Category Archives: Hanwell

‘RH 101…’

'Just before the penny dropped..'


‘A lesson in Humidity…’

The concept of ‘Condensation’ is a perfect example of humidity. The temperature at which this occurs depends on the level of moisture in the air. Inside this can be seen on window panes as the temperature of the glass falls, and outside, when the temperature drops, condensation can form on blades of grass, and given rise to the term ‘dew point’. Localised RH fluctuations can also affect objects and these can be simple things like sunlight or a radiator. Ever been in a stuffy attic in the summer? That’ll be the low RH then… Dramatic fluctuations can have dramatic results and cause canvas and wood to expand and contract at different rates to the paint layer, so cracks will appear and in worse cases flake and fall off. Among the many gadgets that are available to the humble conservator are those produced by the good people of Hanwell. A system of wireless units placed throughout the house sends info to our main computer to be displayed in a graph format. This is, once decoded; a quick reference for maintaining a happy house as you can monitor the temperature and RH from the comfort of an office chair before any decision about adjustments needs to be made. It did take a little while for my tiny brain to re-organise what I was looking at, hoping I didn’t try Katy’s ‘saint-like’ patience with me in the process! It’s a great tool to use, to gage results from days to weeks to months and even look at results from one year to the next. A handy warning light on the system tells you instantly when areas are in the danger zone and a relentless ‘bleep’ doesn’t let you forget!

‘An education in fluctuation…’

‘It’s all Relative…’

One of the most important aspects of conservation in an historic property is the concept of ‘Relative Humidity’ or RH for short. This was and still is a slightly tricky concept for me to get my head round, as maths and physics were never my strong subjects, so I’ll try my best to explain. (Mentor Katy D took great delight in firing off pop-quiz style questions, and watching me squirm…) Humidity is a fairly destructive mistress and can have a myriad of effects on a collection. A simple way to dive in to the subject is with a definition: ‘‘Warm air, holds more moisture than cold air’’. We keep an eye on this with percentages, and the dictionary definition of Relative Humidity is the ‘moisture content of the air, divided by the maximum amount of moisture the air can hold at that temperature’. 0 = very dry, and 100 = very damp. The target range for historic properties is between 40-65%. This safe zone is to fend off mould and corrosion when the air is too damp, and if too dry, the air can draw moisture from organic objects, and cause cracking in wood. So the temperature alone is not an issue but its effect on RH is. During winter months, when heating gets switched on, the air get’s drier and stuffy and it tries to absorb moisture from objects. Then when the heating is off, the air gets damper and the materials try to absorb the extra moisture. But, the RH can also increase when the temperature remains the same, such as on a rainy day. Of which we have many to due to our ‘unique intermittent weather system’.