Quote:
Originally Posted by Christ
Do the test. Then think about what you're saying.
In order for heat to radiate effectively, there has to be a differential between the area that is heated, and the area that is being heated. Hence the term radiation.
We know from physics that everything will follow the path of least resistance, which in this case, is the area with the largest temperature differential.
The heat is actually radiating OFF the roof, into the open air. The headliner is there so you can't touch the hot roof, it's there for aesthetics, and for sound insulation. It's not there for temperature insulation. (Cardboard is crappy insulation anyway.)
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How I see it,is that the ultraviolet radiation,which is streaming in from the sun,is converted to infrared as it is intercepted by the metal roof.For the albedo of the roof,heat will build until it reaches some equilibrium temp.Inside the car,you'd be exposed to infrared radiation eminating from the sheetmetal above,and since the roof and headliner has a composite heat transfer coefficient ( the inverse of it's R-value) the air inside the car would also build do to conduction across the temp differential(delta-T)from the roof to cabin air temp,until some equilibrium is reached ( about 140-degrees F around here in the summer).The headliner(mine has some polypropylene foam in it) acts as a buffer,providing some shielding from the infrared,and also slowing heat transfer,to give the AC a little break.So I'll think of the headliner in terms of heat gain or heat loss for either the heater or AC to deal with.