I have never tryed coffee filters it was a brain storm that is a couple of days old. I did speak with worms way in regards to the capillary mat and they said the roots have no problems growing thru it and it is almost a quarter inch thick. I was like is it kind of like a coffee filter in regards to the material and he told me it was so I wish I could tell you I have tryed it, but I cant. You could be the first if you are down for an experiment. I saw some one use a nylon in one of these threads and the roots did not seem to have a problem growing thru it. It hung off a bit so I would only assume the air inside would be constantly humid.

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c