News wrote:

I always thought the shiny side reflects, so needs to be facing where heat
needs to be reflected back

When heat radiation strikes a surface, it's either transmitted, absorbed,
or reflected. Kirchoff said "It has to go somewhere," ie T + A + R = 1. If
T = 0 (an opaque surface with no transmission), A + R = 1. If the surface
emits as much power as it absorbs, E = A, integrated over the whole spectrum
(R is an energy conservation wash.) So a foil has reflectivity 1-E, which
is large if E is small, ie it's a good heat mirror. It can stay cool because
it doesn't absorb much heat, and it won't lose much heat because it's at
a low temp and it emits poorly.

and there needs to be a 1" gap between that and any other surface.

Big gaps with less still-air conductance are good for downwards heatflow.
A 1-1.5" gap is good for sideways heatflow. Smaller gaps have more still-air
conductance and larger gaps have slightly more "convection conductance."

Having it under floors facing down should not be effective.

It should be, if there's an air gap beneath the foil.

Nick