Monday, 17 January 2011

humidity and mold

By and large people in Europe (and many parts of the USA) have totally no knowledge of how humidity and mold effects life in tropical and sub tropical areas. Of course for these people probably won't benefit much from this article but the point is that the views and understandings of things in the world is often shaped by these peoples.

Queensland is an excellent example of why this is a problem. Here we have a population that acts and thinks as if its in Central Europe when it comes to many (if not most) things.

Humidity is the cause of mold (or mould for the GB spelling), you can't blame the spoors because they're everywhere anywhere.

So if you're trying to store or keep in good condition:
  • paintings
  • stamps
  • prints
  • camera equipment
  • lenses and optical equipment
  • down products
then you need to think about the humidity you store things in or you just have to accept that mold is going to get to it and destroy it sooner or later.

Mold spores start to grow when the humidity goes over 60% and love it when it goes over 70% ... the more the happier it is in growing. You need to go below 40% to kill it. Beware that I extended storage at below 50% can begin to damage papers and oil based paint dyes (as well as oils in lubrication of camera parts which is my main concern).

I can not recall my source of information on this, but other sources such as this one support my thoughts.


  • Keep the humidity level in your home between 40% and 60%. Use an air conditioner or a dehumidifier during humid months and in damp spaces, like basements.

In this part of the world in summer we seldom get that low. For example if you look at the BOM site for the Brisbane humidity: The graphs below come from this site earlier today.

If you look at the Humidity graph you'll see it goes up and down daily:

Perhaps you'll notice from the times (on the X axis) you'll that its at the driest time after lunch till nightfall. Actually it drops all day from dawn, especially on sunny days (like today was).

There is a relationship between the significance of relative humidity and temperature .. the actual amounts of water in the air do not suddenly change (this is why its relative humidity not absolute humidity).

So as the temperature of the air drops the amount of water which the air can hold reduces. This means that its closer to falling out of the air and becoming moisture.

Paper (as well as dust collected on surfaces and wood) is a sponge for this sucking it up this now more available moisture. The way to see when things are likely to get dry and when they'll stay moist and damp can be seen when comparing the dew point (the point where water falls out of the air and starts forming on things) to the temperature. The closer the ambient temperature is to the dew point the the more things will start to feel 'damp'.

You can see that as the temperature drops during the day that the dew point becomes closer in this graph: Temperature and Dew Point

I've marked the worst point and the best point (for keeping things dry and mold free), its the wors time because the dew point is just 2 degrees C away from the ambient temperature. So the slightest cooling will start condensation forming.

Unlike forming on a gold glass (if you live here you know what I mean) this is subtle, less obvious and not pooling on the table.

Today the dew point is around 18 degrees C, while the ambient temp is 26 ... great for drying things outside (as your gut feeling would tell you anyway).

Now we overlay the graphs upon each other we'll see that when the relative humidity (the blue line) is at its lowest which was (about 50%) we were at the point where the ambient air temp was higher.

This is important because we try to keep our homes and rooms cooler ... note that the dew point has actually gone up a little ... so this means that if your stuff is kept in a cool place it will become more damp than the warmer sunny places. There are no cool dry places in our environment.

How do you deal with this?

People can delude themselves to the worth of desiccants like Silica Gel but the only real way to deal with this is to use a dehumidifier system or an Air Conditioner.

Why won't desiccants work? Well for instance in my cabinet for my lenses (which is about 1000mm x 400mm x 400mm I'll pull 500ml of water out on a humid day. Yes that's right half a liter out of a sealed cabinet.

I built a lens cabinet incorporating a dehumidifier for just my camera gear, but if you have more stuff I suggest putting it into a room that has a small window mounted AC unit. In fact this works out better in some ways as the benefit you'll get increases as the scale increases. The old "surface area to volume ratio" comes in to effect working in your favor.

If you operate and AC or a dehumidifier it will consume energy. If you're interested in tuning the system so its not consuming energy (money) then your going to get your best effect turning it on just on dusk as the ambient air temperature starts to drop.

You will suck out more of the moisture as the air temperature starts to cool with the least energy because its getting closer to the dew point and the AC will not need to drop its metal surfaces (where the moisture forms) in temperature so far.

Remember AC and dehumidification requires energy ... the more you need to cool (or heat) something the more energy needed ... energy costs money. I use a simple device which has no fancy remote control system because I can plug it in via a clockwork timer and then forget about it.

Hope that helps

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