LearJersey

Sorry for the delay - have been away so not been looking at the site. Yes to both, an internal and external membrane. My SIP supplier was just the same - supplied the SIP structure and external membrane only. I sourced the internal one myself. Like you, I was in two-minds, so I looked around a lot and it ~seemed~ that most garden room builders weren't doing the internal one, on the basis that the rooms are not for intensive use, therefore moisture content in the air is supposed to be low. In our case though, I expect very frequent use - daily and for many hours at a time - so I opted to included it. It's not a massive additional cost, but a bit labourious putting it up... Again, maybe belts-and-braces, especially as I have created a vented channel on my OSB deck through the oversheet if any moisture does transfer through the roof assembly, but I'd rather do the job well once, than live to regret a step that I can't subsequently rectify. (Made that mistake before!)

Actually one other small benefit of the oversheet that's worth mentioning - Our building merchant stocked some 18mm TnG OSB sheets which we used. The TnG ensured that our top structural surface - that received the EPDM - is completely level and uniform across the surface. Conversely, we found that the roof SIP panels resulted in small variances when interlocked as a monolithic structure. Probably not too great a concern if you're not oversheeting as I believe the EPDM can handle this, but it was a nice result so worth looking for TnG if you do decide to go that way.

No, not at all. In fact, as I understand it, that's how the majority do it and our SIP supplier originally suggested. The oversheeting is a belt-and-braces approach... gives a small channel for any moisture than transfers through the SIP roof to wick away rather than remain stuck under the EPDM. I think this option, together with a good vapour control barrier internally, is probably overkill, but we plan to use this as a garden office for two people year round and with heavy usage, so it's a small additional cost / effort to play it safe. If you don't do the oversheeting and 'just' do an AVCB, then I'm pretty sure it'll be fine for most uses...

Shortly taking delivery of a SIP garden room - all precut as a kit. As agreed with the SIP supplier, the plan is to overdeck the SIP roof with battens running front to back and a top (final) OSB 18mm sheet that will receive the EPDM, so as to promote ventilation across the roof and maximise lifespan of the building. An AVCL will be fitted carefully internally, but the oversheeting is being done as belt and braces just in case of any residual warm air condenses to the cold side of the SIP roof. Question is: would there be any reason not to apply the same breather membrane to the flat SIP roof (before battens for oversheeting are fixed) as will be put on the wall elevations? We're using Tyvek UV facade product as we're going for an open rainscreen cladding system (larch) for the aestheics and to promote good ventilation to the wood. Thanks in advance