Crack in plaster under window behind radiator.

Under many of my windows, behind the radiator, the plaster has a hair line crack. This is a standard wall with proper plaster, not plaster board.
I recently redecorated a room, raked out the crack filled with polyfilla and the cracked returned within months.
The cracks never get big so it doesn't seem to be structural movement. May be it is heat expansion.
So I hit on the idea of caulking the crack instead of filling. It is mainly hidden behind the radiator so a minor imperfection doesn't notice.
Was this a sensible idea of will the emulsion paint crack on top of the caulk. I just read the caulk tube and it explicitly says not to do this :o(.
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Not paint over decorator's caulk? Not much point in it, then, I'd say. ;-)
My problem was cracks in the ceiling between sheets of plasterboard fitted and skimmed in the mists of time. Polyfilla was useless - but a good caulk has worked beautifully. But took several layers to get a good level surface.
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On 28/04/2017 15:01, Dave Plowman (News) wrote:

I finally did two layers of caulk before painting. There is still a slight indent where the caulk shrank.
Anyway the experiment is on.
I noticed that every window with a radiator below has cracked vertically behind the radiator, none of the windows without a radiator has such a crack. Also the walls with radiators on but no window haven't cracked. Well one wall that has a radiator both sides has cracked but I pollyfilled it years ago and the crack never came back.
It seems there is something special about the combination of window and radiator.
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Yes. As I said can take several goes to get it flat. But worth the effort, since none of the cracks here have opened up again. As they did with any of the regular fillers I tried before.
If only someone could produce a flexible grippy filler that didn't shrink. ;-)
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman wrote:

That's why I mentioned the one-strike stuff, the only thing is it doesn't produce a plaster-smooth finish and doesn't take sanding well, hence topping it off with something else.
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replying to Nick, Dain1 wrote: If you heat a wall up, then cool it off, then heat it up....eventually, with all the expansion and contraction, it will crack. Because we live in rabbit hutches someone had the bright idea of making slim rads that could be bolted to the wall - so the walls crack.
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On Monday, 8 January 2018 18:44:07 UTC, Dain1 wrote:

I expect it's been sorted since it was asked 8 months ago. Get yourself a sensible portal to this place. http://wiki.diyfaq.org.uk/index.php/Newsgroup_access_tips
NT
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I'kd not worry, they are just covering themselves I think. Most of the walls in a nearby public building have this problem behind the radiators. Didnt they used to glue some naff looking alluminium stuff over the wall to hide it all? Brian
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Brian Gaff submitted this idea :

No, the idea of the foil, is to try to reflect heat from the radiator back into the room - rather than the radiator warming up the wall which would lose heat to the outdoors.
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What about the air rising from the rad meeting that cold wall? With central heating rads it's convection which does most of the heating - not radiation.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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on 29/04/2017, Dave Plowman (News) supposed :

Yes I know, but that fact doesn't stop people selling them, nor people buying them assuming they must work.
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So should we really refer to them as convectors instead of radiators?
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Graeme explained :

Really, yes. They warm the air which flows past them by convection air currents. Cool air comes in at the bottom, then collects warmth as it rises past the radiator. The warmed air will warm the wall behind the radiator to some extent, but alloy foil alone will not prevent that - insulation of the wall will, so thin polystyrene sheet would be much more effective.
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Yes, I have vaguely thought about a sheet of thin 3 ply behind the rad, just with slots to hang over the brackets. Polystyrene glued to the ply, and possibly foil glued to the polystyrene.
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I am never very brave when it comes to plumbing stuff, although it all sounds easy in theory. Turn off valves, loosen joints, catch water in old ice cream container, job done. Hmm...
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Graeme

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I'd love to see some test figures on how this sort of thing performs. Given the large wall area as opposed to the bit behind the rad. My gut feeling is you wouldn't be able to measure any difference. If it was worthwhile, would be easy enough to make a rad which included insulation to the rear.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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I tend to agree, which is why it is a vague thought. Given that a radiator is really a convector, as discussed, is that much heat really radiating out into or through the wall? The heat is mainly going upwards, not outwards.
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Graeme wrote:

    There is some heat loss through the wall behind the radiator which is at a higher temperature than the rest of the wall. Using a foam/foil pad will reduce this to some extent, however IMO it is not worth doing as the dust build up will reduce the reflectivity of the foil after a few months.
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When I wallpapered the room, I removed the radiator and stuck baking foil on the wall behind the radiator before repapering.
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wrote:

Not true of foam. Still not worth doing tho, too small an area to matter.
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