Studwork cavity wall

[Tewy81]

I'm converting my garage, my architect has detailed a stud wall to seperate store room from utility room using 4x2 timbers. This wall is double thickness. what gap should i leave between the 2 walls as its in effect a cavity wall. Would 50mm be ok?
Andy

 

[^woody^]

Why had he specified a double wall when a single will do?

 

[Tewy81]

He said because the small store room has a door opening to outside, its better to do a double skin wall, he said "its in essence a cavity wall and requires thermal properties" i've got the timber to do it but i dont know what gap to leave between them, surely not 100mm, would 50mm be ok?
Thanks
Andy

 

[^woody^]

Your architect does not know much.

A double cavity timber wall is a nonsense, and liable to rot.

You can achieve the thermal requirements of an external wall with a 100mm frame and 100mm celotex insulation.

There is no advantage in making the wall any thicker.

If you are intent on doing it double, then don't have a cavity, else that will cause you problems with detailing ventilation and drainage of the cavity, and the vapour check and membrane for both leafs. You will also need to fill the frame entirely with insulation.

 

[Tewy81]

You got me worried now Woody, i was all up for building this but have my doubts now especially if it could be less effective than a singlle 100mm wall.
I've got the celotex ready and was all prepped to build it, presuming double wall = double sound/heat insulation.

 

[^woody^]

You have to be careful with timber frames - prevent moisture getting in from the inside and outside, and allowing moisture to evaporate out and not trapping it. You do not want a cavity, or any air voids between two timber frames, or within a frame.

You could build it 200mm thick and completely fill it with your celotex . But ideally you should have insulation between the frame and then across the inside of the timbers before plasterboarding, to prevent thermal bridges on the timbers.

100mm timbers filled with celotex will meet b/regs. But a further 25 or 50mm across the internal face will prevent thermal bridging of the timbers - and allow for any sockets or switches. If its within 1m of a boundary, then you need to sheath it with a cement fire board (eg Supalux) instead of OSB or ply, and then clad it with whatever you are cladding it with - and this should be on laths to form a ventilated cavity between the cladding and the frame

Your architect should have detailed this, as it is very important to get right

 

[Tewy81]

Thanks Woody, I have just double checked this, it isn't actually a cavity wall, its a 220mm stud wall(completely filled with either rockwool or celotex, then theres 25mm celotex(or similar) across the whole wall then double plasterboard.