Insulating a truck roof
This is only a small 1940 Chevy cab roof- just bought some 1/4″ closed cell foam w/alum on both sides- they claim its Low e, but I am not expecting it to be perfect, just maybe slow the suns rays a little and also to lower that roof bottom so the headliner material will blend into the sides better? My idea is that I will attach the cloth headliner material directly to this stuff, but my instinct tells me it would be best to have an air gap between the bottom of the steel roof and this foil/foam. Not sure how to do that and keep the stuff looking flat, but maybe you can till me in this instance I dont need an air gap since Im not expecting too much protection anyway. I have considered laying some sort of strips under it for a slight gap, but not sure how or if it would be worth the trouble.
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George,
It's going to be hard to maintain an air gap in this situation. The insulation material you are choosing is quite thin -- understandably, because you don't have much room to work with in a truck.
Installing 1/4" of foam can't hurt. Don't overthink this -- just give it a try.
The solar gain through the windows is a much bigger issue than conducted heat through the roof, but a low-E headliner might provide a subtle improvement in comfort. If you cut some 1" squares of the 1/4" goods and glue them to the roof you can probably maintain at least a small gap. There may also be a bit of improvement if you paint the underside of the roof with a low-E paint before installing the foam.
I like DD's 1" square piece idea. I wouldn't bother with the low-e paint--the low-e on the foam is going drop the radiation heat transfer down so low that it wouldn't matter anymore. And those paints are both expensive and not very good.