Insulating walls of barn conversion

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BarnConvo

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Hi, I hope I’m posting in the correct place.

I am insulating existing exterior walls of my barn conversion. Building regs wants us to use 55mm celotex, 12.5mm plasterboard and 25mmx47mm battens.

I am completely confused on how to do this, so many different opinions from different builders.
The latest said to build a frame 25mm from the wall and put celotex in between battens of frame. If the celotex is 55mm thick does that mean it will touch the existing walls? And if so, is that ok?
 
55mm celotex - that seems very odd and won’t meet the requirements. What is the required U-value you need to hit?
 
Building regs have asked for 55mm celotex ga4000, 12.5mm knauf plasterboard, 25mmx47mm battens. Taped to create VPL. U Value 0.28
 
It doesn’t come in 55mm so you’ll need 60mm. You might want to ask if you can use plasterboard foam instead of battens. It makes life a lot easier as you spray the foam on the back and glue to the wall. Same for the plasterboard- no need for timber and higher u value achieved. Dot n dab is also approved method.

it’s ok to touch the wall as it’s foil backed. It must be sealed with tape/foam to stope thermal bridging.
 
Thank you. Building regs want a 25mm air gap, so would your method provide that?
They originally said build a frame 25mm off the wall and then put celotex between the battens. I’m confused…as the celotex is 55mm, wouldn’t it touch the wall (as the frame is only 25mm off the wall) and then no air gap?
 
Screenshot_2021-11-16-19-06-26-80_f541918c7893c52dbd1ee5d319333948.jpg
 
What is the external material? Is this some kind of timber overlap cladding? In which case the air gap makes sense. If this is a solid block, celotex can be fixed directly to the brickwork - foam, mechanical or dot n dab. The battens are to create the air gap. I.e they are flat against the wall to space 25mm. This isn’t needed if the external wall is weather tight. Most insulation providers provide standard approved methods
 
You've got two basic options; one is to build a wall liner system and insulate between. Pretty much what you've been advised but you could also consider using Dritherm 34 slab, which is cheaper and much easier to deal with. You'd use 125mm and it can touch the wall. A wall liner can be timber or metal stud. There are loads of systems. Also note that 55mm PIR does not give you 0.28 - you'd need at least 65mm between timber studs.

The other option is to use thermal laminate board which can be fixed mechanically or dot and dabbed to the wall. This does not need a frame - it can be fixed direct.

Thermal laminate is easy and effective but depends on the quality/condition of the stone walls? If the walls are friable you may need to render with a rough coat and fix to that. Bear in mind that fixing to the wall later will be either plasterboard fixings (limited load) or you will need long screws to fix through to the stone. A liner system is more fiddly and the thermal performance is not as good. But the timber framework makes it easier to fix things later.
 
I suspect the building control officer is counting the R value of the existing structure as part of the U value calculations:
On its own 60mm Celotex has an R of 2.7 + about .45 tops for the plasterboard so U=0.32. If the structure is some sort of concrete block then its another 0.4 - 0.5, which gets you to under 0.28
 
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