You need Nigel. It's not not just shiny side down for water resistance. Get an old one and try burning it. The foam side melts straight away with heat applied. The shiny side doesn't. Likewise you could just try holding it close to an exhaust. Ask me how I know... Shiny side down
I fitted mine in replacement of the engine lid seal. The old rubber one was too rubbery. Seems to work well and the shiney/shiny side facing out leaves somewhere for me to wipe my fingers after eating cheesey/cheesy balls in the general vicinity of the rear of my camper.
There is no tick box for "lets try both ways" was shiny up as web site, now shiny down as it makes more sense!