Recommended air gap for radiator

[Elerek]

If I want to mount a radiator directly against and facing a surface (in this case, the plate the motherboard is mounted to) What would be the recommended air gap to maintain to allow adequate airflow out the sides? Is there some ratio or radiator surface area to ventilation surface area I should maintain? I'm assuming it doesn't need to be 1:1 since a radiator provides a lot more resistance than a bare opening.

Some implementation specifics:
I'm thing about either a 280 rad or 2x 240 rads in either push or pull (not both) to cool a high end cpu and 2 gpus (zotac 1080 mini or vega nano)
They'd be mounted under or on top of a custom console style case, so the less clearance I can get away with the shorter the build will be.

 

[SeñorDonut]

As far as I know, there is no clearance necessary as long as there's ventilation holes.

 

[Elerek]

As far as I know, there is no clearance necessary as long as there's ventilation holes.

This would basically be up against a metal or acrylic sheet with the motherboard, psu, and gpus directly on the other side, so not really anywhere to put ventilation holes. Airflow will almost exclusively have to go through the air gap/standoff space between the radiator and the sheet it's mounted to.

 

[SeñorDonut]

This would basically be up against a metal or acrylic sheet with the motherboard, psu, and gpus directly on the other side, so not really anywhere to put ventilation holes. Airflow will almost exclusively have to go through the air gap/standoff space between the radiator and the sheet it's mounted to.

In that case, I'd give it at least 20mm of room, 10 might be fine, but I can't say for sure until I see how it's laid out. Do you have any pictures?

 

[Elerek]

In that case, I'd give it at least 20mm of room, 10 might be fine, but I can't say for sure until I see how it's laid out. Do you have any pictures?

It's just a hypothetical case right now, I might have some 3d models soon, but no pictures. I think I might flip it and put the radiators on the populated side of the motherboard so I can have some extra ventilation and gain some airflow over the chipset and vram.

 

[Biowarejak]

Should be fine, just expirement! My only concern is mounting, you'll have to go through the other side with long screws and use spacers. The 25mm-fan screws used for your typical radiator should be fine.