Rigid, Semi-Rigid or Foil: What Your Dryer Duct Is Made Of Matters

Most of a dryer run is buried in the structure and nobody chooses it twice. The last few feet — the
bit between the machine and the wall — get chosen by whoever delivered the appliance, and that is the
section that causes disproportionate trouble.

Why the surface matters

Air moving along a smooth wall loses relatively little energy to friction. Air moving along a
ribbed wall is disrupted by every corrugation: the flow becomes turbulent, the effective speed drops,
and each ridge is a small ledge for lint to land on.

Multiply a small effect by the length of a run and it stops being small. A ribbed section is not
just worse per foot; it makes the whole run behave as though it were longer, which is the same
mechanism described in
how long a dryer vent run should be.

The three you will actually encounter

Rigid metal

Smooth-walled and dimensionally stable. It cannot crush, cannot sag, and gives lint nothing to
catch on. This is what most of a run should be, and it is what appliance instructions generally
specify for the concealed portion.

Semi-rigid metal

Bendable but holds its shape, used for the short connection where flexibility is genuinely needed.
Better than flexible foil and worse than rigid, which is exactly the trade it is making.

Flexible foil or plastic

Cheap, universally available, and the material most appliance manufacturers restrict or prohibit
outright. It is ribbed along its whole length, it sags between supports, and it crushes when the
dryer is pushed back against the wall.

The transition is where the damage is

The failure is rarely the material in the abstract. It is what happens to it when the machine goes
back into place: a metre of flexible hose folded into a loop behind the dryer, one section flattened
against the wall, another kinked where it turns.

That geometry does not appear on any drawing and it is not visible from the room. It is, in our
experience, one of the most common single causes of a run that loads far faster than its length
suggests it should.

What to check, and what to leave

You can pull the machine out and look at the connection. What you are looking for is the length of
the flexible section, whether it is crushed or kinked, and whether the clamp at each end is still
doing its job.

What is not visible that way is everything past the wall, and that is where a
dryer vent inspection earns its place — the run, the
material changes along it, and the termination at the far end.

What we do at the machine

The connection at the appliance is disconnected, cleared and refitted as part of a
cleaning, and the area behind the machine is part of
the work. Cleaning the concealed run and leaving a crushed hose behind the dryer would restore the
duct and leave the restriction.