Low-Emissivity Surfaces in Membrane Structures: Radiative Heat Transfer, Spectral Behaviour and Engineering Application

This Technical Note examines the physical and engineering role of low-emissivity (Low-E) surfaces in membrane structures. It extends the thermal-transmittance framework established in Technical Note 01 by focusing on the long-wave radiative heat-transfer mechanisms that determine the effect of emissivity at a single membrane surface and across an air space between two membrane layers. The Note distinguishes long-wave thermal emissivity from short-wave solar and visible optical properties, develops the relevant radiation relationships from fundamental principles, and applies them to membrane envelopes using a consistent S1–S4 surface convention. Engineering formulations are aligned with EN ISO 6946:2017 where appropriate. Representative emissivity cases, measured spectral data, manufacturer declarations, technical reports and author-defined calculations are kept explicitly separate. Practical aspects including surface position, weldability, ageing, contamination and thermographic interpretation are also addressed. The analysis shows that Low-E is a surface mechanism that can materially reduce radiative heat transfer in thin membrane envelopes, but does not replace conventional bulk insulation and does not eliminate conductive or convective heat-transfer paths.