Loft Steel frame boxed in with vapour board AND CELOTEX?

[Robbie72]

I have architects plans for our loft conversion which state that the steel frame, where showing, is to be boxed in with 25mm Celotex T-Break and then 12.5mm Foil Backed Plasterboard and skim.

I can see the point of the 12.5mm vapour board to provide the building regulations fire protection requirement, however I can’t see the point of the Celotex insulation?

If anyone knows why this has been recommended, and if it is actually necessary, it would be much appreciated, as the extra 25mm headroom would be quite handy if it’s not actually required!

Looking forward to comments! Many thanks.

 

[freddiemercurystwin]

Why don't you ask your architect? Got a detail you can scan and load up? Are these steels in your roof? That's sorta how it sounds!

May be reducing the cold bridge from the outside but can't say for certain without seeing where its located/how its detailed.

 

[Robbie72]

Why don't you ask your architect? Got a detail you can scan and load up? Are these steels in your roof? That's sorta how it sounds!

May be reducing the cold bridge from the outside but can't say for certain without seeing where its located/how its detailed.

Hi Freddy, many thanks for your response.

I've gave up with the Architect ages ago so not going to bother contacting them!

Yes the steels are in the roof, but not entirely hidden by the roof timbers. I've attached a photo if that is any help:

Untitled

• Robbie72
• 12 Oct 2011
• 1

I'm just over 6ft tall and the Celotex and Plasterboard layers would reduce clearance under the horizontal steel to around 5cm so I'm trying to gain as much headroom as possible!

Thanks again, Rob

 

[freddiemercurystwin]

OK, regarding the sloping steels the insulation is there to reduce the cold bridge, as they're pretty close to the outside. Is that a breathable membrane? If so it looks like you may be able to get some in above the steel although maybe not it looks pretty confined. Without any insulation you're likely to end up with greyish and maybe damp marks under the steels showing on the plasterboard as those steels are gonna be seriously cold and any moisture within the loft once its finished is gonna wanna condense on those steel chappy's.

The roof insulation is going to follow the line of the rafters so there would be little advantage in insulating underneath the horizontal steel although strictly speaking this does need insulating under Building Regs as that one is also allowing a cold bridge from each end. As ever its probably best to talk it through with your inspector on site, this is one of those that's somewhat open to your inspectors discretion.