations (Standard Assessment Procedure) Appendix K as derived from linear thermal transmission.

Psi (Ψ) value, or linear thermal transmittance

Measurement of heat loss due to a thermal bridge is called linear thermal transmittance (as opposed to’ area’ thermal transmittance otherwise called U-value), with the units of measurement again being W/m2K. Psi values are used to

Thermal mass

Hitherto ignored in the UK construction industry, thermal mass (unlike thermal admittance) is derived from the specific heat capacity (the ability of a material to keep heat relative to its mass), density and thermal conductivity (how heat can pass through a material). SAP Calculations use thermal conductivity as the’ k’ (or kappa) factor in measuring the thermal mass parameter (TMP). The’ k’ value is the heat capacity per unit area of the building element’s’ active’ component (only the first 50 mm or so of the element’s thickness has a real impact on thermal mass, as it decreases the element’s depth by increasing; the effect is negligible beyond 100 mm). It should be noted that the’ k’ value is an estimate, as assumptions are made about the size of a material’s active volumes; yet, it ignores the effect of thermal conductivity in measuring the time during which heat is absorbed and released from the material. BS EN ISO 13786 offers a more effective thermal mass determination process. Isolation should not confuse thermal mass.

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