Short answer
The whole-building test averages leakage across every unit, so we did not need to seal all 11. We picked three vacant units, masked the finished interiors, and sealed each one with its neighbours held at the same pressure. On our guarded multi-point test, the three units went from 2.78 to 0.67 ACH50, 2.30 to 0.90, and 2.26 to 0.73.
The job
The building was done: flooring down, cabinets and appliances in, and some units already lived in. It failed its Step 4 final on airtightness. The builder's energy advisor brought us in to close the gap without opening walls or touching the occupied units.
Why three units were enough
A multi-unit result is the total leakage of every unit (CFM50) divided by the total volume. Taking leakage out of any one unit lowers the building number, so the leakiest vacant units do the most work. We set a leakage budget from the energy advisor's unit-by-unit readings, then chose the three units we could get into.
One planned unit had a tenant move in before the seal, so we swapped in another unit of the same size. The budget barely moved, because the building average only cares about total leakage and total volume.
Protecting a finished interior
Sealing a finished unit means covering everything the sealant should not land on. Before each seal, floors, stairs, cabinets, appliances, glass and plumbing fixtures were masked. The sealant only builds up where air is escaping, which is at the leaks.
The seal
These are attached units, so a normal test would also count air moving through the shared walls. We ran every seal and every test guarded: blower doors in the adjoining units held each neighbour at the same pressure as the unit we were sealing. That way the number reflects leakage to the outdoors, which is what the building result measures.
The gauge reads in real time through the seal, so we watched each unit's number fall and stopped when it was where it needed to be. Two units were sealed on September 29 and the third on October 1.
Results
| Unit | Start of seal | Final | Change |
|---|---|---|---|
| Unit A | 2.78 | 0.67 | 76% lower |
| Unit B | 2.30 | 0.90 | 61% lower |
| Unit C | 2.26 | 0.73 | 68% lower |
The energy advisor's retest of the whole building is the official result, and we report our numbers alongside it, never in place of it.
What this means for your build
- A failed final on a finished building does not have to mean opening drywall. Sealing from the inside reaches the gaps you can no longer get to.
- On a multi-unit building, you may not need to seal every unit. The test sheet shows which units carry the most leakage.
- Finished units need a masking day before the seal. Plan for it in the schedule.
- Your energy advisor sets the target and runs the official test. We seal to that target and share our readings with them.
If your building just missed its number, call me at 250-864-8727 with the test sheet and I will tell you whether sealing can close the gap.
Written by Sebastian Motora, owner-operator. Builder and project details withheld.