Likely moisture trapped between the floor and the mats.

Unless there is poly and insulation under the concrete, extremely unlikely, there is an inevitable upward movement of water vapour through the concrete (high pressure to low pressure) eventually trapped under the mats and absorbed by them.

Delta FL or equivalent systems will solve the rising moisture problem, though you will still need mechanical dehumidification to keep rh below 50%.

Hi Worthy,

Do you think a vapour barrier would solve my problem, or do I have to go with something that won't absorb moisture?

Thanks,

...... john

A poly vapour barrier on top of the concrete only means that the rising moisture will be trapped under the poly and feed mould growth under it. As well, it doesn't do anything to raise the temperature of the floor above the dew point of the interior air. Impermeable flooring installed directly on concrete in basements and on un-insulated slabs is--unless you're in a desert--a recipe for eventual problems. See here for further details.

Thanks Worthy. I appreciate your advice.

... john