If one side has a metallic surface it's probably there to protect against heat. The heat is coming from the cylinder head so I would guess that would be the side to face the exhaust ports. Besides, if you've ever had to scrape a header gasket off a cylinder head before :: you'll appreciate the metal face.
I always thought the foil went against the exhaust manifolds or headers. I've seen my headers glow red hot so I think that's the hotter side. The heads are liquid cooled so I don't think they get as hot.
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