ᐅ Condensation and Mold on Supply Air Duct – What to Do?

Hello forum,

I have a mechanical ventilation system from Vallox (model 350SE).
The supply air duct (cold outside air) goes directly outside next to the basement door.
Unfortunately, condensation forms there and eventually causes mold.
The basement temperature is on average around 17-20°C (63-68°F).

I would like to ask for your opinion on which corrective measure makes the most sense.

1. Insulate the duct more (I have already used insulation tape, but it didn’t help)
2. Build a styrofoam box around the duct (up to the door) to maximize insulation
3. Install a door with better insulation properties (currently it’s a cheap door from a hardware store with little or no insulation)
4. Route the supply air duct further away from the door frame to the outside (this wouldn’t require drilling a core hole or using angled ducts)

Thanks in advance for your feedback!

In my case, the spiral duct pipes run through the unheated garage to the outside.
I solved the problem by insulating both pipes separately. For this, I wrapped 40mm (1.5 inches) 0.35 density partition mats around the pipes and generously covered them with clear plastic foil. This works perfectly, with no condensation forming on either the supply or exhaust air.
Alternatively, you can replace the spiral ducts with foam ducts.

He already has foam pipe insulation... at least it seems that way.

Is the pipe sealed where it passes through the wall? Cold air might be entering there, cooling the pipe and insulation on the interior side. This can cause condensation to form.

The problem could be at the wall penetration, which must be completely sealed.
Just wrapping narrow strips around it is ineffective.

In my opinion, the issue is not with the pipe. If it were, the outer surface of the pipe inside the room would also be cold, causing condensation and dripping water. This would be visible on the pipe and the floor. It seems that the wall penetration is not sealed properly. Additionally, the adjacent door creates a thermal bridge.

Next steps
- Remove the sealant (or insulating material) wrapped around the pipe on the inside. It doesn’t serve a purpose there.
- Check whether the wall penetration is sealed properly and repair if necessary.
- Inspect the door installation to see if the door frame forms a cavity against the wall.
- Consider running a climate logger in the room to monitor humidity levels for confirmation.