Building Science and Multifamily Fundamentals
Why building science comes first
Every retrofit decision in multifamily housing rests on a few physical principles. The building enclosure is a system of four control layers: water (bulk and vapor), air, thermal, and - in some assemblies - radiant. A candidate must be able to explain how these layers interact and where they belong in a wall or roof assembly. Air leakage matters more than many people assume: in multifamily buildings, infiltration and exfiltration can drive a large share of heating and cooling loads and also carry moisture into assemblies where it condenses.
Multifamily-specific concepts
Understand stack effect and how it scales with building height, temperature difference, and shaft connectivity. In a tall building, the pressure difference at the top and bottom can be substantial, which is why lobbies, stairwells, and elevator shafts are common leakage pathways. Know the difference between compartmentalization (separating dwelling units from each other and from common space) and guard/conditioning of common areas. Recognize thermal bridging at slab edges, balcony penetrations, and steel studs, and understand why continuous insulation performs differently from cavity insulation.
Also know the basics of ventilation and pressure balance - exhaust-only, supply, and balanced systems - and how bathroom and kitchen exhaust interacts with tight envelopes. Be able to discuss combustion safety and the risk of depressurization in units with natural-draft equipment.
Traps to watch for
Questions often swap terms: vapor barrier vs. air barrier, permeance vs. permeability, R-value vs. U-factor. Another common trap is assuming a measure that works in a single-family house transfers unchanged to a multifamily building - shared walls, central systems, and common-area loads change the analysis.
Worked example
A 10-story building with a 60 °F indoor-outdoor temperature difference shows strong lobby door pull. Stack pressure scales with height and temperature difference; the fix is not just a stronger door closer but reducing shaft leakage and improving compartmentalization.