Interesting take on physics

[nightjar]

That is more or less what I assumed from the comment, although I did not know the downlighter was in a ceiling void directly below the roof. I also made the assumption that the downlighter can vent freely to the space above the ceiling. In that case, the lamp heats the air above it, which rises. That draws more air in from the room, creating a rising column of air. The column will be wider than the fitting, so some of the rising air will hit the ceiling around the fitting. If that is cooler than the dew point of the rising air, you will get condensation around the fitting. If insulation has been cut back to avoid it impinging on the fitting, the ceiling around the fitting might well be cold enough.

I would not expect that to be a direct result of the lamp, although there could be some secondary effects from drawing moist air into the ceiling void. Fitting a fire resistant enclosure above the fitting ought to cure the first problem and might sort the second.

Colin Bignell

[nightjar]

I'm not sure where you get the idea I said it would draw up cold air. I said it could draw up moist air, which is more usually warm than cold. See my other reply posting for a more detailed explanation of how I would expect it to work.

Colin Bignell

[Rod]

nightjar

I agree that the lamp should have a "plantpot", that if it had one and the cause was as described (warm, moist air rising past the lamp and condensing), that would fix it. But I actually think there is a minor roof problem.

[DM]

That would not really fix anything if the light is not on you would just get condensation inside the pot. However if the light is on , and is a nice LV halogen then the pot might just get warm enough to prevent condensation forming.

cheers

David

[Rod]

Possibly. But I figured that without a significant airflow *through* the fitting, any airflow up into (moist air) and down out of (air which has been dried by condensing onto the plantpot) would be minimal - and would certainly not spread across any the rest of the ceiling. (It is possible that the airflow is caused almost entirely by wind over the roof sucking air upwards. That too would be massively reduced by a plantpot.) Of course, it all depends on the conductivity of the plantpot.