[Christ]

Adding mesh is a good idea to protect your investment. Adding window screen will not restrict vacuumed airflow, but will help to redirect pressure flow.

It's like water... surface tension on water is very easy to break with a vacuum, but the more inertia you apply with an object, the harder the water becomes.

When you add something like a window screen in front of your radiator, it acts like a pre-filter, and won't allow high inertia flows, such as those coming at your car at 70mph, due to pressure buildup in front of the restrictive mesh. Vacuum flow, however, will be mostly unhindered, especially if the mesh isn't flat.

I'm not sure if that fully explains it...

I know - shoot a water hose at a window screen. The water coming out the other side is going significantly slower. Now angle that screen, and shoot the water at it again. You'll see that more of the water goes over the top of the screen, especially if you add pressure to the water. (Put your thumb over the hose) Obviously, some water will flow into the screen, but not nearly as much as if there wasn't a screen there.

Adding a vacuum to the back side of the screen reduces the pressure behind it, meaning it will "pull" more of the active high-pressure air through it before it goes up/over/around it.

Does that make more sense?