I have used for my vivaro conversion professional duct insulatio " ARMAFLEX"  30mm ,fitted with spray glu . This stuff is briliant, silver foil outer, soundproof as well, verry soft to fit in and around corners. This ( i have finished my conversion just in July) cepps the van cool in the heat and warm at the cold, damm effective. At one night at snowdon , a fue weeks ago, i had at night 20 c in the vat at outsite 8 c.  I am a professional master plumber and heating engineer since over 30 years, so i know this material from industrial ducting at some shopping centres i was involved.      Hope this helps.   Mario