Originally Posted by seiyafan
you can buy a MERV 13 filter and cut to the size, but air flow will suffer.
If You base Yourself on a comparison of 1-layer glass microfibre media, You're right.
But not as a general rule, it all depends on the type of media (fibre diameters + mix, gradient structure). Triple layer PES media or double/triple layer Meltblown media gives You the most possibilities to build the optimal mix regarding efficiency and permeability.
There are e.g. 2-layer MERV13 (F9-EN779) media having a 10-15% higher permeability than a classic 1-layer MERV8 (F5-EN779) media.
Nontheless, even MERV13 holds only 80-90% of 0,4 micron particulate.
You would need Hepa filters (H11/MERV16) to obtain a certain tranquility that soot or similar stuff does not reach You through the air vent.
And in that case - being usually made of glass microfibre - air flow WILL suffer due to lower permeability => install a more powerful ventilator ?