cold bridges in internal insulation

[mpooley]

I.m asking this for my nephew again as he is about to add 50mm cellotex internally with some 2x2 studs around windows and in other strategic positions.
He will then use a 20mm frame to hold them in position thus leaving a 20mm cavity before plasterboarding.

My question is:
How important will the un-insulated studs be, to the total insulation value of the wall?

It will be difficult to insulate them as the cavity will be used for wiring etc but I don't want him to spend so much time and money if the studs will compromise the U value so much as to make it useless.

thanks

Mike

[JohnS]

Why use solid studs?

I 'instafixed' 50mm Celotex/Kingspan to the walls and then over battened it with 25mm battens fixed to the walls with masonary screws.  I filled between the battens with 25mm celotex and then covered with Fermacell instead of plasterboard.

Also, I recall that Knauf have an internal wall insulation using composite studs which are 50mm of some sort of foam insulation and 10 or 15 mm of OSB.  Between the studs is filled with glasswool type insulation before plasterboarding.


Found it !
http://www.knaufinsulation.co.uk/products/thermoshell-solid_wall_insulat/thermoshell_iwi.aspx

John

[mpooley]

Thanks
But I think he wants to use timber studs around doors and windows to make it easier to finish around them.
Also he is using them in places to hang kitchen units etc.

mike

[wookey]

I'd say the same - there is no need for studs - indeed they are a bad idea. If he wants to hang kitchen units then just use frame fixings right through to the wall - easy with only 50mm insulaton (120mm frame fixings).

If he insist on studs then at least get the eco-studs knauf do. If I need to put in something solid to screw things too I cut out a slot in the insulation and put the stud in there, then finish the vapour barrier over the front before installing plasterboard. This works fine, but is not a problematic thermal bridge because there is still insulation behind (studs thinner than insulation depth). You can attach them through to the wall to make them really solid, or just use them as plasterboard-backing for intermediate weight items.

I'm just about to do a garage full of incredibly heavy shelving. That will have 50mm insulation continuous, then 50mm more insulation with 50mm studding between, then boarding (multipro or ply).

Full-depth timber noggins are a bad plan and there should always be at least some insulation behind or in front - even if only 20mm.

[mpooley]

OK Thanks
I will tell him 

[MR GUS]

Wookey, a few step by step photo's would be nice if you can spare the time!
I can't quite get my inadequate head around it without a few visuals...

[Baz]

I think you can get a bit too paranoid about bridging and risk having the cabinets drop on someone, or at least sag on inadequate fixing. Admittedly the U value of wood is twice as bad as the best foam board but actually close to that of the roll of  fluffy stuff i just got so not to be sniffed at.
 However 2x2 is more than necessary and pretty expensive too these days. Consider a build up of 15mm softwood strips for nail/screw on 6mm ply for depth.
 If you do go for a stud free design get a 2 in hole drill to make a hole at the cabinet fixing points and matching FULL DEPTH wooden plugs. It you try a long screw just through foam it will hold it in but not up and in a few months they will sag out of line.

[wookey]

Yes, you can get too carried away, and things do need to be properly fixed - but they need to be properly fixed without serious thermal bridging, which is usually not hard to arrange. Your numbers are wrong though: wood is 5-6 times worse than the best foams, and 3-4 times as bad as fluff. see http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

I picked 2x2 because it's the same depth as the foam so it's easy to put up, and I expect my garage shelves to have a _huge_ amount of Oh, how I wish it was better on them, so this is being overengineered. I agree that I could get away with smaller timbers. The wood is nearly free though in comparison to the nearly 2 grand of insulation, panels and floor coverings.

Long screws will be through the face panels (plasterboard/ply/cementition board) and then insulation. The boards may well be sufficient to hold stuff up. But yes, wooden plugs would work well.