Debugging foggy apartment windows by measuring air pressure difference between indoors and outdoors

Interior side of exterior windows in my apartment gets occasionally very foggy, especially during mornings. Too high air pressure indoors can cause this, but one time measurement by the constructor showed the air pressure to be fine, so lower indoors than outdoors. The constructor thought that drying concrete might cause the issue. I want to debug the cause of the fog in more detail. The fog appears at particular times of the day, so I thought to follow the air pressure and humidity in detail in time at three locations: indoors, between the windows, and outdoors.

Have you tried a similar application or do you have tips on how to execute? I already got a tip to calibrate first the Ruuvis so you know if there is a constant drift between their measurements. Should I just tape the outdoor Ruuvi to the outdoor wall? Does snow interfere with measurements?


I have actually done this once. I took two RuuviTags and calibrated the pressure readings to show same value when they were next to each other. I then put one RuuviTag outside and saw that the indoor pressure was too high, which lead to repairs in the roof ventilation unit and my fogging windows were fixed. It sounds like the constructor has ruled the pressure out in your case though.

The sensor readings, especially pressure, drift over time and temperature. As a tip Iā€™d suggest to calibrate the sensors to match indoors and then check it again outdoors, especially during cold season.

1 Like

First measurements indicate 3pa overpressure indoors. Unfortunately, Ruuvitag promised accuracy is 10pa, though indoors after calibration the differences were about 1pa. This is not conclusive, but I suspect that an occasional indoor overpressure explains the fog, and also why ice in between windows sometimes melts even if the temperature outdoors is constant: overpressure indoors warms the space between the windows. If true then air pressure differences can be followed by tracking temperature between windows. Also, windows exposed to wind can experience more underpressure as high wind can cause up to 60pa difference. Lets see how numbers look after tracking for few weeks :slight_smile:

1 Like

This is very interesting.
I have sometime had window condensation due to high humidity and large temperature between indoors and outdoors especially when cooking.
Can you provide some measurements of temperature and humidity.

I would be very surprised to find an air pressure difference between indoors and outdoors AT THE SAME ALTITUDE and time. That would require very well sealed building. I have found that bme280 included in a Ruuvi can detect a change in elevation. between the floor and a wall shelf (like 2 meters).
In addition to altitude changes in weather over a few hours will cause change in air pressure.

Can you provide a reference regarding air pressure causing fogging.


Here in Finland air pressure difference between outdoors and indoors can occur in new apartments which are sealed well. I have understood that the recommended underpressure indoors is 0 to 5 pa, but Iā€™m not an expert.

What matters for foggy windows is not the absolute pressure, but overpressure indoors, which pushes warm moist air to the space between the windows. Here the exterior window is cold due to weather, which then condenses the moist.

The overpressure indoors was confirmed by an accurate sensor for measuring air pressure differences. What is odd is that the air pressure difference seems to change in time. Today the window fog disappeared and at the same time Ruuvitag shows underpressure indoors, though I cant verify it with a proper sensor.