Studio roof build up humidity readings.

I've installed temperature and humidity sensors in my cabin. They've logged a summer's worth of data.
I'll write a fuller post at some point, analysing the data in more detail, however here's the most important thing I was interested in:

The temperature and humidity readings of the cabin roof buildup, in particular the outer deck of OSB.

The cabin roof is been through it's second year so readings should be indicative enough of a long-term situation.

I basically have a warm deck roof, OSB ontop of foil-face foam board insulation. EPDM single-ply membrane adheared to the OSB. No vapour barrier though, and a gas cooker inside.

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EPDM

OSB

50mm Foam Board Insulation

Joists.

Plasterboard.

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Readings for the inner face of the OSB have not gone below 81% relative humidity the whole summer.

I take a general conclusion that it's best to ventilate the OSB layer with counter battens, if timber deck is required for the outer layer.

It's increasingly common with warm-deck construction to do away with the OSB outer deck and instead select a closed-cell insulation board with inherent compressive strength, and adhere the EPDM directly to that. There is then no organic material on the cold, and potentially damp, upper side. For refurbishment in particular this seems a fine enough choice.

However I'm increasingly of the preference of ventilation and breathability.

In new build, I favour timber I-joist construction for a 400mm deep roof structure, insulated between joists with a generous quantity of sheep's wool or recycled cellulose (wood/paper/cellulose) fibre insulation, for naturally breathable construction.

Thinner new roof can utilise foam-boards again, ventilated on the outer side, with typical counter-batten arrangements.

That said, there are systems combining roof covering with insulation that require no ventilation of vapour barrier. Big expensive systems developer are interested in, if they're building a large heated shed or ghastly shopping mall.