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Nail on head - It's all to do with fire containment time > http://www.labc.co.uk/sites/default/files/labcpd0914_techg_dlight_dwellings.pdf
Secondary effects are a lowering of U value due to air leakage (fire draught) paths and a thinning/removing of insulation where down-lighters are installed
Tertiary effect can be a lowering of sound insulation performance
rive_gauche
@Airhead
From my recent reading of the regs the main concern with downlighters is the holes allow for passage of fire from one 'compartment' to another. I'm often replacing downlighter "sugar cubes" and the terminal connectors quite often arc out and melt, transformers also go over time. It does make you shudder when you see a melted arced out terminal block and some bare 12v wires above the ceiling.