Windows Open When Humidity Outside is High?

Adamc

Hi all 

We had a condensation issue last week and I just wanted to improve my understanding of indoor and outdoor humidities. 

I tend to try and let let fresh air into the house where I can to try to keep it well ventilated. Obviously this can lead to increased heating bills etc.  

However, if I want to lower or limit the humidity in the house, is this a lost cause when the humidity outside the house is higher than inside? Will more moisture move inside?

What is the best way to find out outdoor humidity? The weather updates on my phone denote very high humidities (80s and 90s) currently where as my hygrometer in the house has the humidity within as 47 degrees currently. I'm assuming these aren't simply comparable?

theoretica

It depends on the temperature of the air as well as humidity.  Humidity is a percentage of the maximum moisture the air will hold, and it will hold a lot more when warm and very little when cold.  So opening the windows on a hot muggy day won't lower indoor humidity.  But cold air will, once it has warmed up.

tacpot12

If the humidity outside is higher than that indoors, the humidity in the house will rise until it equalises. The rate at which it will do will depend on how well ventilated the house it. If it is well ventilate it will equalise more quickly. The same is true of the humidity outdoors is lower than that inside the house. 

The measurement of humidity is normally given as a percentage of relative humidity. 80 and 90% is high and 47% is quite a low level relatively speaking. You are right that they are not directly comparable, because they are relative measures of how much water the air can hold at the current temperature of the air - the air outside the house usually much cooler than that the air in your home (at least in the winter), so 90% relative humidity (RH) at 2 degrees C could mean that the air actual contains the same amount of water at air with 47% RH at 16 degrees C.

I can see that this doesn't really help you understand whether ventilating your home during times of high humidity outside will help reduce condensation. During the winter the air outside is rarely so humid that using that air to ventilate your home will cause any problems. On a mild winter day after heavy rain, and on cold, foggy days you might get a detectable increase in humidity if you open the windows, but generally you will not. The  solution to condensation is insulation on the walls, to stop the walls getting so cold that air that water condenses on the walls, and adequate heating, to keep the RH level lower. Both techniques stop the air at the surface of the wall reaching 100% RH which is when you get condensations. 

twopenny

I open my windows or doors most days for a while. If the heating is on I turn the radiator thermostat down or off in a room, open the window for a while depending on how cold it is outside and close the door of that room so any cold air isn't causing the heating to run. It has to come up to temperature afterwards but not by much.

It's come rain or shine.

The only time I get condensation is when it's freezing temperature and water condenses on the glass because of the glass being very cold.

It doesn't happen because it's raining or in summer, humid.

theoretica

tacpot12 said:

so 90% relative humidity (RH) at 2 degrees C could mean that the air actual contains the same amount of water at air with 47% RH at 16 degrees C.

I make it about 35% RH actually.  10 degrees and 90% RH outside wouldn't work well for drying inside, but 2 degrees would be fine.

Smithy99

I find the humidity in the house always drops with some ventilation regardless of what the outside humidity is.