Floor void with high humidity

[bigcolin]

We have a Victorian terraced house, with timber joists/floor on the ground floor.
We have 3 double (flood proof) airbricks to the front of the property, however, at some point in the past a large extension was built across the entire back of the house with concrete floors and we do not have any though flowing (front to rear) ventilation.

The extension now contains a kitchen with slate flooring which i'd rather not have to take up.

I have fitted a fan in the understairs cupboard to blow air down under the floor, which has dried out the timbers below the cupboard, however under the floor by the front door we are still seeing relative humidity at 92% at 11c.

It feels like a more powerful fan would help??

I'm guessing we are not the only people to have this problem of new extensions blocking airflow.

Above ground level we are starting to see paint flaking off the internal walls - which is why we started investigating.

Colin.

 

[John D v2.0]

If the air is stagnant you'll get rot under there but not necessity damp walls, that could be condensation on cold walls or penetrating from high ground outside.
Regarding the humidity, you'll need a through draught which could be a powerful fan or could be a lesser fan with ducting to blow to the right parts. Probably better in the long term to sorry the ventilation naturally, perhaps you have a redundant chimney you can connect to the void at the back? Then the air rising up the chimney will take air in from outside.
Another option if you have space is duct to the back of the house above floor level, would work better if you have kitchen units to go behind

 

[endecotp]

The air from inside that you're blowing down may not be any drier than 92% RH once it has cooled. (Do you understand relative humidity?)
You really need to vent to the ouside atmosphere.

 

[bigcolin]

Thanks both for your replies. I have an access panel in the oak flooring by the front door, so have been thinking about installing a fan under the floor there extracting air to outside. I guess we're still likely to pull some air in from above the floor boards, but I'm hoping the fan will pull air from under neighbours houses ( who all seem to have air bricks front and back of their houses, and who don't suffer with damp/condensation as we are) as there are big gaps in the party walls under the flow.

 

[vinn]

Do you need "flood proof air bricks"? There are far simpler precautions against rising waters entering air bricks.
Three air bricks along the front elevation might not be enough - but without a pic of the front who knows?

IMO: You should avoid using any elec device in your subarea.
As suggested above, natural ventilation is the way to go.

Gaps in party walls are not usually a good idea.
Do you have honeycombed knee walls - you should.
Condensation can form in sheets of dripping moisture in an unventilated sub-area.
Forget about measuring humidity - perhaps you should crawl your sub-area and examine the conditions down there?