Looking at this info, it seems like the DS2438 reports a higher temp when its powered.
The board may be heating up and/or the DS2438 itself heats up with VDD power.
You could try running the humidity board unpowered (and pull JP1), and see what happens.
Owfs has a slight timing problem with hobby-boards hubs and parasitic sensors. It is suppose to be fixed in CVS, so will have to see. Before I powered my DS18S20 on the board it reported 185, nope not 85, 185. Anyway, works now since powered it. So I am going to leave it powered.