Can you use a foil barrier for a thermal break?

They forgot to put blueboard on outside of house over studs around windows so we're trying to create a thermal break on the inside on the studs...

Foil is not a thermal break. Aluminum foil is an excellent thermal conductor (just the opposite of what you want).

We were trying to avoid XPS because of the flame retardant and HCFCs so someone had suggested Reflectix (foil faced bubble wrap).  If that won't work, what else can help?

If we do use XPS, which is the "least toxic" if there is such a thing (this is a "healthy house" - or trying to be)?

These studs are on south side of house between windows -- interior -- they did not put insulation on exterior.

Your original question was about foil barrier, not foil faced bubble wrap. There is a difference as the bubble wrap does provide some resistance to heat flow.

How much space do you have to work with here? How much space from the studs to the inside edge of the wall? What is your inside wall covering material? Drywall or what?

Inside material is drywall.
We're defining space between stud and wall so we were thinking 1 inch foam if we did foam. For asethetics we wouldn't want to go too thick.
This is a wall of windows with wood framing in between. The rest of the house (exterior walls) is blocks.
Currently the wood studs are very cold. I keep hearing different opinions -- e.g. if foam wasn't used on exterior studs, anything we put on inside won't make much difference.

I'd use foam if I knew it would not offgas flame retardant - but I don't think drywall will prevent it from doing so. They don't smell, but I know that stuff bioaccumulates. However, we also don't want to be cold when in the living room.

It doesn't really matter which side of the studs have the thermal break.

How many square feet of surface area are we talking about here?

What about 1" fiberglass duct board or 1" rockwool board? I'll let you google them to see if they might meet your needs/wants.

What makes you think something like EPS foam will off-gas flame retardant under normal conditions experienced inside the wall of a home? I assume you read this somewhere. Got any links or references?

Posted By woksawi on 15 Aug 2011 01:14 AM
...Currently the wood studs are very cold...

Where are you located?  It is pretty warm where I am.  We hit an all time record high of 114F last week.

It's warm now but they were very cold in winter (our project was on hold for a bit). We're in mountains of Colorado. They need an answer today so I might have to just use the rigid foam which isn't very green - oh well.

A low-e material such as aluminum foil has an emissivity value of 0.03 and a reflectance value of 0.97, meaning it reflects 97% of radiant energy and emits only 3%.