1) yes thinner tends to buckle opening up air paths
2)I would use t&g and tape joints
3)yes
4)foam?

use non expanding foam only a little is needed, most use none after all, it will be better than the glue I think but care is still needed.

9mm OSB3 is not available in T&G, so you'll need noggins where the board edges meet creating potential thermal bridges. 18mm T&G boards don't require noggins & avoid the thermal bridges.

18mm sarking boards should also avoid the need for diagonal & longitudinal bracing below the rafters, making it easier to fit the air barrier/VCL membrane.

I would always use a VCL membrane when placing the OSB part way through the insulation. It's not necessary if the OSB is on the warm side, as long as you glue &/or tape the OSB joints.

David

Posted By: davidfreeborough9mm OSB3 is not available in T&G, so you'll need noggins where the board edges meet creating potential thermal bridges

Been using dryliner's 0.7mm galv angles and flatstraps, tacked across the face of the studs/rafters, as 'nogging' to back the butt joints of 9 OSB3, with power-screws to pull the metal flat against the back of the OSB and gap filling expanding polyurethane adhesive. No thermal bridging results, and in fact with site discipline, a great deal of the customary bits and pieces of additional timbers can be eliminated from the insulation zone. Including no need for 2 abutting stud/rafters at angles (wall verticals, hips, wall-to-roof at eaves etc) to support 2 abutting board edges - one member is enough, with a metal angle attached, to pick up the second board edge. Same principle applied borth to plastereboard lining and to OSB sheathing.

On the other hand, using 18 T&G OSB3 instead of 9 straight edged, means that much (but not all) of the metal flatstrapping (or trad noggings) is unecessary. Take your pick. Vikinghouse prefers the 18 way.

Having had builders each choose differently, between 9 + flatsraps, and 18 T&G + other noggings, it's clear to me that 9 is the way to go - cheaper, far less handling, easy cutting, easy everything in fact - pics attached. Andy Dodden, an02ew of this forum, the builder.

There is other of factors in this debate.

Weather- Given the nature of OSB to degrade in wet condition a thicker board would cope better, corners and edges would crumble easy on the 9mm.

Fixing- 9mm OSB could be fixed with either colated screws or power nails. 18mm could only be fixed with nails and each nail would require checkingit had driven home.

Cost- 9mm per M2 would be less than 18mm t&G and less glue

Cutting - 9MM again easier also with 18mmT&G the joint requires handing

Posted By: fostertomThat rules out using non-thermal bridging metal flatstraps/angles for the many edges/joints that the T&G still wouldn't take care of. So back to thermal-bridging timber noggings and additional members at plane changes.

Thats right, and i hadn`t realy thought it through but fixing metal strips, corner through such a robust sheet would be a real pain, somtimes it can be awkward when using much thinner board 9/11mm and a very liberal application of expanding glue is a godsend for taking up suspected bad compression between metal and sheet(often cant be seen behind work piece)

i will have to rethink a trial of 18mm sheathing on my next job, just wanted to see if its as quick and asy as 9/11mm

EPS will allow water vapour through.

It will travel round it