I started with field goals. Home teams are slightly more accurate, a difference that amounts to .12 points per game. The advantage is larger in pressure situations, so that home/road field goal accuracy has very little effect in blowouts and early in games and a larger (but still not overwhelming) effect late in close games.
Today I look at penalties. On average, home teams are penalized 2.22 fewer yards per game than road teams which, when we account for when and where those penalties occur, are worth about .16 points per game. Breaking it down by penalties on the offense and defense (and adjusting for the number of plays run by the offense and the defense), 75% of the home field advantage in penalties comes on defense (more on that later).
Next, we look at specific situations: high leverage plays are those late in games in which the outcome is still undetermined and the purple zone is the last 10 yards before the end zone. The penalty advantage for home teams is much larger in high leverage situations; about 8 yards (.5 points) if an entire game were high leverage. Surprisingly, the same is not true of purple zone situations. Home teams are penalized more inside the 10 yard line.
Finally, I break it down by call. Consistent with my expectations, road teams are much more likely to get false start and delay of game penalties against them. This difference is worth about 1.4 yards per game. (In case you're wondering, that gap is not larger in high leverage or purple zone situations.) Holding calls, on the other hand, are more likely to go against the home team. Extra pass interference calls against the road team are worth 1.1 yards per game and offsides calls add .16 penalty yards to the road team tally.
Roughing the passer (.22 yards/game), face mask (.13), and kick catching interference (.12) calls are also substantially more likely to go against the road team. Unsportsmanlike conduct (.52) and illegal block (.22) are more likely to go against the home team. I'm open to any good theories about why that might be the case.
Adding home field advantage in penalties to field goal accuracy, we have not accounted for .28 points of the 3.15 points of home field advantage. With more than 90% remaining, I will turn next to turnovers.