Hi,

You might want to try prefab aluminum insulated panels, usually sold with
porcelain finish on one face for a nice exterior appearance, but probably
available on special order without, and less expensive that way too.   They
consist of two sheets of aluminum bonded to a closed cell foam--very high R-
values.

Mapes Industries in Lincoln, Nebraska, USA, is a manufacturer of such.  Since
the panels are rigid and lightweight, they can be applied in many different
ways, to suit the substrate, and the substrate doesn't have to be structural
or the surface sound, although in your case that's probably not an issue.
:-)   The aluminum is a pretty stable material also, not subject to water
damage, mold, etc,  as masonite panels are.

The only drawback might be if your building has a lot of rounded components or
irregular architectural details, like towers and such.  In that case, maybe
there is a way you could use a flexible membrane/insulation system similar to
those used in  commercial roofing.  They are usually horizontal and
ballasted, but you could probably get a manufacturer to engineer the
application and recommend a different method of  installation.   The key is to
stick with closed cell foam material so as to get the higher R-values and
avoid condensation problems down the road.

There is probably some type of spray on product, but I would be reluctant to
spray anything permanent and gooey on something from the 14th century that ppl
in the 23rd century might want to restore to it's original condition.   ;)

FYI, from an energy standpoint, you usually _would_ want to insulate the
outside of any massive masonry structure, as you can better control the
thermal flywheel effect that way.  When you insulate masonry structures on the
inside, you can have problems with condensation, deterioration, and mold at
the interface, and so have to pay quite a bit of attention to installing and
maintaining a seamless vapor barrior, which is rather difficult to do.  That
is not such an issue with exterior panels, because the difference in temp is
not so great from inside to out, once it's been spread out and tempered by all
that mass.

On the downside, when you insulate a massive structure, you lose any exterior
passive solar gain benefits you might have had, once you isolate the masonry
from the sun.  That is not always an issue,  perhaps you have big south facing
windows and so can trap the gain inside the structure rather than collecting
through the surface of the building, but something to consider in some
situations and climates.

Please post updates on the project!

Deb Bledsoe, lurker
Reily Twp,  Ohio