The Natural Philosopher wrote:

Even 3mm of foam is a heck of a sight better than 9" of brick.

Wrong. Thermal resistance (R-value) is thickness divided by the thermal
conductivity of the material: R = t/k.

- for 3mm foam: t = 0.003m, k = 0.03 W/mK typically,
hence R = 0.1 m^2K/W approx.

- for 9 in. brick: t = 0.225 m, k ~ 1 W/mK,
hence R = 0.2 m^2K/W approx.

So 9 in. of brick is about twice as good as 3mm of foam, so far as
steady state heat loss is concerned.

--
Andy