Single-seal doors vs hermetic doors: differences
Seals or a pressure system? What sets a single-seal door apart from a hermetic door, where each is needed and which specification mistakes to avoid.
In specifications and requests for quotation you often read "sealed door" with no further details. It seems a minor point, but it is one of the most frequent causes of change orders during works, disputed acceptance tests and prices that are hard to compare.
The reason is simple: on the market, the term "sealed" refers to two very different products.
- Single-seal door: reduces the passage of air, dust and noise thanks to perimeter seals.
- Hermetic door: designed to maintain a pressure difference between two rooms, with low, measurable air permeability.
The difference shows in three aspects: how it is built, what performance it guarantees and where it should be installed.
Construction: seals or a pressure system
Single-seal door
Sealing relies on perimeter seals (rubber, brush or soft lip) that lightly touch the frame. On sliding doors the leaf slides parallel to the wall and always leaves a small gap. At the bottom there is usually only a brush.
The result is good control of draughts, dust and noise, but not a true closure: air keeps passing through, especially when the two rooms are at different pressures.
Hermetic door
On closing, the leaf is pushed against the frame and compresses a continuous seal around the entire perimeter:
- on hermetic sliding doors, a dedicated mechanism moves the leaf towards the wall in the last part of its travel and presses it against the seal;
- on hermetic hinged doors, the seal is compressed by the leaf closing onto the frame, with precise adjustment of the closure;
- at floor level, on hinged doors the seal is provided by a drop-down (guillotine) seal built into the leaf, which lowers onto the floor on closing and lifts on opening; on sliding doors it is the leaf itself that lowers and presses the bottom seal onto the floor.
When closed, the door becomes a continuous barrier. This requires greater installation accuracy, rigid frames and periodic maintenance of seals and adjustments.
Performance: "looks closed" or "is measured"
This is the most important difference for designers.
- For a single-seal door, an air permeability class is not usually declared. The sealing is "perceived".
- For a hermetic door, air permeability is tested and declared. The reference is the UNI EN 12207 classification (test to UNI EN 1026), which measures air leakage at different pressures.
For the HVAC designer this figure is not a detail. Door leakage is part of the air balance calculation. With a single-seal door instead of a hermetic door, the design pressure differential may not be maintained, or the AHU has to work harder to compensate.
Intended use: where it is really needed
Where a single-seal door is enough
A single-seal door is suitable where draughts, dust and noise need to be limited but there is no pressure difference to maintain:
- corridors and internal routes;
- outpatient clinics and consulting rooms;
- standard inpatient wards;
- offices, storerooms and service rooms.
In these rooms it is the most sensible choice: it costs less, is easier to install and only requires routine maintenance.
Where a hermetic door is needed
A hermetic door is necessary when the room works under overpressure or underpressure compared with nearby rooms and the ventilation system must maintain the design differential:
- operating rooms, under overpressure to protect the surgical field;
- intensive and sub-intensive care units;
- isolation rooms, at positive or negative pressure depending on the patient;
- infectious disease wards, where underpressure prevents contaminated air from escaping;
- laboratories and cleanrooms.
Here the door is part of the system. It must have tested and classified air permeability and a floor closure (drop-down seal on hinged doors, a lowering leaf on sliding doors) that shuts off the airflow at the bottom too. Scheduled maintenance of seals, adjustments and automation is also needed.
How to choose
There is no "best" door in absolute terms. There is the right door for the room. Installing a hermetic door in a corridor is a waste. Installing a single-seal door in a negative-pressure isolation room is a safety issue.
One question is enough to decide: does this room need to maintain a different pressure from the rooms next to it? If the answer is yes, you need a hermetic door.
Single-seal or hermetic: the most common mistakes
Mixing up the two products is costly: sometimes on site, more often at commissioning.
These are the mistakes we see most often.
1. Generic specification "Sealed door" is written without stating the air permeability class or the pressure differential. Each supplier reads it their own way, and a single-seal door may arrive on site where a hermetic door was needed.
2. Single seal in pressure-controlled rooms To save money, a single-seal door is chosen for an operating room, an isolation room or an infectious disease ward. The door looks closed, but air gets through. At commissioning the pressure differential is not reached, or the system has to work harder to compensate. Replacing the door after completion costs far more than was saved.
3. Hermetic door where it is not needed This is the opposite mistake: hermetic doors in corridors, outpatient clinics or standard wards, where there is no pressure to maintain. You pay for a more expensive product, a more complex installation and scheduled maintenance that bring no benefit.
4. Quotes that cannot be compared Two quotes for a "sealed door" may describe different products: one with seals and a brush, the other with the leaf pressed onto the seal and a floor closure. If you compare only the price, the single-seal door almost always wins, even where it is not enough.
5. "Just add seals" A single-seal door does not become a hermetic door by adding seals or brushes. Without the system that presses the leaf against the frame and without the floor closure, the gap between leaf and wall remains. Sealing depends on how the door is designed, not on an accessory.
6. Installed like an ordinary door Even when the product is right, the site treats it like any other door:
- walls and floor out of true: the seal is compressed only at some points and the bottom closure does not seal on an uneven floor; the finished floor must be perfectly level;
- missing structural provisions: no wall reinforcement for the track and automation, no space for the leaf to slide or no height for the header box;
- missing electrical provisions: no dedicated power supply, conduits for push buttons and sensors, interlocks or access control connections.
The result is work on finished walls, delays and a hermetic door that does not achieve the performance it was paid for.
Our advice: decide at design stage which rooms really need a hermetic door. Write it into the specification with class and pressure differential, and involve the supplier before the walls are closed.