Whilst on the Summer Field park job the other day, part of the program of work is to rip out an old suspended floor which seemed a little springy under the vinyl floor.
When we started I couldn't believe we'd been walking on it for so long, it was completely rotten. Every step we took we put our foot through, and you could squeeze the water out of the flooring. But it hadn't even rained in over a month so what caused it?
Well many years ago they had tarmaced round the outside of the property, covering the air bricks. At around the same time they concreted some (not all) of the floors inside. What this did was to completely cut off the air flow that is essential for a suspended timber ground floor to breath. This caused wet rot to set in as the moisture from the ground is trapped in there - there were no damp membranes in those days - The sweating timber soon reaches and maintains the plus 20 - 25% moisture content needed for a good attack of wet rot
To fix the problem I was told to install 4 air bricks and reinstall the timber floor. But I have refused as the room is a corner of the house, so complete through flow of air would no longer be possible. I've put my foot down (through the floor) so now its going to be concrete. 
This is the second floor in this house like this, I think we're lucky to only be dealing with wet rot!
Good point. I usually prefer proper envelope sealing to venting, but it's important to do one and do it right or else you'll run into the problems like you found.
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