I brewed a Founders Breakfast Stout clone, pretty much following the recipe at http://www.homebrewtalk.com/f68/founders-breakfast-stout-clone-139078/ except that I didn't have enough cocoa powder, so I used what cocoa powder I had and did the rest with baking nibs.
This was my first time with my new rig, a BIAB setup with a 15 gallon kettle. Brew day went pretty well, the only problem was that I didn't realize that you get significantly less grain absorption when you BIAB, and when you have 20 lbs of grain that can make a big difference . I ended up with a gallon and a half more wort than I expected, and so my OG was low (expecting 1.094, got 1.072). I suppose I should have tried to boil off some water, but I didn't have a way to measure volume in the brew kettle (something that I will change before the next brew).
So, on to my question. The recipe calls for a mash temperature of 155. I mashed in at 156, I stirred after half an hour and the temp went down to 154. I added some heat, got the mash back up to 156 and then it went back down to 154 during the second half-hour, so I hit the temperature pretty spot-on. My thermometer is calibrated and accurate. I did not have iodine to do a conversion test (something else I will probably start doing) but I very much doubt that I had any conversion problems.
I aerated the wort with an aquarium pump and an airstone, there was quite a bit of foam on top of the wort after 10 minutes. I pitched a starter of Wyeast 1056, enough cells for the 1.094 I was expecting, so I am definitely good for the 1.072 I got. I had my fermentation fridge set for 64 degrees with the probe insulated against the side of the bucket. I had bubbles after 3 hours and it was really rocking for about 3 days. My basement is cold and I didn't have a heater in the fermentation chamber, so after fermentation slowed the temperature went down to about 60. After about a week and a half I took a gravity reading and I was at 1.026, and my brewing software had me expecting a FG around 1.017. I stuck a heater in there to bring the temps up to around 68 to see if I could rouse the yeast a little. It's been there for two weeks. I took a sample this morning and it was down to 1.024.
So, my question. I know that higher mash temps = less fermentable wort, but I can't find information on how much less. 155 is on the high side, but not ridiculously so. My attenuation is sitting around 65% and 1056 should be between 73-77%. Can it all be explained by the higher mash temperature, or is there something I should be looking at for why I finished so high? It's been 3 1/2 weeks since brew day, so I doubt it just needs more time.
In any case, the sample I took tastes absolutely divine and I am planning on bottling this week. I'm not worried about the beer, just want to make sure I am not missing anything in my process.