When you create a new wall below the wall dividing the split level (reducing the above wall height to the remainder of the lower level ceiling height and making the new lower wall the balance of the height to the lower floor level) and then do similar for the upstairs as per the user manual you are then referencing a level as the 0 point.
What I’m wondering is can it be correct to use either level as a reference level? For instance, set the new bottom wall as zero and reference up from the lower split level or set the new bottom wall as negative and reference from the higher split level?
Do I need to go into the zones/walls and set the remainder of the walls in the non-reference level to the new datum? Common sense tells me I should, but the user manual doesn’t mention it.
I do not need to worry about adjusting the base height of the remaining walls on the changed level?
The thing to keep in mind to assist in dealing with split levels is that NatHERS programs measure two dimensional heat flows. This means that where a wall is split into horizontal bands a lower portion of wall will have connection to sub-floor the mid-section will be connected to the wall in the adjoining zone and an upper portion will be connected to roof space or external air.
Accordingly the base height does not hold any relevance for split walls between zones and may be ignored.
Following the procedure in the FirstRate manual should achieve the required relationships.