I just re-read this thread and have to summarize.
1) headspace is a concern in some cases. During normal primary fermentation, enough CO2 is produced to fill a reasonable space.
5 gallons in a 6 or 6.5 g fermenter is plenty of headspace. I consider it normal, as I'm sure most of is do.
As far as the OP is concerned, the fact that 5 g in a 6/6.5 leaves minimal headspace IS the very reason that a blowoff tube is a sensible idea in pretty much every case.
2) Notty yeast is another fantastic reason to use a blowoff every time.
The only normal ferment that I have ever gotten with Notty was when I started the wort at 60 and fermented for the first week at 62 degrees and then brought it up to 67 for the next 2 wks.
After the first week, there was about an inch of krausen and then the warming caused those suckers to wake up and finish the job. In less than 12 hours, I had 4" and the blowoff was bubbling about 4x per hour.
Notty is a beast and should never be disregarded as a potential mess maker.
To recap: a blowoff is a great idea every batch. Simple and sensible.
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