I am no expert on this but I suspect heat radiating off the heads contributes a good bit of the heat...
1) the surface area of the bottom of the UIM is many times larger than the relatively small LIM/engine contact
2) There are 2 plastic gaskets to reduce convection (above & below LIM) a bit.
I am sure convection transmits a good bit of heat but I think radiation does too.. As "anacdotal" evidence...I have had thermotec heat shielding on the underside of my UIM for over a year. It "seemed" to reduce the touch temp of the manifold to a noticable degree (for only $10 or so)..
