One layer of Reflectix needs a minimum 2 cm airgap in order to achieve it's insulation "R"-rating, and in many applications up to two 3/4" air gaps (sorry to mix metric and 'Merican) or combination with insulation batting.
The internet is full of people decrying the misleading marketing and advertising of Reflectlix.
You can build little 1" standoffs by cutting 1" squares from foamboard and gluing them onto the kettle side of the Reflectix insulation wrap.
The other option is to use THREE wraps of Reflectix. Three wraps were tested by @fermware (http://fermware.com/mash-tun-insulation-comparisons-complete// and found to be more effective than having a 2 cm gap. It's not a big deal because one roll of Reflectix contains more than enough to wrap any reasonable size vessel three times.
@squeegeethree's experience bears this out, too.
BTW, adding a cap or dome of Reflectix is the killer move (but can lead to condensation issues).