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I have a vacation house that I visit a few weeks a year, which faces a condensation issue.

Indeed, when I stay there in winter, where the outside temperature is well below freezing, I measure around 24°C/75°F and 40% humidity (absolute: 0.009 kg/m3). (Heating with a fireplace, hence the high temp and low humidity)

When I leave and set the heaters to frost protection, in a couple days, the house is back to 7°C and 50% humidity (absolute: 0.004 kg/m3).

The difference between the two is condensation that is ruining the lime plaster on the stone walls.

Wall picture from comment

What are best practices to cool down a house without condensation (and without spending the GDP of a small country to heat the house, nor bursting all water pipes.

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    The simplest and cheapest way is to turn off the main water supply and open all faucets. You should allow some airflow in the house so no mold and bad smell will develope.
    – r13
    Aug 15, 2021 at 20:23
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    "lime plaster on the stone walls" i.e. uninsulated? Then "drain the plumbing and ventilate" is indeed the right answer, or "insulate the building properly" is. 40% is not particularly "low" for heating season...and 50% is not high. But the walls are clearly cold in heating season if condensate is forming from 50% RH 7C air.
    – Ecnerwal
    Aug 16, 2021 at 18:03
  • Not sure I understand the cause of condensation here. Are you saying it happens only when you leave the house in the winter? If that's correct, is the "stone wall" uninsulated? Perhaps the cooling down of the house is happening significantly through that wall? IE, the stone becomes cold before the air in the house does, and so the air is cooled by the stone, causing condensation? Understanding this would help. Maybe insulation would help or maybe just quickly admitting cold air into the house would help eg open all windows for a few hours before leaving.
    – jay613
    Aug 16, 2021 at 18:08
  • @jay613 The walls are 1m / 3 feet of basalt, no insulation whatsoever. For sure, the house is cooling down very significantly through the walls, I am not sure about ventilating, since I guess -15°C / 0°F for weeks is not really better than condensation (?), but opening the windows before leaving seems like a good idea, seems like good material for an answer
    – Maxime
    Aug 16, 2021 at 18:25
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    Before I make it an answer .... Does the condensation occur 1) during your short stays 2) for a short period after you depart, 3) throughout your absence? I wonder now if the wall is the primary path for 7C inside air and -15C outside temps to meet, if rapid cooling of the air might not help. You might need insulation for that wall or a dehumidifier.
    – jay613
    Aug 16, 2021 at 18:52

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I would just get portable dehumidifier and set it to the desired humidity. Some have hose connections so you don't have to dump a bucket of condensate every day, which would be impossible in a vacation home. You'd have to find a convenient drain for the hose, maybe a shower base? I wouldn't want to rely on a condensate pump in a vacation home... too much can go wrong with those little guys. A side benefit of a dehumidifier is they do put out a small amount of heat, not a lot, but would offset your winter time heating bill a bit.

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