Sealing round light fixtures

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I want to seal around the electrical cables coming through the ceiling in our bedrooms and bathroom to limit the moist air getting into the loft (where we have a minor condensation problem). What’s the best material to use? Caulk? Silicone? Something else?

Thanks.
 
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Moist air is not blowing through these tiny gaps. Heat going through the whole ceiling is your problem - together with lack of ventilation in the loft.
 
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Yep, we are on to all of that (I am investigating vent roof tiles or those flat lap “easyvent” things, and the loft is fully insulated) but we have been recommended to seal up any gaps in the ceiling around pipes and cables. As it should be an easy fix I’m willing to give it a go; but I take your point and if it’s awkward or expensive I will probably not bother. Any suggestions for best product to seal round electrical cables would be great. Thanks.
 
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Caulk or a silicone should do. Some types of external grade products might be a bit reactive with certain plastics or rubbers, but these will always have a warning and tend to be marketed for roofing repairs.
 
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Moist air is not blowing through these tiny gaps. Heat going through the whole ceiling is your problem - together with lack of ventilation in the loft.
Agree with your first and last point but the heat itself isn't an issue, just the water vapour. Sealing all the gaps will indeed help. The more heat that gets through the warmer the loft will be so the dryer it will be.
Especially focus on the wet rooms eg bathroom and kitchen.
 
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Agree with your first and last point but the heat itself isn't an issue, just the water vapour. Sealing all the gaps will indeed help. The more heat that gets through the warmer the loft will be so the dryer it will be.
Especially focus on the wet rooms eg bathroom and kitchen.
Water vapour does not tend to travel through the ceiling in significant quantities to cause issues. Any that is absorbed into the plasterboard is held and will then evaporate back on the warm side rather than the cold side.

Heat however, will be the issue and despite warm air holding more water like the books say it wont cause dryness, but will hit the very cold surfaces and release moisture more freely as condensation.
 
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Hi woody agreed most of the water vapour will be checked by the plasterboard/skim. Hotter air indeed can, but not necesarily will, be holding more water, hence why condensation happens when it cools below its dew point. However cooler air in the loft being heated indirectly by the upside of the ceiling won't have had any way to gain moisture, so the water content will be the same, thus the relative humidity will be lower. Therefore when it hits the cold surfaces, it will go back to its original temperature and relative humidity with the same absolute water content which should still be above the dew point.
Furthermore, due to crap insulation, the cold surfaces will actually be a bit warmer, so there's a bit of additional protection.
That's why adding more insulation but without making sure decent vcl can cause condensation where there was none before.

However none of this invalidates your good advice to the OP which is to go ahead and seal up the holes!
 

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