1 x XR5 temp will equal 5 x XR temp when they are all warmed up and working.
Every physical engineer can tell you that what you state here is not the case. 1x a certain heat source is not the same as 5x the same heat source. If that would be the case one radiator in my house would be enough to keep my house warm....
5 XR's will produce 5 times the amount of heat (measured in calories, joules or ultimately Watts).
Thus 5 cards will in the end heat the inner temp of the enclosure more then only one.
Off course, the hotter the enclosure get the more heat loss towards the outside environment, and the lower the outside environment temperature the more heat loss from within the box.
Also, if your enclosure is receiving direct sun light the sun's infra-red radiation will heat up the enclosure to higher temps then the actual outside air temp. would suggest. It is just a greenhouse made of plastic/alu/steel.
On the other hand, wind will cool down a enclosure faster if the air temp is lower then the enclosure. Off course it will heat up the enclosure if the air temp is higher then the enclosure.
It's all about balancing heat production against heat loss.
So, factors rising the radio temps are:
- heat produced by the radios themselves
- plus the heat received by the sun
- the outside air temp. if this is higher then the inside temp.
- the thermal insulation value of the enclosure.
On the other side of the balance we have the heat loss factors:
- heat loss to the outside air temp. if this is lower then the inside temp.
- the thermal insulation value of the enclosure.
Temperature=energy is always looking for a balance. That is a physical law nobody can deny. Thus the more heat you produce with the radios the higher the temperature will rise. But the higher the temperature of the enclosure rises above the surrounding air mass the more heat loss will take place, until the heat loss balances out with the heat production.
In this respect I think it well possible that if a enclosure is placed in the sun, in a hot surrounding environment, like in a desert, no wind and you have one or more radio's working hard, the inside temp can reaches temps that easy outrun the max. working temps of the radio's.
Theoretically there is no max temp the enclosure would reach. In real life their will be a maximum, probably the temp the radios will die with. (If they are dead the temp probably drops since no more heat is produced by them.)
Food for thought....