Insulation question, how far should I go?

I have used 13mm armaflex to insulate the outer walls (see second picture). Should I use thinner armaflex to also do the rest, as seen in the first picture? And then place the battons over that thin layer?

I just did wintercamping and I noticed that my 2kw diesel heater is struggling a tiny bit to keep my van warm in -13° Celsius. It does the trick but it will take some time to get the van to 20°.