I used to live in a seriously damp house. I bought a variety of humidity monitors and they all showed between 78% and 90%, sometimes more. Water streamed down the walls. I became obsessed with preventing damp and read everything I could about it. I kept the windows open permanently, never left wet towels or anything damp in the house, never dried clothes inside etc. It got to the point were I was too scared to boil a kettle and only ate cold food as I was scared cooking would create damp. None of this made any difference.
I’ve since moved and now have a nice dry house. I’ve still got the humidity monitors and they generally show around 50-55%. Recently with all the wet weather, they have showed 70% or more which made me panic. It’s very warm so I don’t need the heat on. All the advice says to open the windows, but when I do that the humidity rises which seems obvious to me since its wet outside. Does anyone know why this is always recommended?
Is it normal for humidity to rise and fall with the outside weather? There’s no sign of damp in this house and when I mention this issue to other people they tend to find it weird that I even think about it. I’ve thought about buying a dehumidifier but then I think it’s excessive since I only need it occasionally, when the weather is like this.
New homes that are energy efficient through having a strong seal from the outdoors have a side effect of trapping humidity inside because there is nowhere for it to go. Hence why guides may recommend allowing it to escape by opening windows.
If your humidity is fluctuating based on the outdoor humidity level then maybe you have the opposite problem?
ETA: If your meter shows 70% humidity (anything over 50% is probably too much for the indoors) and it doesn’t seem humid to you then maybe the meter is malfunctioning. I’d get a second one to verify.
The windows are double glazed but they all have trickle vents which I leave permanently open. I also have the windows themselves open almost all the time I’m home if it’s not too cold. I can understand the advice about opening windows if the outside was likely to be drier but it often isn’t.
Eg. this morning when I got up it was 61%. Within 15 minutes of opening the windows it was 68% and soon rose to 72%. It was raining outside, but it’s been drummed into me that you must open windows as much as possible to prevent damp. It’s the same advice given all the time.
I’ve got 4 humidity monitors. They show much the same reading so I don’t think they’re faulty. The house does feel humid due to this warm sticky weather. It feels the same in friends houses at the minute, but they don’t have humidity monitors to notice. I haven’t noticed any condensation or mould in my house at all. Not yet anyway!
I would guess this typically varies depending on the areas of the house.
Sounds like you put more thought into this than most people.
I live in Georgia (US) and it is quite humid, but my main housing area is showing 42% right now. The area where the AC condenser is located is quite a bit higher (typically has moisture on the AC unit in the summer. I did place a dehumidifier in that area to help with the moisture.
Due to the heat and the humidity, I keep the windows closed all day this time of year and I would expect the humidity to rise if I opened the windows in general and definitely if it is raining outside as that would be 100% humidity outside.
You possibly have an issue with humidity getting in, but I would say the opening of the windows to equalize it is your main problem.
To answer your question, if it is damp out, opening the windows would increase your in house humidity, not decrease it.
If you open the windows, you allow faster air exchange between the inside of the house and the outside. If it is raining outside, then the humidity outside is likely higher than the humidity inside. Opening the windows in this case will cause the humidity inside the house to increase faster.
On the other hand, after the rain has stopped, if the humidity drops outside, opening the windows then will allow the humidity to escape from the house faster.
By trying to keep the house as open as possible, you are making the house get damp more quickly when the humidity outside is high, but also are ridding the house of humidity more quickly when the humidity outside drops. So you are both partially helping and hurting the situation.
You can do better by keeping things sealed as much as possible when the humidity outside is higher than the humidity inside to slow the rate of humidity increase inside the house.
ETA: A tighter sealed house with dehumidifiers or air conditioning (which dehumidifies just by the way it cools) running will tend to have a lower humidity on average than the outside air. The more open a house is to the outside air, the more the air inside the house will match the humidity outside.
It’s not a blanket rule. It might be a blanket rule in Phoenix, where the outside relative humidity is < 40% most of the time, but (for instance) in the soggy Southeast, there’d be no rational reason to expect exposing a slightly humid interior to much more humid external air would decrease internal humidity.
Logical extreme: outside humidity is 100%. In other words, it’s raining. Just for fun, it’s also blowing hard.
Your inside RH of 60% is pretty much guaranteed to rise sharply if you keep the window open and all that rain blows in.
