Insulated the roof with 25mm polyiso cut between the ribs, foil tape on the seams, ply ceiling over the top. Every cold morning there's condensation running down the ribs and pooling on the ledges, and I can see damp patches at the edges of the panels. Two people sleeping in there, no heater running overnight. Did I do the insulation wrong or is this a ventilation problem?
Check your seams too. Foil tape on polyiso is a decent vapour retarder in theory and a poor one in practice, because the tape lifts at the edges within a season of temperature cycling. If warm wet air can get behind the board through a gap, it condenses on the metal skin where you'll never see it until something rusts.
This is the argument for spray foam or for accepting an imperfect envelope and ventilating aggressively. The half-sealed middle ground is the one that rots vans.
Cheap interim measures that help a lot while you plan the proper fix:
Worth checking whether it's condensation at all before you rebuild the ceiling. Water tracking in around a roof vent flange or a solar mount will also appear as drips near the ribs and get blamed on insulation. Run a hose over the roof on a dry day and watch inside.
Did this yesterday, roof is dry, so it's definitely condensation. Good check to rule out first though.
Just get a small dehumidifier, sorts it out completely.
@drainfield_dave · 5mo ago · 3 replies
Both, but mostly the ribs. Polyiso between the ribs leaves the ribs themselves as bare steel connecting inside to outside — that's a thermal bridge, and every one of them is now the coldest surface in the van, so all your moisture condenses there. This is the single most common van insulation mistake and it's exactly what your symptom describes.
Proper fix is a continuous layer over the ribs: even 10mm of foam board or closed cell foam across the whole ceiling before the ply breaks the bridge. Retrofit is annoying but doable if you can drop the ceiling.
Also, two adults breathing overnight puts out somewhere around a litre of water. Even a perfect envelope needs that moisture to go somewhere, so a roof vent cracked open or a window on the vent setting isn't optional in winter, it's the other half of the system.
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@printbed_gremlin · 5mo ago
The ribs being the coldest surface makes the whole pattern make sense — it's exactly and only the ribs. Dropping the ceiling this month.
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@litercount_lou · 5mo ago
A litre a night sounds made up until you leave a dehumidifier running and see the tank fill.
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