Waterproofing has to go on the outside surface to work.
> Any wood on the house was a magnet for termites, and they were happily gnawing away all the woodwork that touched the ground and the concrete. We replaced all damaged wood with treated timber and soaked it in creosote for good measure.
Never have wood in contact with concrete. For one thing, concrete is a sponge for water, and it will wick into the wood. For another, it becomes a highway for hidden termite infestations, as you've experienced. The solution on my house was to put a stainless steel sill plate between the wood and the concrete.
The house has an underground basement, and it's dry (20 years now). I had a wet basement before, and did everything I could think of to keep it dry:
1. gravel under the slab with plastic on top sloping down to a gravity drain. The slap is on top of the plastic.
2. gravity drain, no pumps
3. The exterior concrete walls have a layer of waterproofing, followed by a "drain mat", followed by a foot of gravel wrapped in drain fabric. French gravity drains all around.
4. deep eaves on the roof keep the water away from the ground adjacent to the house.
5. ground slopes away from the house. Driveway slopes away. Deck slopes away.
There are cracks in the walls and floor, but no water. Gotta think of defense in depth, like a castle, with layers of defense. If water pools against the house, it will get in, guaranteed. The design has to prevent any pools from forming.
> Humidity in an underground house needs to stay at no more than 50%, but I get nosebleeds at that level
I got regular nosebleeds from the dry air in Arizona after moving there. Went away after 6 or 9 months.