vapour barrier help


Full Member

Hope everybody's well.

I'm planning my conversion now, from a Ducato panel van, and am currently drawing up all the plans.

I'm already stuck at the first step - insulation. I understand there is a need for a vapour barrier as you don't want condensation to collect on the inside of the vans metal panels, degrading them over time. But surely wherever the vapour barrier is within the wall, moisture will trickle down and pool at the bottom? How do you stop this? Also, how much of a gap is considered a vapour barrier?

Here is what I was planning to do...

-Spray glue Silver Bubble insulation to the interior wall of the van.
-Then on top of that and in-between the structural struts of the van, fill with 25mm Celotex/Kingspan.
-I'd then infill the struts with some form of insulation wool - maybe recycled plastic bottle insulation.
-Then I would cover the Kingspan with a roofing membrane (eq Tyvek), but this would sit over the struts therefore hopefully creating a gap (vapour barrier) between this and the Kingspan.
-I plan to board the inside of my van with cedar cladding (11mm thick), rather than carpet walls, so this cladding would be screwed directly into the vans struts or, into battons that are screwed into the side of the struts.
-The only thing I may change is by placing a sheet of 5mm ply between the Tyvek and the cedar to help with the rigidity of the interior walls as there will be cabinets mounted to the walls. If I did this I would probably screw the cedar boards to this ply from the back, making up wall sized panels that could be lifted and screwed in place in one hit, but keeping the the appearance of the cedar boards.

I was planning to avoid using any form of spray insulation or expanding foam as it would be impossible to remove if I ever needed too.

Insulation wise, is this sufficient? What about that vapour barrier? Do the bubbles in the silver bubble insulation act as enough of a vapour barrier? I'm guessing not. So if not, how much of a gap do people think is needed between the Kingspan and Tyvek to act as a vapour barrier?

Any help on vapour barriers would be great.

Thank you.


Full Member
Unless you intend owning the van for thirty or forty years, vapour barriers are over rated imho. Look for obvious moisture traps before you line the interior and paint them with underseal. If you want to be really careful you can underseal the whole interior and put some vents in top and bottom on whatever you decide to line the van with - much like a standard house cavity wall venting.
My 1984 Westfalia just has rockwool stuck onto the walls and ceiling, it breathes and so far there has been no rust where the insulation is stuck - elsewhere now that's a different story.
I also have an LDV V80 Maxus which I insulated with silvered foam on the roof panels between the cross members and on the walls used rockwool. So far I'm not aware of any moisture issues even in my hot and humid climate with air con running during the day. I undersealed the inside of the lower parts of the walls and doors. Chinese made steel and rust proof dipping is not the best.
I like the rockwool as its also non flammable, easy to install and cut to odd shapes and stuff into voids that have small openings. Whereas spray foam and silvered foam both will catch alight given enough heat. Silvered foam is good for big flat panels but is impossible to get into voids. Spray foam - aargh covers everything, encroaches onto cross members that you need to use to secure the lining and you need to have all the plumbing and power in place before lining.
Think about the type of lining you will use - if its wood, make sure you seal the side that will be facing the van walls. If its a plastic/composite lining you will need to vent somehow.
You are confusing yourself, a vapour barrier is not necessarily an air gap and yes a lot insulation has a vapour barrier built in by way of foil etc
The problem is interstitial condensation which put simply is warm moisture laden air hitting a cold surface and creating damaging damp.
We had one van where literally cups full of water collected inside the roof space and poured down as you drove off.
This will help explain
interstitial condensation - Google Search


Full Member
Save yourself a lot of time and trouble by getting the van spray insulated. Fit the windows and vents first then mask them any other parts you don't want spraying. Fix battens for wall boards and cupboards and channels for cables and pipes. You can fit cupboards and furniture temporary to try the layout before you have the spraying.

Mr B.


Full Member
Just thinking

Too many layers is too much.....Weight and fuss
and impinges on "space in the van"..

I appreciate you planning to do a good job but try not to go OTT.
Heat rises so the roof is most important
I appreciate you also need to reduce condensation problems


Full Member
I think the insulation and vapour barrier is covered above, I.e. Vapour barrier is on inside of insulation and is to stop warm moist air hitting cold surface.
Now let's look at cedar and ply, as you have described it is way ott, and you are likely to lose a lot of payload. Properly constructed either should do the job on own and still be ott, large areas of our walls are just thin plastic. Don't forget it's best to only store light items in overhead lockers if possible.


Full Member
You could join DIYMotorhome, lot's of advice on there. It's free to join!

Myself I de rusted everywhere, then Isoflexed all the joins in the body, then painted it with rustoleum hard hat, I then stuck 10mm thick CLOSED cell foam rubber sheet directly to the panels wherever I could & filled the areas behind the ribs etc., white recycled plastic bottle insulation.

I left a bit of the foam sheet loosely wedged in & a bit of roof bare, because it was this time of the year I didn't have to wait long for the roof to be frosty, the bare patch was very wet but when I removed the foam sheet it was bone dry!

You are only trying stop warm damp air hitting the steel, this air doesn't come from the steel, so after you've fitted the rest of your insulation, you then stick your vapour barrier over the top, but there's no harm in sticking one to the steel first except you don't really want any air gaps.

Hope this helps!



Full Member
I agree with the advice to have your van spray foam insulated. Very good and easy if not cheap.

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