Discover a great idea
The tiled roofs of houses absorb radiant energy by day and re-radiate it back to both loft and the exterior air at night. A white coating should reduce the external radiation and thereby increase the internal amount of energy.
The suggestion has been made that white roofs may have some positive benefit to counter global warming.
Basic physics would also suggest that in theory loft temperatures might go up slightly at night and be reduced slighlty during daytime in hot weather. I think it would be interesting if a group of households could be persuaded to try this as an experiment. If limewash were used I think there would also be a CO2 take-up effect from the chemistry of lime.