Thanks to everyone for great insight I have gotten so far. I have another question.

We are in the Washington, DC area and are adding an addition to our 1938 brick colonial. The addition is over a crawl space (footers poured and block wall going up today). We are planning on using the insulation approach in Figure 7 showing on the building science article here: http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces since this won't be a conditioned space.

For heating, the house has been radiant heat and we were going to use European panel radiators in the new space (Runtals), but have decided we would rather go with radiant floors using Warmboard (www.warmboard.com).

But when I look at warmboard with the seamless aluminum cladding on the top, I wonder if it will act as a vapor barrier and if we will have trouble with this and using the insulation noted above which basically creates a vapor barrier on the bottom. If I understand correctly, it is a bad idea to have a vapor barrier on both sides of a wall or floor. Will the warmboard act as a vapor barrier? Do you think this could create problems? Should we leave small gaps between the sheets of warmboard (1/8 inch or even 1/4) or even drill a series of holes through the warmboard? Or am I getting worried about a non-issue?

I'll be calling Warmboard about this as well, but I'd like thoughts from those here as well.

Thanks.