I recently had an addition added onto the back of my home. The addition 10x'24' is on cinderblock footers. The underflooring has been well insulated and enclosed with marine plywood. To help eliminate any draft (which is little) and to give a finished look I have decided to enclose with underpinning which I understand how to do. My question is once enclosed should I then use heavy guage plastic to cover the ground and side walls to controll moisture or would this be overkill?
The ground under the addition should have a 6 mil vapor barrier and at least 2" of concrete over that. In NY we put 2" of insulated boards over the plastic, then another layer of plastic, then the concrete.
It appears to me that you may have done this the wrong way around...
Generally we would set drainage under the surface of the ground around the wall perimeter and extend the drainage outside to be connected to the stormwater. (I know different systems in different countries)
Then we would vapour barrier and insulate the sub-floor and then pour a concrete floor over (Min 100mm or 4" thick) with basic reinforcement. If the floor is to be structural, then add stronger reinforcement and thickening / strength of the concrete. (Have a structural engineer design floor)
We would add waterproofing to the existing walls and then have a cavity (min 50mm or 2") between the existing walls and new wall. Build our internal walls with a flashing under the wall. The internal wall to have insulation and wall linings to be moisture resistant or waterproof. (Sometimes if there is too much moisture present, we would install an in-line elect fan to ventilate the cavity space)