Likely is to boost output of existing radiator. Hard question to answer, mothers house pre-double glazing had trays on the bottom of the windows and a small hole leading outside, the water would condense on the windows run down them then outside which removed the humidity.
However as long as no leaks our caravan can sit unoccupied all winter with no signs of damp, the damp comes from our breath. Or anything else which burns a hydro carbon fuel.
So an unheated room should not get damp unless your circulating the air with a warmer room, and likely you want to remove the damp anyway not drive it back to another room.
So best is likely a heat recovery unit, this will send damp stale air outside and suck in dry fresh air from outside into the room without causing the room to get two cold, it will also circulate air in the room so the radiator has more of a chance of heating the room, using TRV in every room will reduce the heat to warm rooms meaning more heat available to cold rooms, if not TRV fitted could try adjusting the lock shield valves.
I extended the kitchen and I drilled two holes to an existing radiator and plumbed in an extra radiator using micro bore pipe seem to remember 10 mm so only needed to 12 mm holes.
I fancy a heat recovery unit, but don't fancy using the core drill to make the hole in the wall, so in the bathroom which really could do with a heat recovery unit we just open the window.
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