Sunday, 11 November 2012

Too much humidity in the summer

Some of the thermometers in the house have been dutifully recording humidity for over a year now, but for the first few months I was largely ignoring that, much more interested in the temperature. Humidity is, of course, important for the health of the building and of the people in it. If the humidity is too high, there will be condensation. Condensation provides an ideal habitat for molds and mildews. Dust mites also like humidity, so high humidity means more dust mites, which in turn cause more allergies and asthma for people.

If the humidity is too low, the wood in the building can dry out and shrivel up. This may not have huge structural consequences, but can lead to warped plaster board and cracks in the paint work.

The comfort level for humidity is between 30 and 50%, apparently. Or between 40 and 50% or between 35 and 45% depending on which website you're reading. Our house was usually in that range in the first winter, but over July and August was in the 50 to 70% range. 

The human body generates heat at around 100 watts, and has to lose it somehow to avoid overheating. The main method of heat loss is evaporation, and the more humidity is in the air, the less effective this is. This means that if air is very humid, it feels a few degrees hotter because we judge temperature by the amount of heat we lose. If the air is very dry, it can feel cooler, but this can also lead to dry skin and respiratory problems.

Humidity is not presented as an absolute quantity of moisture in the air, but the amount of moisture relative to the maximum the air can hold. As air gets hotter, it can hold more moisture, just as hotter tea can hold more sugar, although technically speaking the humidity is not dissolved in the air as the sugar is in the water. So as the temperature goes up, we can expect the relative humidity to go down, and vice versa, as we can see on this graph of the temperature and humidity inside and outside on a couple of days in the summer. The total amount of moisture in the air, both inside and outside, is not changing very much.