Tuesday, November 23, 2010

Humidity Control

My "tfa humidity meter 20-99% rH" arrived this morning (pictured) and my first act was to put it outside the front door. It took 40 minutes to converge to a static value showing the temperature as 9 °C and the relative humidity (RH) as 84%. That's damp.

I then took it into our living room which I have been religiously humidifying this last few days (but not this morning), full of anticipation as to how much unsupportable dryness it would show.

Here are some facts about relative humidity (RH).

  • RH below 30%: too low - leads to dry skin, sore eyes, sinus trouble.

  • RH 30% - 40%: some people like it; a bit on the dry side.

  • RH 40% - 50%: OK, comfortable.

  • RH 50% - 60%: OK, comfortable but the dust mites like it too.

  • RH greater than 60%: damp, encourages mould, drains you of energy.

I was hoping for a target RH of 40% - 50% then. After an hour and a half of acclimatising, this is what the meter told me: temperature 22 °C and RH 49%.

So that sounds pretty good, but then I wondered: so was the humidifier actually adding any value at all? After all, it hadn't been on during the morning, had it?

Now there's a rule of thumb which states: "the relative humidity will drop by a factor of 2 for each 10 °C increase in temperature (assuming conservation of absolute moisture). For example, air at 20 °C and 50% relative humidity will become saturated if cooled to 10 °C, its dewpoint. Air at 5 °C and 80% relative humidity warmed to 20 °C will have a relative humidity of only 29% and feel dry." This means that for each drop in temperature of 1 °C, the new RH is 2-0.1 = 93.3% of its previous value.

So taking the outside air at 9 °C and 84% RH, and heating it to 22 °C (13 °C increase) one would expect to decrease the RH from 84% to around 34%. The air in our living room was comfortably more humid than that and I credit my recent acquisition.

Equally on a cold, dry day with an RH of around 60% (forecast for later this week) a temperature increase of, say, 15 °C would decrease the RH to 1/2√2 = 35% of its previous value (i.e. to around 20%) and that would be way too dry without my little humidifier doing its bit.

Find below an (edited) more detailed story on relative humidity.


The ideal indoor humidity level is either 35% to 45% or 40% to 50% depending on who you ask. Thankfully, the human body is quite flexible and you do not have too aim at an exact figure. The important thing is to avoid extremes. Living indoors is not entirely natural. The artificial environments that we create for ourselves can sometimes cause extremes of humidity to occur. For short periods of time this is nothing to worry about, but the long term effects can be quite unpleasant.

If the humidity level regularly exceeds 50% you are likely to experience a rapidly increasing dust mite population, which will affect allergy sufferers. Permanently damp rooms tend to have a musty smell. The damp air is a perfect breeding ground for mold, mildew and fungus, which can cause serious health problems. Where ever possible the cause of such conditions should be removed, but it is not always that simple and a dehumidifier may be required. A modern dehumidifier with built in humidistat can be programmed to maintain a humidity level below 50%.

Consistently low humidity levels are also bad. This tends to occur when the weather is cold outside and we turn up the heat in our homes. The most noticeable effects are a sore throat and sinus pain, symptoms that are common in modern society at certain times of year. You can also get dry skin and itchy eyes. The solution is to buy a humidifier that will put moisture back into the air.

If any of these symptoms sound familiar, maybe you should buy a hygrometer (humidity meter) and take some readings to find out if humidity is the cause. With modern technology at your disposal there is no need for your health to suffer.