quick shroom question

do you mean airtight or with holes
i heard of both but you can take it off since most people put it on to prevent water coming in the jar when in pressure cooker,with a air inlet on the cap of the jar

Yes, the foil ensures that no "diseases" so to say get inside your jars.. you could always put medical tape also.

But, the tin foil route is the customary way

I've read conflicting reports over at the shroomery. Some say leave it on while others say take it off to allow for gas exchange in the jar. The dry vermiculite layer on top is supposed to prevent any contaminations in the jars. I took the foil off of my jars and didn't have any problems but I did have the holes taped.