Beesknees,
Felt underlay is great.
As wardy says, don't use the foam backed carpet and don't use nylon carpet fro long term work - it "breaks down" into long strands of nylon, which don't break down any further.
You can therefore use nylon carpet for around 6 months but then it needs to be removed to the recycling place (don't burn it - it creates all sorts of nasty smells and substances) - remove it when dry - it's very very heavy when wet
Phil