Insulation that actually works

Moisture first, warmth second

Two people sleeping in a van put roughly a litre of water vapour into the air overnight. Insulation slows heat loss. Ventilation and a vapour control layer stop that moisture condensing on cold steel behind your cladding. Skip those and you get damp, mould and rust, even with expensive boards.

Fit a roof vent you will actually use. A closed, insulated van without airflow is a damp box.

Materials, honestly

Material	Why people use it	Watch-outs
PIR board	Strong R-value per mm, widely available	Awkward on curves, gaps need filling
Sheep wool	Forgiving with moisture, nice to work with	Bulkier, dearer
Thinsulate-style mats	Excellent in cavities, breathable	Cost adds up
Closed-cell spray foam	Fills every void	Can trap water, hides rust, miserable to reverse

Most UK DIY builds mix PIR on flat areas with a flexible product in ribs and awkward voids. That is fine. Chasing a perfect R-value while leaving gaps around every rib is not.

Order of work that saves pain later

1. Treat rust first. You will not get back in there once the walls are closed.
2. Sound-deaden about a quarter to a third of each large panel. Full coverage is mostly wasted weight.
3. Fill cavities and ribs, then the flat areas. Keep product dry and compressed as the maker intends.
4. Fit the vapour control layer and tape every seam, gland and penetration. Half-hearted taping defeats the point.
5. Leave a clear path for future cable pulls if you can. Future you will be grateful.

Weight

Insulation usually adds roughly 20 to 30kg on a mid-size van, more if you go heavy on wool or spray. Log it in the weight tracker so the payload gauge stays honest.

Insulation without ventilation is how vans get damp. Put the fan in early and use it.

Figures in this guide are planning estimates. Confirm weights and prices against real products, and have gas and electrical habitation work signed off by a qualified person.