TPS question. Open cell mandatory?

The STS reference manual said that one of the things which makes the
shuttle tile so effective is that their structure allows "internal re-
radiation" of heat between different parts of the tile. It also said
the production method results in an open cell structure.

My question is twofold. Firstly does anybody know if this effect
exists with closed cell ceramic structures or can it only works
because each individual cell is a skelton rather than a full bubble?

I know Aerojet pitched closed cell ceramic foams and I believe they
were rejected on weight grounds (s.g of .34). These foams are readily
available in Europe as construction materials. They are fire rated to
at least 1 hr (believe this would be 1600c, but I have never seen a
temp spec on the std) and lightweight grades are down to s.g. of .17,
with a crush strength of 6atm. However I wonder if they popcorn if the
pressure difference or the rate of change of the pressure difference
(preventing pressure equalising between the cells and the environment)
is too high.

Thanks in advance for any help.

Steve