On December 15th we downloaded the first set of data for the July-December timeframe and sure learned a lot. Here's a graph showing temperature, humidity and dew point. You will definitely want to click on the image below to view a larger version:
"Consider the importance of dew point (DP), which is the temperature at which the air cannot hold all the moisture in it and water condenses. Unless it is mechanically humidified or dehumidified, the air circulating through the building will have the same absolute moisture content, or dew point, as the outdoor air. The dew point determines what combinations of temperature and relative humidity (RH) are possible in the storage environment. At a constant dew point, when the temperature goes up, the RH goes down and when the temperature goes down the RH goes up."
The Image Permanence Institute (IPI) has a great on line tool that allows the user to change one variable (temp, dew point or relative humidity) and solve for the other variables. It then provides a risk summary based on those outputs. To try out the model CLICK HERE It's fun and fascinating. For example, using the recent NCCHP temp of 30 and RH of 70, the model produces a dew point in the low 20's (matching our on site data logger) and the risk evaluation shows no mold risk but a metal corrosion risk (condensation).
Our graph is a textbook example, especially in December where the lower dew point drives humidity over 90%, higher than the muggiest days of August. When the dew point drops with the temperature the humidity percentage is driven up. Without dehumidification equipment there is no way to bring the humidity down to the 30% to 50% recommended humidity range for archives storage. As the IPI reinforces, " controlling dew point is the key to managing to managing material decay."
Setting aside the science, what it means is that one of our long term goals will need to be the achievement of a more stable archive environment. Things like insulation and heat are good starters, with humidity controls and dare we say "air conditioning"? On a day like today in Granville we'd settle just for some heat. So if you are planning on donating a large (or small) bundle of money this year, you still have time to donate to NCCHP!