wet joists underfloor heating

[simon72post]

Hello Please can someone give me some advice on wet floor joists.

We have a 1920s brick built bungalow that has been cavity insulated. We have just converted the upstairs into bedrooms.
The house is situated on land that has a very high water table in the winter the water is about an inch from the surface.

The house has the original single pain glass windows that always get condensation in the winter. Before we started the building work there was a bathroom in the middle of the house with a window that couldn't be opened. So we put the high amount of condensation down to that.
The house is fully rendered on the outside. With lots of air bricks 12 in total all clear.

I have started lifting floorboards downstairs because I would like to install underfloor heating.
But I have noticed the suspended floor joists are dripping wet underneath. Looks like they have been wet a long time.I know there are no water or pipe leaks. There are no leaks from upstairs as it's all new.

My thought is the moisture is coming up from the ground as it's so wet. And as condensation due to the temperature difference between inside and outside.

The internal moisture is not such a problem now as we have a new bathroom with an opening window and extractor fans.

I am looking at ways to reduce the moisture under the floorboards. I am thinking of putting a damp proof membrane on the ground to reduce moisture coming up from the ground. Keeping the air brick clear for ventilation.

And as I would like underfloor heating then pinning a breathable membrane to the underside of the floor joists to allow moisture out but not in. Protecting the underside of the beams from moisture below.
Then draping another breathable membrane over the joists and putting in hygroscopic insulation between the joist as insulation for the underfloor heating. Again allowing moisture in the house to escape while not allowing moisture in.

Please could someone let me know if my plan is a good one one. Or if I should tackle the problem in a different way.

Thanks.

[El carajillo]

Your problem lies with the high water table . The only way I am aware of is to reduce the water table around the bungalow.
This is achieved by digging trenches around the property and forming land drains, by filling with aggregate and leading the land drain (water) away depending on the lay of the land. Or it could be led to a sump and pumped away.
Not lowering the water table will lead o rotten joists and floor in future in addition to a permanently damp house.