Fermentor head space?

Is the amount of headspace above the wort critical during fermenting?

Say 1 gallon of head space. 5.5 gallons of wort in a 6.5 gallon fermentor.

It seems to alleviate using a blow off tube early in fermentation.

I can see where you wouldn’t want an extreme amount like a 2 gallon batch in a 6.5 gallon fermentor.

1-2 gallons of headspace for a 5 gallon batch are usually sufficient. Blowoff depends on a variety of factors, temperature, yeast strain, original gravity. Some of the top fermenting ale yeasts which have been traditionally top-cropped will really build up a layer of thick yeast towards the end of vigorous fermentation which can cause a late blowoff when you might not be expecting it.

From my experience having 2 gallons headspace in primary does not hurt the beer at all thanks to the co2 blanket…i have brewed 3-4 gallons in a 6.5 fermenter with no problem

I myself think more headspace is better then not enough. The way I see it when you fill the fermentation containers up close to the top you will need a blow off tube and I do use them but would rather keep the fermentation clean and all in the bucket. It’s mostly just preference to me. I have set the alarm clock more then a few times to check on a crazy clogging fermentor.