The rules for combat involving buildings are in AS beginning on page 83. The Damage Absorption chart is on page 84, and the rules for units fighting one another whilst inside the same building are on page 85 and are quoted below:
Attacks from Within the Same Building: If attacker and
its target are inside the same building, reduce the building’s
Damage Absorption values by half (round down).
I know this forum isn't for answering
why rules are the way they are, but I do feel the urge to plea to you errata writers finalizing this year's master errata doc to address infantry fighting infantry inside the same building. Almost all infantry are unable to harm one another, even after halving absorption. (for infantry, 1/2 absorption just ends up being everyone else's absorption and the vast majority of infantry only does 1 or 2 damage. That means that only the heaviest armed of infantry can hurt other infantry in the same tent with them, and exceptionally rare are the infantry that can damage other infantry in the same medium grade or sturdier building...) I see it as a problem that warrants being addressed since if the building were instead a mobile structure, you can then use boarding rules instead and none of that nonsense applies anymore :)
I do have a question, though. Regarding timing of absorption reducing damage and HT adding damage:
Does the additional damage from HT get applied before or after damage from absorption is removed? If HT comes after absorption, it'll be at least that HT value in damage since absorption only reduces damage to a minimum of 0 (AS, attacking Infantry Units Inside Buildings, pg 85).
