Summer House DBL Board OSB + Breather Membrane Needed?

[ILikeDIYS]

Hi. Doing the outside of my summer house. Interiors are plasterboard + foil bubble wrap underneath the PB, then rockwool insulated 4x2 frame, then OSB on the outside.

Should I use 18mm OSB or double board in 11mm? I'm trying for sound insulation so wondering if the extra 4mm will be worth it. Sould the 2x 11mm be stuck together with silicon around the perimeter?

Also a question on breathing etc...

I see people are using breather membranes for OSB. But in my case I have 30mm Celotex to stick ON TOP of the OSB on the outside. Celotex won't allow breathing.

I was initially going to do an OSB Celotex sandwich [OSB -- Celotex -- OSB] but have decided on [OSB -- OSB -- Celotex] instead because of weight.

So if I put the Celotex on the outside should I drill holes in it to help with breathing?

This breathing thing confuses me cos OSB is a resin product and shouldn't have to breath anyway? On top of that it won't allow vapour through it anyway so what's the point? What's the point of even having a breather membrane on OSB?

I'm thinking just slap the OSB on, stick/screw the Celotex onto that and vertical battens ready for cladding. No messing with breather membranes or drilling any holes.

Thanks.

 

[ILikeDIYS]

BUMP!

I'm thinking of doing the breathable membrane first then drilling shallow holes in the Celotex just deep enough to get rid of the foil on both sides as I hear the foam part is actually breathable but not the foil... then screwing it on top of the breathable membrane?

 

[dontbelieveawordofit]

the celotex outside is pointless imo , unless i’ve read it wrong.
wrap in breathable membrane batten and clad.
personally id go with 18mm osb.