Just spent an hour at walmart looking at air filters. Andy, I concur that some of the cheap pleated filters have holes in them. In fact, the filter media has an array of holes punched through it, possibly with a hot pin that helps bind the material together.
The $4 filter had the same media as the $9 filter. The $9 filter has slightly more pleats and a nicer looking but equally flimsy cardboard frame.
I didn't see any K&N HVAC filters but they did have K&N vehicle cabin filters. They have a cotton flock-type media tufted into a metal screen. It is quite similar to the media used for chainsaw filters - good for sawdust, useless for fine particulates. The only performance information on the box was a long string of superlatives. I have to give them some credit. At least they didn't use words like Quantum or Unique Molecular Bonding.
I came home with two $4 filters.
My last employer had several filter separators that used socks. They did a good job of keeping fine TiO2, silica, and calcium carbonate dust in check. I considered building something similar for the house but the smallest of these was bigger than a refrigerator. The socks were about $60 each and were 8 inch diameter by six feet long. The pleated bag type had cartridges as small as 20 inches square. Unfortunately these cartridges were several hundred dollars each. They can be cleaned with compressed air but not washed. Thus, they were a good place to start a mold colony.
My quest to find a cost effective HVAC air filter continues.
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