Getting dripping condensation from duct - please advise!

[ToastyToes]

Hi all,

Due to the lack of ventilation under my subfloor, I decided to add some retrospective ventilation ducts behind kitchen units above the outside floor level. however after a few weeks I’ve pull the subfloor up and noticed the duct opening under the floor is soaking wet which I assume is condensation.

The insulation around the duct between the duct and joist is soaked too.

I understand I need to insulate the duct above the subfloor but I am not sure why the duct is so wet below the subfloor.

Please advise as I can’t fit my new kitchen until it’s resolved and it’s causing huge pressure on me.

 

[JohnD]

I can't understand what your picture is showing. Show us the outside as well. How many sides of the floor are ventilated to promote through flow of air?

 

[ToastyToes]

I can't understand what your picture is showing. Show us the outside as well. How many sides of the floor are ventilated to promote through flow of air?

The ground outside is about 12” from the top of that duct. There are existing air bricks on this wall but airflow wasn’t great so I core drilled 4” hole and added a duct which runs through the wall and down between the wall and joist.

The opposite side of this wall in the house is an extension so cross ventilation isn’t possible so I have another one of this holes with a flexible duct which runs to the other side of the floor. This has helped cross ventilation a lot but the problem I have is that the duct which comes through the wall and down in the image I posted before is wet under the subfloor. Also the insulation around the duct is wet too.

 

[ToastyToes]

I can't understand what your picture is showing. Show us the outside as well. How many sides of the floor are ventilated to promote through flow of air?

 

[ToastyToes]

The insulation (now removed temporarily) where is circled red was soaking wet.

Also the underside of the duct all the way around is soaked.

 

[JohnD]

so on one side of the room, air is entering (or perhaps leaving, depending on the direction the wind is blowing). Where does it leave (or perhaps enter)?

Is there a chimney in the room?

 

[ToastyToes]

so on one side of the room, air is entering (or perhaps leaving, depending on the direction the wind is blowing). Where does it leave (or perhaps enter)?

Is there a chimney in the room?

Drawing attached. Let me know if you have any questions about it.

No chimney in the kitchen.

 

[ToastyToes]

A builder neighbour advised I should just insulate the duct from the floor boards up and seal all gaps in the floor with expanding foam and it should be fine.

The insulated duct will prevent warm humid air inside from touching the cold duct and prevent condensation.

What I am concerned about is why the duct under the floorboards is wet.

 

[JohnD]

I think the problem is that there is not enough airflow on the other side of the house to remove the humid air from under the floor.

Your 40mm pipe is not remotely big enough.

I don't know how big the flexible pipe is.

Possibly if the dwarf wall is honeycombed you could put additional vents in the dining room.

 

[ToastyToes]

I think the problem is that there is not enough airflow on the other side of the house to remove the humid air from under the floor.

Your 40mm pipe is not remotely big enough.

I don't know how big the flexible pipe is.

Possibly if the dwarf wall is honeycombed you could put additional vents in the dining room.

Before I added the additional ducts, there was lots of water on the underside of the insulation and the timbers were damp. Since adding the ducts, we have had downpours of rain and all the timbers and insulation is now dry. However by adding the ducts we have now caused a possible issue with condensation.

The ducting which is stretched across is 4” wide.

The dwarf walls are semi honeycombed but the gaps are quite narrow.