So, effectively, without a vacuum (or, at least, a free-space non-touching
environment), the addtion of aluminum foil merely acts to accelerate
(aluminum being an excellent heat conductor) heat transfer?

Yes, the heat would tend to conduct right through the aluminum. However,
given that the materials on each side would tend to have lower levels of
conduction, the aluminum probably has no effect either way. In other words,
if you sandwiched a layer of foil between two layers of insulation, the
overall conduction would be nearly the same as the insulation layers alone.

Thus, for example, the foil-faced building insulation products are no
better
than the level of vacuum behind them?

I'm not sure what the advantage of foil-faced foam board is over non-faced
foam board in housing applications. I have a call in to my brother who is a
building materials guru. I will pass on the information when I hear from
him.

I'd been migrating to the thought of layering heavy foil between the
highest
R-value foam I could find, and then doing a heat-sealed vapor barrier
wrap,
evacuated to the best of my ability. However, your comments suggest
that's
a waste of time.

Yes, I believe that would be a waste of time. My hot tub cover had a heat
sealed vapor barrier around the foam insulation. After a few years it got so
heavy from trapped moisture, I had to replace it. And, that was after
unwrapping it and letting it dry in the sun for three months.

I *think* I understand you to say that foil is counterproductive if not
faced with a vacuum. If so, from that, if I'm not going to spring for the
vacuum panels, simple block foam, encapsulated to prevent moisture, is the
best?

Yes, the foam is then the best way.

Some other thoughts:
1) Wet insulation is an excellent heat conductor
2) When insulation gets wet, it can be almost impossible to dry out.
3) It's almost impossible to totally encapsulate the insulation around a
boat ice box.
4) Imperfections will allow air to move in and out of the insulation.
5) When the air is cool and dense it will migrate into the insulation.
6) When the air is warmed it will expand, release its moisture, and move
out.
7) After enough cycles of cooling and warming, the insulation will be wet
from the released moisture.

This is why you have a vapor barrier in your house between the interior and
the insulation, but not on the outside.

These problems can be minimized by not trying to encapsulate the insulation.
Give it a way to dry out between cycles. Also, try to use closed cell foam
instead of open cell. You can test your insulation before installing it buy
breaking off a small piece, carefully weigh it, put it in a glass of water
for a few days, take it out and weigh it again. If there's no appreciable
weight gain, it should work okay on your boat.

My box exterior (which is a single layer of roving over the hard urethane
2") is currently exposed for most of two sides. I'd thought to put foil
on
that exterior surface. If I understand you properly, that's
counterproductive?

Unless the exterior is exposed to a strong radiant heat source, the foil
won't help.

On to the last:
Is that like the lead foam used in noise control, nearly as expensive as
heat shield :{)) ? Or is there some other noise abatement of which I'm
not
aware (there are probably encyclopediea worth of info of which I'm not
aware!)? Is this an application where a foil-backed insultion board would
help?

Yes, it's like the lead & foam sound insulation. I'm using a sound blocking
product with a back layer of foam, a layer of some type of semi-rigid
plastic, a thicker layer of foam , and faced with shiny Mylar. About an
inch and a quarter overall thicknes. I don't remember the brand name but the
price was not out of line. My local rubber products retailer carries it in
stock.

Foil faced insulation in an engine room would work to keep radiant and
convected heat away from living spaces. But shiny-faced sound control
products with their 'decoupled' layer would do a better overall job.

Low emissivity surfaces are very effective at minimizing heat gain from
non-contact radiant sources. This includes the sun or even a hot engine
block. They are not a solution to other problems.

Rusty O

Thanks again for your knowledgeable input.

L8R

Skip and Lydia