The Air Lock Entry
This pattern is shaped by
Problem
When a single door separates inside from outside, every entry and exit tears open the thermal envelope — flooding the dwelling with -30°C air in January or 200 µg/m³ of wildfire smoke in August. The vestibule provides space for the ritual of arrival, but if both doors can open simultaneously, or if the seals are poor, the airlock fails and the house breathes the street.
Evidence and Discussion
The physics are unforgiving. When a door opens, air exchange occurs through two mechanisms: temperature-driven stack effect and wind-driven pressure. A study by the National Research Council Canada measured air exchange through a single exterior door at 4-8 cubic meters per opening event in winter conditions — enough to replace 5-10% of a small home's volume with outside air each time someone enters. In a household of four making six entries and exits daily, a single-door entry can exchange half the home's air volume through door openings alone.
The double-door airlock interrupts this exchange. The principle is simple: the inner door closes before the outer door opens; the outer door closes before the inner door opens. The vestibule between them holds a captive volume of air — typically 2-4 cubic meters — that serves as the sacrificial exchange zone. When the outer door opens, only this small volume mixes with outdoor air. When the inner door opens, only this now-tempered volume enters the home. The Norwegian Building Research Institute found that a properly sequenced double-door entry reduces air infiltration by 70-85% compared to a single door, with the critical variable being door timing: if both doors are open simultaneously for even two seconds, infiltration approaches single-door levels.
During wildfire smoke events, the airlock serves a second function. Smoke particles behave like gases for infiltration purposes — they follow pressure gradients through any available opening. Environment and Climate Change Canada recorded 15 days of PM2.5 exceeding 100 µg/m³ in Edmonton during the 2023 wildfire season. Each door opening during such events floods the interior with particulate matter that takes hours to clear even with filtration running. The vestibule, if properly sealed and small enough to clear quickly with the home's ventilation system, limits each opening event to a recoverable spike rather than a sustained contamination.
The failure modes are specific and preventable. First: doors that can be propped or held open simultaneously — common in commercial buildings, rare in residential, but devastating when it occurs. Second: weatherstripping that fails within two years of installation, allowing continuous infiltration regardless of door sequencing. Third: vestibules so large (above 6 cubic meters) that they hold too much contaminated air to clear efficiently. Fourth: no interlock or protocol ensuring sequential operation. The Passive House Institut recommends vestibule volumes between 2 and 4 cubic meters for optimal airlock performance — large enough for comfortable use, small enough to exchange quickly.
Therefore
at the primary entry of every cold-climate dwelling, construct a vestibule bounded by two doors that cannot be fully open simultaneously — either through physical interlock, automatic closers with overlapping timing, or a vestibule deep enough (minimum 1.2 meters clear) that a person must close the first door to reach the second. Size the vestibule between 2 and 4 cubic meters of air volume. Seal both doors to the same standard as the exterior envelope: continuous weatherstripping achieving less than 0.5 CFM per linear foot of crack at 75 Pa pressure differential. Test with a smoke pencil at all edges under blower door pressurization. The airlock passes when, with the inner door closed and the outer door open, no smoke movement is visible at the inner door seals.