We're quite excited to be commencing design work on a Grade II listed barn conversion and I'm interested to hear from anybody who has experience with this kind of thing, with a focus on the detailing of the weathertight thermal envelope.

The roof tiles and weatherboard will be lifted, providing opportunity to add a sheathing/sarking, perhaps an insulated sheathing/sarking. The sheathing/sarking racking strength just needs to be sufficient to transfer wind loads as there will be some secondary structure added internally to help support the frame. The roof tiles will be replaced with new tiles but they (and the sarking) will need to follow the existing wobbly roof. The weatherboard will be a mixture of recycled + new. The main focus is preserving and maintaining visual exposure of the timber frame internally. So a small amount of insulation can be added between studwork/rafters, but not full depth.

Are there any standards/guidelines out there for this kind of work?

My initial inclination is to stay away from plastics and sheath the frame in a suitable wood fibre board of some description on the outside, then wood fibre insulation + wet plaster between stud/rafters.

But I did also wonder about EPS. And I did also wonder about a fibre-reinforced plasterboard like Siniat Weather Defence board. This as a vapour resistance of 0.77 MNs/g and would provide a weathertight, fire resistant sheathing with no need for a breather membrane. I haven't checked the figures but I imagine a wood fibre sheathing/sarking + breather membrane would come in about the same figure. Although Weather Defence may not be appropriate for a pitched roof, and a wood fibre sarking might be better able to follow the wobbly roof.

No, some requirements can be relaxed where strict application of the regs would harm the special interest of a listed building. We do the best that we can and apply for a waiver for where we can't meet the regs.

Make it as air tight as possible. Add continuous mechanical extract (because there's nowhere to hide ductwork for MVHR). Underfloor heating + air source heat pump.

Ideally we'd be adding a load of insulation outside of the frame before putting the tiles and weatherboard back on, but that presents all sorts of detailing problems to the external wall and the eaves junction (of which there is very little overhang).

Posted By: fostertomIf you've got the tiles off, the ideal way by far to insulate is over the top of the sarking board on top of the rafters.

I expect that's easier said than done on a listed building?

I should add that there should be a continuous perhaps 75x75 running the length of the ridge, set flush with the EPS, for the downslope battens to hang across - without it the EPS at the ridge would prob crush under load from the downslopes. It might be OK to let the 75x75 simply bear distributed where it's seated on the EPS, or else provide intermittent solid support for it, penetrating the EPS.

Similar short pieces looking like projecting purlin ends could support overhanging verge rafter and boarded soffit; similar looking like projecting rafter ends at the eave.

Whatever you decide throw it out the window and start afresh. This is a grade 2 listed building your talking about and wacky modern ideas do not hack it when faced with an old fashioned conservation officer. Best advice is to join the listed building community and learn from others experience on what actually works on a Listed building. There are normally 2 listed building road shows each year one in London and the other in Harrogate which are a fountain of knowledge and expertise and I would thoroughly recommend attending if only for the contact of experts you will find there. Not knowing your building construction I cannot advise but on your ventilation I would throw your continuous mechanical extract in the bin and install Partel MVHR units instead which have worked very well on our grade 2 listed farmhouse.