Staggering Multi Layered Board Joints

[padstar]

I am going to start internally insulating my external walls over the break. I am using 50mm celotex board over-clad with 12.5mm plasterboard. The ceilings are 2800-3000 so there will eb joints even if the boards are installed vertically.

Given that the boards are the same size i was planning on installing one horizontally and the other vertically so that there is never a joint straight through both boards. This could of course be achieved by staggering vertical joints but i thought it would be easier running at 90 degrees to each other.

3 Questions before i begin;

1) Is 1 skin vertical and the 2nd horizontal common practice and ok to do (i have seen it regularly done on site but with both skins being plasterbaord?
2) Is there a preference of the celotex or plasterboard layer to be laid horizontally/vertically
3) Due to ceiling heights there will always be a 200-300 section on top of a full board. Should this strip be always placed at the top/bottom or should these also be staggered.

 

[^woody^]

You dont need to stagger boards on masonry walls. Otherwise fit the board however you like.

 

[mikeey84]

I've done the 300 strip at both top and bottom. Usually I would change depending on what is going in the room, so for a room where there is a bed, or sideboard across most of the length of the wall, do the join at the bottom. This is only if you are taping and painting. If it's being skimmed it really doesn't matter

Doing it at the top is a lot easier though, as you don't have to lift a full board up, which are quite heavy

 

[John D v2.0]

I can see why it's desirable to stagger the boards, but you don't mention how you plan to fix the plasterboard or what the buildup is e.g. taping the foil or separate plastic barrier.
You would really need a mechanical fix and you need battens to screw the plasterboard into.
If you want to dot and dab you'd have to get the expensive insulation backed plasterboard, otherwise either insulate between new studs, or put the celotex up and screw battens through the lot into the wall, then screw plasterboard to the battens. You'd need to either tape or use a vcl on the warm side of the celotex.

 

[padstar]

Thanks guys. The plan was to bond the celotex to the fairly flat walls fully taping the joints with foil tape. Then bond the plasterboard to the celotex with a few mechanical fixings back to the masonry with the lot being skimmed after.

Was thinking to have the rio at the top for ease as the skin coat should leave a crisp flat finish.

 

[Ian H]

You can get bigger boards, not sure on price though:

https://www.ccfltd.co.uk/British-Gy...-12-5mm-Tapered-Edge-3000mm-x-1200mm/p/796783