When going across to the main stack to vent the bathroom sink. Can the vent pipe go inside the water wall cavity ( it's 2 x 8 ) instead of through the top plate / insulation and then across to the 3 " stack in the attic? Assuming it is well above any fixture drains. It's not a wet vent. Thanks
Sure, the common rule is that any horizontal vent piping needs to be at least 6" above the flood rim of the lavatory and pitched towards the drain.