This is a common misunderstanding often quoted by damp proofing specialists but again it is not true. This statement implies that there is a ‘steam machine’ under the floor pumping water vapour out under pressure that then ‘leaks up’ into a room around the edges because the floor wall junction is not sealed. Just look at the surface area logic and explanation in Condensation 7 above to see that this just would not happen. Correct sealing of a floor wall junction is very important for other reasons but not for preventing significant condensation in a property.