Mike,

I have read their site, but went further and looked into the moisture
absorption of Polyisocyanurate. I think they overstate the problem -
probably to try and sell their vacuum panels.

According to the manufacturers, it absorbs very little moisture (way less
than the building codes require) but when it does, it reduces the R value to
80% of new and then stabilises . But, the R value quoted in the literature
already takes this into account. The reduction takes place mainly around the
edges if they are not sealed (often the case in house construction). The
aluminum foil (both sides) minimises moisture entry through the surface and
if the joints are well taped, this insulation should be fine. It does have a
better R value that the Blueboard and the aluminum foil acts so as to reduce
radiant heat transfer into the box. It is widely used to sheath houses under
the siding and once installed is there for good - Just like our iceboxes!

Besides, I am only adding to existing insulation and I already have the foam


But, you are right - Styrofoam would be fine, but it needs greater thickness
and I don't have much space.

GM

Rethink your foam choice.

Glacier Bay has a cool site at:
http://www.glacierbay.com/Instresult.asp

Here you'll ultimately conclude that anything but Extruded Polystyrene
(Dow "blue" Styrofoam Square Edge) is crap, because it falls apart or
absorbs moisture.

Mike Worrall
Los Angeles