U-Value Calculations

Sustineo are able to provide U Value calculations for your SAP Assessments and SBEM calculations (or for other purposes)

U-values (sometimes referred to as heat transfer coefficients) are used to measure how effective elements of a building’s fabric are as insulators. That is, how effective they are at preventing heat from transmitting between the inside and the outside of a building. R-values (not to be confused with the R Coefficient used to show the rate of transmission of a virus in a community), which measure  thermal resistance rather than thermal transmission, are often described as being the reciprocal of U-values, however R-values do not include surface heat transfers.

The lower the U-value of an element of a building's fabric, the more slowly heat is able to transmit through it, and so the better it performs as an insulator.  Very broadly, the better (ie lower) the U-value of a building’s fabric, the less energy is required to maintain comfortable conditions inside the building. As energy prices increase, and there is greater awareness of sustainability, performance measures such as U-values have become more important and building standards (such as the Building Regulations) have required that lower and lower U-values are achieved. This has required changes in the design of buildings, both in the use of materials (such as insulation), the make-up of the building elements (such cavity walls and double glazing) and the overall make up of a building’s fabric (for example reducing the proportion of glazing).


U-values are measured in watts per square metre per degree Kelvin (W/m²K). So for example, if we consider a double glazed window with a U-value of 2.8, for every degree difference in temperature between the inside and outside of the window, 2.8 watts will be transmitted every square meter.

 

A range of U-values are indicated below for the purposes of comparison only:
 

•Solid brick wall 2.0 W/m²K.
•Cavity wall with no insulation 1.5 W/m²K.
•Insulated wall 0.18 W/m²K.

 

Part L of the Building Regulations (Conservation of fuel and power) now prevents certain forms of construction by setting limiting standards (ie maximum U-values) for building elements.  It should be noted however that these are maximum permitted values, the specification for the notional domestic building referred to in Part L1A has considerably lower values, for example:
 

• External wall 0.18 W/m²K.
• Floor 0.13 W/m²K.
• Roofs 0.13 W/m²K.
• Windows, roof windows, glazed rooflights and glazed doors 1.4

   W/m²K.


The U value of an element (in W/m²K) can be calculated from sum of the thermal resistances (Rvalues in m²K/W) of the layers that make up the element plus its inside and outside surface thermal resistances (Ri and Ro).  U-value = 1 / (ΣR + Ri + Ro).

Where the thermal resistance of the layers of the element R = the thickness of each layer / the thermal conductivity of that layer (its k-value or lambda value (λ) in W/mK). This can become a complicated calculation when there are a large number of layers, ventilated or unventilated cavities are introduced, or the element is inclined. Manufacturers will generally provide U-values for products that they supply. The conventions for calculating U-values are established by BR 443 – Conventions for U-values.

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