Managing Relative Humidity in an English Home

Having lived in England for the past two decades I have heard quite often: the humidity in our house is terrible in the cold seasons, condensation and mould is all over the place. Interestingly enough, in all properties we have lived so far we did not experience that. I wonder whether this might be due to our strict ventilation regime: airing long enough for a complete air exchange with all windows open several times a day, at least in the early morning and late evening, and also after every cooking session. In the following I would like to present an explanation of the positive effects when airing thoroughly and conversely, the negative consequences of not airing enough, both using simple concepts of thermodynamics such as relative and absolute humidity.

Keeping the Windows Shut

If you happen to live in the same street, please forgive me to share my observation that most windows of nearly all houses appear to remain closed all day or at least are not being opened for an airing period. Apart from some trickle vents and other leaks, the air and the water vapour contained in those houses remains mostly constant. That being said, for the cold seasons a daily cycle starts when the heating switches on in the morning:

1. Heating up with decreasing relative humidity. The indoor air is almost at the dew point in the early morning hours, laden with water vapour at a relative humidity close to 100%. As the heating system gradually heats up the air (and the water vapour therein) to say 20°C, the absolute amount of water contained in that air remains almost constant during this period, since the house is nearly sealed. Because warmer air can keep more water vapour, the relative humidity decreases. Once the target temperature has been reached, the air will feel comparatively dry, say 70%.
2. Maintaining the temperature with increasing relative humidity. During the day, the heating system maintains the constant temperature of said 20°C. Due to perspiration and transpiration of all inhabitants, cooking and showers, or drying cloths indoors (in airing cupboards), the air inside the house will absorb more and more water vapour increasing both the absolute and relative humidity. In the example below, about 2g water per kg air are assumed. Towards the end of the day, the relative humidity would have climbed up to about 85% in this example.
3. Cooling Down with increasing relative humidity. During the night the heating is usually set to a lower temperature, say 15°C. As the air cools down its capacity to store water vapour decreases causing the relative humidity to increase.
4. Cooling Down with condensation. Once 100% relative humidity is reached, condensation will set in, on the windows, or in all places that are cooler, like outside walls or behind cupboards. This will continue until the heating is switched on again.

The whole cycle is depicted on the following temperature humidity diagram, 1) heating up at constant absolute humidity, 2) absorbing humidity at constant temperature, 3) cooling down with increasing relative humidity, 4) cooling down with condensation. The lines in shades of blue show the absolute humidity [g water per kg air] for constant relative humidity [%] as function of temperature [°C]; the lines in shades of green show absolute humidity for constant enthalpy [kJ/kg] as function of temperature. Enthalpy is a measure of energy contained in a substance, here the humid air.

Daily Relative Humidity Cycle with Windows Closed at All Times

As a consequence of this cycle, inhabitants and their water vapour producing habits will add water vapour to the indoor air every day, and every night some of this water vapour will condensate somewhere. The result is a rather unhealthy indoors climate which is great for fungus and mould to grow in all areas where condensation forms. These fungus will produces spores which may affect the overall health of the inhabitants and exacerbate health issues such as Asthma and other respiratory diseases.

Airing at Least Twice a Day

When I was a boy it was my duty to open all windows in the house in the morning for 10 minutes and close them again, in the cold Austrian winter with -20°C outside. I hated that chore and only now I understand the purpose and the benefits as we apply the same in our household. For instance, yesterday morning at 7am the temperature outside in Cambridge, UK, was about 5°C with a relative humidity of 80%, which results in about 5g of water in 1 kg of air. This is the beginning of our sequence (not quite a cycle):

1. Heating up cold air. Make sure that all the stale, humid air leaves the house and is replaced by the cold air from the outside; even though its relative humidity is quite high, the absolute humidity is low. Once the windows are closed and the heating kicks in, the cold air is heated up to 20°C. Given the low absolute vapour content to start with, the relative humidity will drop to about 30–40%, i.e. very very dry.
2. Maintaining the temperature with increasing relative humidity. During the day, while the heating system will maintain a constant temperature of 20°C, too. Similar to the above case, the air inside the house will absorb more and more water vapour increasing both the absolute and relative humidity. Again, about 2g water per kg air are assumed to be added. Towards the end of the day, the relative humidity would only have climbed up to about 50% in this example.
3. Cooling Down with increasing relative humidity. During the night the heating is usually set to a lower temperature of 15°C, too. Similar to the above case, the relative humidity will increase, but not as much as before, perhaps only to about 70%. In the morning, however, all that humid air will be expelled and the cycle can restart.

The whole cycle is depicted on the following diagram, 1) heating up at constant (low) absolute humidity, 2) absorbing humidity at constant temperature, 3) cooling down with increasing relative humidity.
Humid air is expelled at the end of the cycle.

Daily Relative Humidity Cycle with Complete Air Exchange.

As a consequence of this cycle, water vapour produced by the inhabitants and their water vapour producing habits will be expelled several times a day; over a longer period all indoor humidity will be shifted to the outside. This habit acts as a water vapour pump shifting humidity from the inside to the outside. The result is a rather healthy indoors climate.

Conclusion

If you want to maintain a dry indoors climate, allow 10 minutes airing for a complete air exchange with all windows open at least in the early morning and late evening (when the outside temperature is lowest and the water content in the air at the absolute minimum), and also after every cooking session, bath or shower. Even though it may sound counter-intuitive, the energy you loose is not much compared to the quality of life you gain. In addition, the decreased relative humidity in the house results in a smaller thermal capacity of the indoors air which is easier to heat. In due course you will have no issues with mould any more.

It would of course be better to have an HVAC system, with controlled air circulation and heat recovery. Retrofitting such as system in an old house, however, would be difficult and costly. Easier it is just to keep airing.