All Grain Biab With Top-up Post Boil

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Hi - I've completed 2 partial BIAB (approx 3 kg of Grains) using a 20l pot, adding LME at the end of the boil and then topping in the fermentation bucket.

Now, I'm eager to go all grain and am wondering if this would work....

I'm thinking about mashing approx 4.5 - 5kg grain in around d 14 - 15 litres (whatever I can fit into my 20l pot) and then adding top-up water into the fermenter for the desired final volume and gravity.

I'm aware that a higher gravity boil will effect hop utilization, and I can work around this, but my question is about mash thickness and efficiency. Is 5 kilos too much for 14 litres??

I have been doing a mash-out (despite the controversy) and giving the bag a squeeze - my last batch was 75% efficiency (I excluded the extract from my calculations)

Is topping up a higher gravity wort post boil a viable option for BIAB?? Any other things I need to be aware of???
The key to brewing Maxi-BIAB is to not be volume-centric in your recipe formulation, but to be SG-centric.

If you want to make an IPA at 7%, you make a 12L batch; if you want a 3.5% mild, you get 24L.

~3.7kg is the sweet spot for the 19L pots.

I have a Dr. Smurto's JSGA clone fermenting right now. It went into fermentation with a slightly higher OG of 1049 (was shooting for 1046). This brew has now been fermenting in a controlled environment since Thursday morning, last week.

There is 21.5l of beer in the fermenter.

I have boiled up 1.4l of water, which is now chilling in an ice bath.

I am about to drop in my dry hopping addition... do people recommend gently adding the water to dilute, at this stage of fermentation? I have to open the lid to add the hops anyway, being that fermentation is still ongoing, I am hoping the yeast will eat up any possible O2 that is added, to prevent oxidation... thoughts?

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