"Solar Flare" wrote in message
...
Isn't the reflective surface only effective if there is an air gap next to
it?


That's my understanding. It reduces the amount of heat transferred across
the air-gap by radiant heat transfer.

Since convection is the larger heat transfer mechanism for heat flow upward
(such as in an attic in a cold climate), they are not as effective when
trying to stop heat loss. So you usually only see them touted in situations
to stop the heat flow downward. Such as a hot attic to an air-conditioned
space, or from a heated room downward to an unheated crawl-space/cellar.

I have read some testing reports that show those 3/8" doubvle radiant
barriers under slabs to have a non-measureable difference to no insultaion
at all. ****ed off my concrete guy but I gave him a print out of the
report and now I can't find it again.


Yeah, putting some other material in direct contact with the foil, such as a
layer of wallboard, or a concrete floor pretty much nullifies the affects of
the foil. Of course, if the foil is over one inch of foam board, you still
have the one inch of foam and its insulation value. But not worth paying
any extra to get the foil.

daestrom