My mats have ridges/folds in them when they are laid – is this okay? 

All mats should be laid as flat as possible. Whenever possible unfold the mats 24 hrs before they are laid - this will help the mats “relax”. If you still encounter ridges or folds stretch the mats as flat as possible and use adhesive tape to secure the edges of the mats to the insulation material. You could also lay the Duct tape across the mat (along the fold/ridge) to assist in levelling the mats.