The therm software can be used to create a 2D thermal analysis of a particular detail.  The software is free and can be combined with the "window" software to produce a variety of U-value results.  The downside is that it is a bit buggy and can have crash out half way through, if you persevere you can get it to work well but it does take time.  If you are willing to pay then Bisco is a better sollution on account of the mesher and DXF auto definition.

Follow the link to get your copy of therm - below are 3 examples

http://windows.lbl.gov/software/therm/therm.html