Originally Posted by Scott Davis

Here is a great list with pros and cons of many glazing options:

http://www.builditsolar.com/Projects/SpaceHeating/Glazing.htm

- Use only high temperature insulation.  Polyisocyanurate insulation works well (1).  Don't use polystyrene, as it is only good up to about 130F.  The pink, blue, and white rigid sheets of foam board are normally polystyrene

(1) Polyisocyanurate (sometimes called polyiso) is a high temperature polyurethane based insulation that works well in collectors.  Many lumber yards carry it, but they often don't know it by the name polyiso.  Ask the lumber yard person to look at what they carry -- if its polyiso, each sheet will say polyisocyanurate somewhere on the face.  This insulation is usual a tan color, and normally comes with face sheets -- often reflective aluminum foil.  Its slightly more expensive than the polystyrene, but also has a higher R value per inch.