vapour barrier - what coverage?

[Bubble Meister]

I'm about to install polythene vapour barrier. On the rear side panels I'm gonna cover the voids above the rear wheel arch but should it continue to the very back where there is no void? It's on the inside of where the rear engine bay vents sit.

[Bubble Meister]

Anyone....? The vapour barrier is gonna be fitted late afternoon.

[a1winchester]

I'm no expert, but vapour barriers only work if there's good insulation on the outside of them. All to do with temperature gradient from inside to outside, and the dewpoint of the air. At a given temperature, air becomes saturated with water, and below that point the water condenses to form dew (condensation). Provided that that temperature (the dew point) is outside the vapour barrier, all is fine, but with little or no insulation, the dew point will be on the inner surface of the vapour barrier and you will still get internal condensation.

Cavity walls are like this in houses. We aim to get the dew point in the cavity / outer skin, not the inner skin.

The question in my mind about vans is - what happens to the condensed water between the vapour barrier and the body panels? If there's no air flow to evapourate it, will it just run down inside the walls and rust something out?

[clift_d]

The water vapour wouldn't be able to get between the vapour barrier and the bodyshell unless there were gaps in the vapour barrier - it would just condense on the inner face of the vapour barrier (if there was no insulation).

[Bubble Meister]

Can you clarify my thoughts - the vapour is on the inside if the van and produced mainly by breathing? Hence I'm trying to stop that vapour from reaching the cold outer skin and condensing.

[Bubble Meister]

I tried to load sone images so you can see the built up: reckhorn sound deadening, 50mm kingspan then polythene vapour barrier.

When I submit the image it says the quotas exceeded!

[Tobias13]

I have lined all my panels with twin foil bubble, this also acts as a vapour barrier!!

There are 4 of us sleeping in the van and its soaking in the mornings on glass and exposed metal, I would hate to see the damage it causes by getting into the cavity, so def def vapour barrier!!

[clift_d]

Can you clarify my thoughts - the vapour is on the inside if the van and produced mainly by breathing? Hence I'm trying to stop that vapour from reaching the cold outer skin and condensing.

Exactly.

If you fit insulation then you get a temperature gradient across the insulation. The inside face might be say 20°C while the outside face might be at -3°C, so there's a continuous drop in temperature through the insulation to the outside. If water vapour gets into the insulation from the inside - people breathing, cooking, sweating, etc, then it fills the voids in the insulation and where the insulation is cold enough it condenses and turns back into water. The vapour barrier just stops the water vapour going any further than the warm face of the insulation hence no condensation. Simples.

[Bubble Meister]

Ahh, that makes sense. I guess that at the very back (adjacent to the black side grills) I'll get condensation on the inside but caused by the fact that there's no space for insulation.