I’m in Northern Ireland so it’s raining a lot, but generally cold as well. With the heating on the humidity is no problem. It’s just this unusual warm wet weather recently.
I do think more about this than most people. It’s a fixation since living in the old damp house. All the advice agencies kept repeating to keep the windows open no matter what. It seemed crazy but I just believed them and got into the habit, even after moving.
Maybe I should go ahead and buy a dehumidifer. I’d dread the electric bills, but I’d only be using it for a short time.
I’m going to suggest that your earlier “wet” house had other things going on, perhaps it was built on top of a spring or something. You, your family’s, your pets and even your houseplants give off water vapor; opening a window will vent that out as long as the outdoor humidity is lower. “Water streamed down the walls” … that’s not normal in almost all circumstances, I didn’t even have that in the tropics.
Yes, the old house had issues. It was 350 years old, with very thick solid stone walls, no insulation, a roof that flooded the place regularly and an ineffective heating system that took 3 days to warm the place to 15 Celsius. The landlord kept saying I should open the windows more. I lived there for over a decade since I couldn’t afford anything better, so I’m totally paranoid about damp.
The new house is great and I was amazed that it warms up in about 20 mins! I guess I’m causing the problem with my old habit of keeping windows open.
The seemingly blanket policy of opening windows to reduce humidity might come from the thought that apart from local ambient humidity, houses with excess humidity problems usually have an additional water infiltration problem such as leaky plumbing, high water table, or leaky roofs which let in more water and cause the inside humidity to rise as this excess water evaporates but has nowhere to go since its enclosed by the walls… And starts condensing and running down the walls. In those cases since there’s a constant inside source of water it would make sense to open windows so it can vent. Doesn’t solve the problem but keeps the humidity down until the problem is solved.
Early in my days living on my own (in California), I was given the advice not to open windows, but to leave fans on in the house that exhaust outside, such as the bathroom fan or range hood fan (though not all hoods exhaust outside). Since these fans tend to be located in the rooms most likely to generate humidity for the house (a modern house anyway), you’re getting a better effect than merely opening windows. So you might not just have the fan on while you showered, but leave it on for an hour or two afterward.
In an environment that is cold and damp (I live near Seattle, so I sympathize), usually you can correct the humidity during the winter just by keeping it comfortably warm in the house. It may be 100% humidity outside at 5C, but indoors at room temperature you should be fine.
I’m guessing this is all lousy advice for a 350-year-old stone house, but it should be good for anything built in the last 50 years or so.
But you didn’t make it clear where you meant relative humidity (RH) and where you meant absolute humidity (AH)
In the warm humid place, it might be that the building is actually cooling the air, bringing the humidity up to 100% RH.
But if its raining, then its likely the air outside is cool. and when it is cool, and it comes inside it warms up, thus dropping the RH below 100%… It may be that the cool air at the moment has a lower absolute humidity than the warm air from the warm of the day.
Warming up the inside air doesn’t do anything for the absolute humidity inside, so running a heater doesn’t ensure that the inside doesn’t fall back to 100% RH when it cools down…
The humidity has now been at around 70% for over 10 days with warm temperatures, particularly the last few days at 24 Celsius. There’s no sign of any mould growth in the house which has got me thinking. I know at 70% humidity in cold temperatures, mould would be flourishing. Does the high temperatures prevent this?
Mold needs liquid water to flourish. High relative humidity may cause water to condense on the walls or whatnot, but mold growth is far more typical of a roof leak (like in your old home) or a leaking plumbing pipe … and of course build up in your bathroom due to hot showers (so open a window or run a vent fan and you should be fine).
Very few organisms can draw water vapor out of the atmosphere like that, closest I can think of is Coastal Redwoods and really they just absorb the dew off their foliage.
I’m thinking that at night, once the temperatures start to cool down, this high humidity could end up condensing on the walls which might feed any mould. I imagine keeping windows open all night would help but I’m too scared of bugs getting in to do that!
I’ve now got a dehumidifier but it’s one of the dessicant types and the extra heat it creates in this warm weather is unbearable.
Thanks watchwolf. I understand it better now. There’s no mould problem here at all. It’s just my paranoia after the old house. I’ve got trickle vents on all the windows which hopefully should create a bit of airflow and there’s a nightlock on them which I’ll use.
Btw, I had to google Captain Cook. I just thought he had something to do with fish fingers but I know more now!