Still looking for explanations for my low mash efficiency with BIAB. I did a near full volume mash starting with 7.75 gallons water and an 18 lb grain bill (15.75 lb 2 row and the rest various specialty grains) for a Black IPA. Expected a preboil SG of 1.076 at 75% but got only 1.058 (61%). All my temps were great and held fairly steady. Had my LHBS grind the grain as fine as they could and the crush looked great with a lot of powder (more crushed than in the past). I squeezed the grain bag hard to extract the remaining liquid. I ended up adding a pound of DME during the boil (that's what I had on hand) but still missed my OG by about 0.01. I have had no better than 65% in about 8 BIAB brews. Pretty mystified how folks get 80% or greater and what I am doing wrong. That said, I have been getting very good attenuation so for most I haven't suffered too much ;-) There is nothing wrong with the final product.
So now that I took you on a stroll down "theoretical lane" you should be able to see that there are things that you can do to improve your efficiency, but I would argue that it's less about what efficiency you achieved, and more about "what efficiency you expect to achieve". This is the true factor that you actually want to be consistent as you are formulating recipes and want to have it where you get the numbers you "expect" and not what you dream about (also lets you avoid having to add extract in order to correct your gravity just because your expectations are off in the first place).
Simple fact is that your expected efficiency changes as well based on the size of your grain bill and sparge technique (Grist Weight vs Sparge technique in some circles).
Remember that article I linked to on efficiency located here:
Understanding Efficiency - German brewing and more
At the bottom of the page in the section "related materials" he links to a presentation (he offers to us as a PDF) he gave at NHC 2010 which can be downloaded directly here:
http://braukaiser.com/download/Troester_NHC_2010_Efficiency.pdf
scroll to page 19 and you will see the chart on "Batch sparging and grist weight" which gives you "expected efficiencies" for batches of different grain bill sizes and number of sparges for a theoretical batch with a 6.5 gal preboil volume.
Since true BIAB is technically a "no sparge" technique when done as intended, look at the blue line for "no sparge" and you will see that for your grain bill of 18 lbs, lo and behold there is an expected efficiency of somewhere right around 65% (my best guess since the chart isn't detailed enough to plot 18lbs exactly). Since this chart is for 6.5 gal preboil volume the reason your efficiency is still a little low (you stated 62%) compared to this chart goes back to the "you didn't have enough water to start with" observation I started with, since the 1.5 gal absorption of your 18 lbs of grain would required you to start with 8 gal of water to use this model and you only started with 7.25.
The really cool thing about this chart is that it does a better job of clarifying what people mean when they say that BIAB is a little less efficient than traditional AG and so you might have to throw in a few more lbs of base malt to get the numbers you want compared to traditional AG (yet you hear about these insane feats of 98% efficiency). See the gap that is pretty consistant between the blue and red lines, that is what they are talking about, not conversion efficiency per se, but the gap of roughly 8% in lauter efficiency between a no sparge and even a single sparge approach. This is the reason that I minimally rinse my grains no mater what, since it closes that gap quite a bit as a way to compensate, or you either need to increase your water volume in the mash to spike your conversion efficiency or compensate with a few more lbs of grain in order to close that gap to hit desired numbers (this is most often likely done with extract by people who thought their efficiency was way low instead of compensating in the numbers up front).
So let's just assume that you have been doing similar sized grain bills for the 8 BIAB brews that you have done so far in which you stated that you haven't had an efficiency better than 65%, I would argue that your efficiency is pretty damn good compared to what should be expected for the batch given the grain bill, and what you really need to do instead of grinding your grain finer or even adjusting the PH of your water is to go into what ever brewing software you are using and adjust the expected efficiency to be in line with what you should expect based on the chart provided so you can tweak all your other numbers to get in line with the style you are shooting for.
If you were expecting 75% because that's what the kit instructions said you should get, you will have to throw the notion of those numbers as correct out the window since most AG kits or LHBS employees assume a traditional 3 vesel approach to AG brewing with typically 2 sparges unless stated in the instructions (if you go back to that same chart, you will see that for 18 lbs of grist, a two sparge method will come in at about 75% which would be consistent with what you were expecting if that's what they told you to expect).
At least now I hope you understand a little better why you're getting 65% which is in no way shape or form "bad", just not what you were told or thought to expect.
If you need an efficiency ego boost, try a simple session beer like a cream ale. Grind the hell out of 7 lb of 2 row pale, 3/4 lb of Honey malt, 1/4 lb of Biscuit (use sparingly since it has a distinct flavor that gets overbearing so if you tweak don't change this unless you really like the tast of cornflakes). With 8-8.5 gal of water, mash in at 135 let it raise to 155 and sit for an hour, mash out at 175 for at least 10 minutes (rinse the grains a bit if you need more volume to start your boil). Boil for an hour with 1 oz Cluster and ferment with American Ale 1056 and you only need about a week in secondary to mellow the honey and corn flavors together.
This would be the brew I make for my brew groupies as it is fast and easy (also very cheap to make), and you should easily score some efficiencies in the high 90s which will you feel like a BIAB efficiency pro as you start to understand the in some cases 65% is really damn good!
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