Stopping mould on items stored in the attic of a new build

It would have been better in terms of the thermal efficiency of your home if you'd left the insulation as per build spec and raised your flooring so it was higher than the depth of the insulation. There are several methods of doing this. There could be a whole list of reasons why your stored items in the loft space are getting mouldy. Could be a problem with ventilation. Without being able to inspect the property, this would be my bet. You may have created cold spots in your insulation when fitting your flooring. The loft insulation could have even been laid badly by the housebuilder. I've seen many properties where the contractors or house builders who have installed the insulation in a new build have actually blocked the intended ventilation measures in the eaves. Sorry I cannot be of more help without a visit.