[Exeric]

Something that very few people realize is that XPS, EPS, and Polyisocyanurate foam insulation have significant off gassing potential when used in the wrong applications. My understanding is that some of those gasses can be released at accelerated rates when in a high temperature environment. The situation is even worse when in a closed system in which the air is continually recycled from solar heater to house and back to solar heater. Some of the effects of that off gassing have been found to sensitize individuals over time and can result in chronic asthma, etc. Not good.

On the solar heating that I'm working on I'm trying to be careful not to use any of those products that will be exposed to continuous warm or hot temps. A good substitute is ductboard. It is a fiberglass product that is very stiff and has one side with a foil backing that you can paint black. You don't have to worry about fiberglass flaking off like regular fiberglass insulation. They've been using it for years in return air ducts of closed system HVAC systems with no bad results reported. Only problem: they are expensive compared to the foam boards.

Edit: I guess I should add that there are plenty of applications of using foam boards for insulation that wouldn't be problematic. But there is a reason besides fire hazards that foam board usually must be covered with drywall when in a living environment. You might even be able to get away with out getting sick using foam material that you are directly exposed to in some situations. But a situation where air is continually moving past foam board in a heated closed system does not seem like an ideal, or even a GOOD application for it.