Last edited 01 Mar 2020

R-value

Contents

[edit] Introduction

R-values can refer to:

[edit] Thermal resistance

R-values are a measure of the thermal resistance of a material of a specific thickness, that is, its resistance to the transfer of heat across it. The higher the R-value of a material, the more effective it is as an insulator.

R-values can be used as part of a labelling system to enable comparison of the thermal performance of different materials, such as insulation, or as part of the calculation of heat transfer across the fabric of a building.

R-values can be calculated by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/mK). R-values are therefore expressed in m2K/W (or ft2·°F·hr/Btu in the USA). The overall R-value of a multi-layered element can be calculated by adding the R-values of its component materials.

R-values are sometimes described as being the reciprocal of U-values (sometimes referred to as heat transfer coefficients) which describe the thermal conductivity of a building element. However, unlike U-values, R-values do not include surface heat transfers at the boundary of the element by convection and radiation.

U-value = 1 / (the sum of the R-values of the layers of the element + the resistance of the internal and external surfaces of the element)

[edit] Indoor air quality

R-value is the: 'Sum of all Ri values. Ri is the ratio Ci or LCIi, where Ci is the mass concentration of the individual VOC i. in the air of the reference room, and LCIi is the LCI value of VOC i. The LCI value is the ‘Lowest Concentration of Interest’, which is the substance-specific value for health-related evaluation of the emission from construction products as agreed by the EU-LCI Working Group https://ec.europa.eu/growth/sectors/construction/eu-lci_en.'

Ref BREEAM UK New Construction, Non-domestic Buildings (United Kingdom), Technical Manual, SD5078: BREEAM UK New Construction 2018 3.0, published by BRE Global Limited

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