I live in a Victorian gable end terrace which is classed as 'hard to heat' and sufferers from mould/ condensation . I have taken all the insulation measures I can afford & treated all obvious sources of damp and leaks. In winter I have no choice but to close off some rooms as I cannot afford to heat them - not ideal I know but I check them daily for damp and mould & clean off if necessary .

My question is on a typical wet winter day in England ie currently 9 degrees centigrade with a Relative Humidity of 70 % , dew point 2degrees, and an internal room temperature in an unheated room typically a 3 -4 degrees higher , is following advice to open windows likely to help or create even more condensation? Also , same question but is it better to constantly leave windows opened just a crack , bearing in mind temperature drop to 0 degrees and even higher relative humidity during the night - or just open once or twice during the day?

I have tried to research this myself but cannot find a straightforward answer ( least one that I can understand ). I do understand that this is good advice for kitchen and bathroom whilst cooking / showering etc.

Any advice appreciated

  • An air/heat exchanger might help. Commented Mar 10, 2016 at 16:53
  • If you do want to open windows for a specified period every day, I bet you'd want to avoid dawn and dusk, since the allergist told me that's when the outdoor mold count is the highest. // It seems like a dehumidifier on wheels would be the way to go. Note that the dehumidifier does heat up a room somewhat, so you could afford to lower the thermostat somewhat in those unused rooms. // I wonder if mold may have colonized those rooms already.... Commented Nov 19, 2019 at 20:49

2 Answers 2


Ok, basic gas laws. Warm air can hold a lot more water than cold air. (That's why it rains on windward side of mountains). We define "relative" humidity as the percentage of water air can hold, at its current temperature.

Fill a trash bag with outside air at 9 degree C 70% relative humidity. Bring it indoors and let it warm to 25C. The relative humidity goes down - and becomes so "dry" that it's uncomfortable, even though it's the same air and it didn't feel dry when you were outside. Your indoor air is wetter than that, because of exhalation, bathing, cooking, and water penetration through floors and roof leaks.

Take a bag of your wet 25C indoor air, outside - at 9C the relative humidity will go over 100% and water must condense on the surface of the bag.

That is what is happening in your closed-off rooms. Warm, wet household air is leaking into those rooms.

So the advantage to opening a window is mixing in the dry air from outside. However that is only competing with the internal leaks between the room and the rest of the house. You'll need to open the window (or seal the room) enough to win. Of course then, you will have water ingress whenever it rains, and that water will add to the problem.

The American way is to run a dehumidifier, retrofit insulation, or just heat the house anyway. But then, the American way is to pay 20 pence per litre for oil.


That's a tough way to go. The problem with either & both turning the heat completely off to those rooms &/or especially opening windows at all. Is, that you just made your interior walls your new exterior walls & left your exterior area fully vulnerable. This all makes condensation & mold even more likely to occur.

Insulation doesn't "stop" anything, it just slows it down. Check your heating system. You shouldn't have any high enough humidity for non-baths & kitchens to be molding...your building breathes tremendously well & yes too well. If you have a Humidifier, then kill it permanently. They aren't needed in the slightest once you're outside of an arid region.

You can try a De-humidifier that drains to the outside or into a sink or other drain. But, that would be more expense now & later. Or, try a dose of Desiccant as another method of De-humidification. It can be expensive, but it's free "to run".

Otherwise, I'd say keep the heat on & just turn-down those rooms to half. If it's somehow a forced-air system, then the HVAC returns will remove the humidity much faster than the heat will dry it out, but both together will do it best. Also, keep the heat on & let it just maintain a temperature. This is typically less expensive than turning it up & down all of the time, where it has to burn a lot of fuel to reach the desired temperature.

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