Using Celotex on solid brick external walls

14 Dec 2008
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United Kingdom
The solid-brick (no cavity) house we've just bought currently has no insulation on its external walls - something we'd like to change.

After research elsewhere online and in this forum I thought we'd dot & dab 25mm Celotex PL3000 (the Celotex that's combined with 12.5 mm plasterboard) to minimize installation time. I'm thinking 25mm as Celotex claims to be twice as efficient as traditional insulation so we'd effectively achieve a 50mm insulation layer.

But do we also _need_ to apply a breathable membrane on to the wall before fitting the Celotex? If we did fit the membrane then presumably we'd need to use batons rather than dot & dab on to the membrane.

Then my head really started to spin; if we line the walls with the breathable membrane but then screw batons to the wall in order to fix the Celotex.... aren't we breaking the membrane by screwing through it?

Comments would be much appreciated.


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You don't need a membrane when using foil laminated insulation boards.

I think you would need to use something like 50mm celotex plus plasterboard to achieve thermal standards.
Thanks for clearing that up about the membrane.

I take your point about the 50mm, but if Celotex is as good as they claim ('twice as efficient as traditional insulation') do you not think we could get away with their 25mm size (2 x 25=50 and all that).


By the way i didn't mean to mislead you with the foil/celotex thing. If you buy foil laminated insulation boards then they will need to be fixed to battens and can not be dabbed.

You can fit what you like, so long as it is not being inspected! :p

Seriously though, we recently upgraded a 9" solid brick building and the stipulation was for 62.5mm kingspan kooltherm k18 insulated plasterboards to be fitted.

At the time (6 months ago) this adhered to reg's.
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Hi Steve
I have got a single brick wall which I had to insulate and as I didn't want to lose room inside I worked externally. Building Regs advised me to use 50mm Celotex onto the brick between horzontal battons then Tyvek and vertical battons then weatherboarding onto that. Not much use to you if you are working internally but maybe you could do the same replacing the weatherboarding with plasterboard? I've also heard that the foil backed Tyvek has insulating properties which might cut back the amount of Celotex you need but then may cause problems if its not breathable...
Thanks to you both - I'm learning a lot!

The board I've been looking into is the combined Celotex / plasterboard product (PL3000) - basically a combination of Celotex at your desired thickness AND a 12.5mm plasterboard, which cuts down installation time as only a single board needs to be fixed before plastering.

Thing is, it doesn't appear to have a foil layer so as I understand it I'd be back to having to fit a membrane against my walls and thus not really saving any installation time anyway.

There must be a specific market for the Celotex PL3000 but it doesn't sound like I'm it!



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