OK so i pretty much agree that a metalised “space blanket” will reflect the body heat back (and so will the metalised babble wrap stuff for insulation). But does this not then conduct well when placed on the body? THinking sweathy runner wrapped in foil some benefit from reflected heat but is there a loss from conducted heat (or indeed van conversion)?

it is made of BoPET (similar plastic to a coke bottle) and has a very low thermal conductivity.

for a van conversion it’s mostly used against the metal of the van as a vapour barrier, to stop moisture rusting the body work and then PIR foam on top.

Although the space blanket will have a lower thermal conductivity it is too thin to really make use of this so PIR (kingspan etc) is used to bulk out a bit. hope that makes sense?

The space blanket only works when it’s wrapped loosely around the person. The air gap between provides the insulation, and the emitted body heat is reflected back into the cavity.