Split-level homes: which spaces belong in the volume calculation? 

A split-level house is easy to walk through and surprisingly easy to describe badly on paper. A half-flight of stairs changes the floor elevation. It does not, by itself, tell us where the heated building ends.

Before comparing airtightness results or using an energy model to inform a heating-system decision, ask for one simple thing: a sketch that shows which spaces belong to the building being assessed. The floor areas, air volume and exterior surfaces should all make sense against that sketch.

Start with the boundary, not the total square footage

Natural Resources Canada describes the building envelope as the shell that separates the indoor environment from outdoors and controls heat, air and moisture movement. That shell includes more than the walls you can see from the street. Floors over an unheated space and changes in roof height matter too. NRCan: how your house works.

For a split-level or a home with an addition, mark the spaces on a plan and a simple side view. Label the lower level, main level, upper level and any taller ceiling section. Then identify what is on the other side of each surface: outdoors, ground, an unheated space or another included room.

A wall between two included rooms is not an exterior wall just because one room sits half a storey higher. Equally, a wall segment above a neighbouring roof may really be exposed to outdoors. The side view helps distinguish the two.

Make a volume ledger

Floor area is two-dimensional. Air volume is three-dimensional. Multiplying one floor-area total by one ceiling height can miss a tall room or misrepresent a lower level. Instead, divide the included space into non-overlapping shapes and account for each one once.

The example below is invented to show the arithmetic. It is not a measured house, a client result or a prescribed EnerGuide measurement convention. Actual evaluations must use the dimensions and inclusion rules required by the applicable method.

The ledger makes a useful check possible: can every part of the total be pointed to on the sketch? Watch for an omitted lower level, a double-counted stairwell or a vaulted ceiling counted both as a full-height box and as an extra wedge.

Keep the test boundary and the model boundary clear

A blower-door result describes a particular test configuration. Closing off an attached space can change which leakage paths the fan sees. It is not enough to compare two air-change numbers and assume the difference belongs entirely to that space.

Ask the advisor to reconcile three items: the spaces included in each test, the volume used to calculate each result, and the envelope represented in each model. A report showing the same floor area does not prove that its volume is correct or that two tests used the same boundary.

This is also why a lower ACH50 number is not automatically evidence of an improvement. ACH50 normalizes the airflow at a 50-pascal pressure difference by the stated volume. It is a test-pressure metric, not the home’s everyday ventilation rate. PNNL: enclosure leakage testing.

What to request before the equipment decision

Ask for the marked-up plan, the volume breakdown and a clear statement of the included spaces. Have unresolved geometry or test-configuration questions settled before relying on comparisons.

Then keep equipment sizing as its own job. NRCan recommends a recognized heating and cooling load calculation method, such as CSA F280, rather than choosing equipment from floor area alone. A whole-building load addresses the total; room-by-room loads help the designer assess distribution needs. NRCan: heating and cooling with a heat pump; BC handbook: block and room-by-room loads.

The useful question is not just “How big is the house?” It is “Which spaces, which surfaces and which volume does this result describe?”