Normally people are looking at dew point in a gas or for precision equipment, in which case you need to know the atmospheric pressure.
But for your purposes, you can just use a pretty easy approximation based on temperature and humidity:
If you are interested in a simpler calculation that gives an approximation of dew point temperature if you know the observed temperature and relative humidity, the following formula was proposed in a 2005 article by Mark G. Lawrence in the Bulletin of the American Meteorological Society:
Td = T - ((100 - RH)/5.)
So you can use any ST-compatible sensor that gives you temperature and humidity, put it on the floor of the attic and use Webcore to calculate a value you can then trigger off of.