batterns for 50mm celotex?

[Andy7]

I'm building a garden office, and have just changed my mind in the insulation for the exterior wall design from 100mm rockwool to 50mm celotex (I can explain the thinking later, when I've gotten to the bottom of this question)...

So, Wickes sell 50mm celotex, okay price, but nowhere in their stock can I see timber 50mm wide. 47mm or 70mm is the closest.

What am I missing here? Do I have a 20mm 'air gap' then?

 

[noseall]

Use 47mm and be careful when pulling the board screws in so you don't pop 'em.

 

[Andy7]

Use 47mm and be careful when pulling the board screws in so you don't pop 'em.

You're a braver man than I.
That was my first thought, but wouldn't it give the PB the excuse to bow?
I might be placing my batterns at 1220 centres so I don't have to cut the celotex (except where there's windows & doors).

Did you manage to get away with that yourself at all?

 

[maltaron]

With battens at 1220 it will bow!!

 

[seasickstevie]

The last time I tried it, Celotex wasn't too difficult to cut.

 

[Andy7]

The last time I tried it, Celotex wasn't too difficult to cut.

Yep, not worried about cutting it - a handsaw goes through it like butter, but guessing that they make it that wide for a reason. Plus, less studs, less cold-spots

... oh and less studs, less studs to BUY! The 18mm OSB3 sheath will give the structure plenty of strength.

 

[Andy7]

100mm rockwool between 600mm centres is suddenly looking attractive again!

 

[Katharine1471]

the 47mm stuff says + / - 3mm so some might actually be 50 mm. (equally could be 44mm!!)

I am intending on using this stuff for 50mm celotex.

Most likely the battens will end up with small wedges behind them to make the wall plumb anyway so my thinking was its likely to be about 50mm at least for it to fit and just will be aware of he sizing when arrived and at what point I put those battens tat are +3mm and -3mm.

 

[Andy7]

the 47mm stuff says + / - 3mm so some might actually be 50 mm. (equally could be 44mm!!)

I am intending on using this stuff for 50mm celotex.

Most likely the battens will end up with small wedges behind them to make the wall plumb anyway so my thinking was its likely to be about 50mm at least for it to fit and just will be aware of he sizing when arrived and at what point I put those battens tat are +3mm and -3mm.

Yeah, I can see that'll work if you're doing that. But in a new build with lovely crisp flat straight angles everywhere(!) I suspect that's goig to bite me.
There's a lumberyard near that does 50mm but I'm just kind of phased by the fact that Wickes don't sell "part A to fit Part B".

One advantage to 100mm might be that there's more depth to bury cables.

 

[seasickstevie]

One advantage to 100mm might be that there's more depth to bury cables.

That's a very good point. If the Celotex is tight, you're gonna have NO space at all for any cabling. I'd use >70mm battens, then get your wiring in, then poke 'em through the insulation where needed, then plasterboard.

That way, your cabling is all over 50mm deep from finished surface, and you don't need to be concerned about 'safe zones'.

 

[Andy7]

One advantage to 100mm might be that there's more depth to bury cables.

That's a very good point. If the Celotex is tight, you're gonna have NO space at all for any cabling. I'd use >70mm battens, then get your wiring in, then poke 'em through the insulation where needed, then plasterboard.

That way, your cabling is all over 50mm deep from finished surface, and you don't need to be concerned about 'safe zones'.

Righto, 70mm battens it is then.

Now, the next layer in will be my vapour barrier and then PB.
BUT... does anyone know if I NEED to have strapping between the VB and the PB? for the life of me I can't think why it's needed here and I cant seem to find any good reason for strapping other than, say holding insulation in place in ceilings before PB or where the battens are too far apart.

I'm SORELY tempted to miss out the strapping... am I wrong?