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I'm working on a project with a set of individual LEDs (datasheet) permanently set up in an outside setting in winter, at night.

I need to build separate enclosures for each of the LEDs because they are only rated IP20, but they will need a heatsink too as I will be driving all three of the channels at the same time.

The whole setup will be under a cover but outside, and therefore there is naturally the chance that moisture will get into the system, so I want to waterproof the enclosures. The enclosure will just contain the heatsink and the LED - the driver will be housed elsewhere and connect to it via an as-yet-to-be-decided waterproof solution.

How can I achieve this without heat from the heatsink building up on the inside of the enclosure and causing condensation due to the daily heating up and cooling down? Should I perhaps use an aluminium enclosure and connect the LED heatsink to that on the inside, or even connect the LED directly to the inside of the aluminium enclosure, so that the heat leaves the system more easily, or am I going about this the wrong way and humid air will always get in due to the daily pressure changes, and I should instead use a "leaks more on the bottom than the top" strategy? Could silica gel be an option?

I also wondered how one would work out the correct heatsink to use if the enclosure is sealed. I know that heatsinks generally assume some kind of convection in the system, which this will not have.

If it's useful, the average maximum temperature of the outside environment will be 9.0 °C | 50 °F and minimum - 1.0 °C | 30.0°F, definitely no colder than -10.0° | 14.0° F.) Also, I'll be connecting the LED to the heatsink using 3M Thermally Conductive Adhesive Transfer Tapes 8810 (datasheet.)

There is a chance I will use even higher powered LEDs such as these (datasheet), so it would be great to know the kind of calculations required.

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The standard technique is to thermally couple the heatsink to the enclosure and then dissipate to the outside the enclosure. Typically: one aluminum wall of the enclosure is often 'heatsink shaped' and you can attach to it to dissipate.

As for the thermal dilatation of the air there are special 'pressure compensation' bolts which are more or less filtered air vents. Humidity can be an issue, usually enclosure manufacturers make provision for that. First component that come up in a search (not endorsing or whatever) is something like this: https://hoffman.nvent.com/en-us/products/pressure-compensation-0

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