Your cleanroom passes its particle count at rest, then fails the room-pressure test: the 10 Pa differential between the corridor and the ISO 7 suite bleeds away within seconds of closing the door. The leaf is fine — so where does the air go? Almost always through the four perimeter gaps: head, jambs, meeting stile, and above all the undercut at the floor. Sealing is a system of components and interfaces, not a single gasket.

10.1 Where leakage happens

Air leakage through a doorset scales with gap area and with pressure difference raised to roughly the power 1.5–2 — doubling the differential more than doubles the leak. The standard test classification for windows and doors, EN 12207, grades air permeability Classes 0–4 (Class 4 being tightest, tested per EN 1026); many national specifications for operating theatres and cleanrooms call for Class 3–4 doorsets or project-specific leakage rates (e.g., m³/h·m² at 50 Pa). Related standards cover watertightness (EN 12208) and wind resistance (EN 12210), but for interior medical doors the air-permeability class is the operative metric.

Typical weak points in descending order of measured leakage: floor undercut (largest gap), meeting stile of pairs, hardware penetrations (lock cases, drop-seal channels), and frame-to-wall joints.

10.2 The component toolkit

  • Perimeter gaskets: compression seals (EPDM, silicone, or TPE bulbs) fitted to frame stop or leaf edge. Silicone tolerates hospital disinfectants and temperature cycling better than EPDM but costs more and tears more easily; specify replaceable, clip-in profiles — glued seals turn a 30-minute maintenance job into a door removal.
  • Automatic drop-down seals: a spring-loaded bar that drops to the floor as the leaf closes and retracts on opening. They seal undercuts of 5–20 mm without a raised threshold and are the standard solution for acoustic, smoke, and pressure control on hinged medical doors. Quality units compensate for sloped floors via an adjustable actuating plunger; cheap ones jam or scuff.
  • Thresholds: a flat or ramped saddle threshold gives a positive sealing surface and is preferable where the pressure class is high; it conflicts with bed/trolley passage and accessibility (max heights of ~13–15 mm with bevels are typical limits under ADA/EN 17210-type rules). The trade-off: threshold + surface seal = best tightness; drop seal + no threshold = best trafficability.
  • Frame interfaces: the frame-to-wall joint is outside most doorset test certificates but leaks as much as the door itself. Specify sealed frame fixing: backfilled grout or intumescent/acrylic sealant both sides, continuous at the head.
  • Meeting stiles: pairs need an astragal or interlocking stile seal; on hermetic pairs this is usually a double bulb arrangement. Coordinators (Chapter 7) must not compromise the seal path.

10.3 Hermetic sealing

True hermetic sliding doors add a vertical motion: at the end of travel the leaf moves inward (typically 10–20 mm) to compress a full-perimeter gasket against the frame, often with the floor track profile providing the bottom seal. This achieves leakage rates low enough for pressure-critical isolation rooms and some containment applications — at the cost of a more complex drive (see Chapter 8's end-stroke torque discussion) and a wearing compression surface. Limitation: "hermetic" is a marketing word unless a leakage rate at a stated pressure differential is certified. Ask for the test value (e.g., per EN 1026 methodology or a project chamber test), not the adjective.

10.4 Integration with pressure control

The door is the largest moving element in the room's pressure envelope. Coordination points:

  1. Direction of swing vs. pressure sign — as covered in Chapter 6, positive-pressure rooms push hinged doors open; the closer (Chapter 7) and latch must be sized accordingly, and gaskets see unseating rather than seating load.
  2. Interlocks — airlock doors (Chapter 9) preserve the cascade; the sealing system determines how well the closed door holds it.
  3. Leakage budget — HVAC designers size make-up air assuming a doorset leakage figure; if the installed door leaks more, the differential sags and alarms trip. Supply the HVAC engineer with the door's certified leakage at design pressure, not a generic class.
  4. Seal wear and re-test — gasket compression set typically degrades leakage performance noticeably within 2–5 years of heavy use; build re-measurement into validation schedules (ISO 14644-3 includes doorset-related leakage and recovery tests).

Practical checklist

  1. State the required leakage rate at a stated ΔP (or EN 12207 class) in the specification.
  2. Choose the floor strategy: threshold + seal (tightest) or drop seal (accessible) — decide before frames are set.
  3. Specify replaceable silicone or TPE gaskets; disinfectant-compatibility certificate from the seal supplier.
  4. Seal the frame-to-wall joint on both faces; include it in site inspection.
  5. For "hermetic" doors, demand a certified leakage value and the test report.
  6. Hand the certified leakage figure to the HVAC balancer before commissioning.

Sources