Damp soil under future sprung floor

[Bobthedino83]

Hi, All. I'm not from the US and timber construction is not a super popular thing where I'm from (Cape Town) so my terminology might be wrong. Though we do have a a LOT of victorian and mid century houses with sprung (suspended?) floors the tendency is to rip them up and pour a slab for tiles or engineered wood. I like to restore houses and find myself in my 3rd house with oregon pine suspended floors.

There's a formal lounge area that was under carpet and we expected most of the floor under the carpet to be suspended oregon. Instead about a 3rd was the latter and the rest was slab. So I pulled out the oregon (douglas fir) and used it to repair floors elsewhere in the house, then I chopped up the slab.

Now I've got a large room with exposed soil for a floor and the next step is to build the substructure from pine beams and then to put reclaimed oregon pine boards on top.
My concern is regarding the moisture levels in this room in the meantime. Granted, we've had exceptionally heavy rains the last month plus the room has been completely sealed up around the windows and door with masking tape to keep the icy draft out until I've gotten double glazed upvc windows and doors which are just as air tight.

After a day or two of the room being closed like this it smells distinctly humid. Like damp earth.

This is a concern as I don't want my floor boards cupping from the moisture differential between the top and bottom. I can't put a dehumidifier in the suspended space nor is there an external wall suitable for a vent (though maybe there is one spot).
Someone suggested DPC which is a thick plastic sheet over the soil. Sounds like a good idea to me however do I need to seal the DPC up the sides of the walls as well so that absolutely zero moisture comes through?

Any advice would be appreciated.

 

[jj4091]

In the UK with a suspended floor a damproof membrane laid under concrete and perhaps insulation would be the way to go. Then an air gap between that and the joists.But you would need the underfloor to have through ventilation. Sounds like you might be a lot better if you did "pour a slab" if you have ripped out all the joists as you would probably need dwarf walls to support the new joists as well. With a slab you would still need a DPC and insulation.

 

[Bobthedino83]

Yea hell no I'm not pouring a slab, that adds a lot of costs to an already unplanned project. Underneath all my previous suspended floors there was nothing but dirt and building rubble, though there was always a vent hole somewhere as one could feel the wind coming up through the floor when it really blew.