Rough formula for calculating moisture content of air (works for me):
%RH*4.1561*EXP(0.0619*T)
T is dry bulb air temp in °C
result is in grams of water per m3
The rest should be plain sailing
edit: IIRC, this is useable (ie <10% error) between 0 and 30°C
edit2: pretty good stuff here-
http://www.coolit.co.za/psychart/chapter1.htm
(I think the Magnus formula is more trustworthy than mine...)