robgraham wrote:

My workshop is relatively air tight so as insolation raises the
temperature there is not in ingress of moist air to match this rising
temperature, and hence cause condensation when it cools in the
evening. That's a possible theory anyway !!

Very approximately, the number of grams of water per cubic metre in
saturated air is the numerical equivalent of the temperatu at 15
degC the air can hold 15g of water per cubic metre.

Let's say your shed has small swings in temperature, from say 6 degC
overnight to 10 degC during the day. If the humidity remained constant
at 50 percent, then even if the machinery remained at 6 degC because
it warmed up slowly, there would be no condensation as then the dew
point at 10 degC and 50 percent (relative) humidity is 5 degC:

The air is 50 percent saturated (50 percent RH at 10 degC); and 50
percent of 10 grams (saturated air at 10 degC) is 5 grams. but 5 grams
is enough to saturate air at 5 degC, and so 5 degC is the dew-point.

If, however, the temperature swing was from 6 degC overnight to 13
degC all at 50 percent RH, then the dewpoint would be 6.5 degC and
condensation would take place.


TF