Insulating arched window header.

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In the process of renovating a "renovation" in a old church.

Situation: arch has been "built" around window header using pieces of celotex 50mm dot/dabbed onto existing stone work, with no mechanical fixings, gaps sealed with foam, and some fibre glass wooll shoved into gaps. Covered in hardboard to form the complex arch (walls are around 2 feet thick and the internal arch is of larger radius to the arch around the stained glass window. Tons of filler, mesh tape, and the longevity of a stage set in the theatre.

Everything is glued with dot dab adhesive, gaps filled with vast quantities of what I assume is some kind of undercoat plaster, and it fits basically where it touches.

I've a couple of options to sort it out, having ripped it out ready to start again.

1. Use battens to form the arch, screwed to the brickwork, with celotex pieces between the battens, taped to complete the vapour barrier. Cover with plasterboard pieces to reform the arch, bonding and skim coat. Disadvantage - cold bridging.

2. Similar to the above, but fix expamet mesh to the battens and use bonding and skim to form the arched reveal. Same disadvantage.

3.. Use pieces of celotex backed plasterboard, dot/dabbed to the stone work, avoiding battens completely and fix to the original stonework underneath. This is probably the least complex way of rough forming these arches, and I can bond/skim to finish. However, I'm conscious that because of the way ill have to construct the arch in pieces 'Ill have lots of joints that aren't sealed behind the plasterboard surface, so worried about vapour control with this approach. But no cold bridging.

I've got 10 of these arches in total to sort, so ease of building is important to me as although I'm a competent diyer, with only weekends available to me, less complex is generally better!

Thoughts and suggestions welcome. Thanks, Simon
 
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Joined
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Coventry
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United Kingdom
In the process of renovating a "renovation" in a old church.

Situation: arch has been "built" around window header using pieces of celotex 50mm dot/dabbed onto existing stone work, with no mechanical fixings, gaps sealed with foam, and some fibre glass wooll shoved into gaps. Covered in hardboard to form the complex arch (walls are around 2 feet thick and the internal arch is of larger radius to the arch around the stained glass window. Tons of filler, mesh tape, and the longevity of a stage set in the theatre.

Everything is glued with dot dab adhesive, gaps filled with vast quantities of what I assume is some kind of undercoat plaster, and it fits basically where it touches.

I've a couple of options to sort it out, having ripped it out ready to start again.

1. Use battens to form the arch, screwed to the brickwork, with celotex pieces between the battens, taped to complete the vapour barrier. Cover with plasterboard pieces to reform the arch, bonding and skim coat. Disadvantage - cold bridging.

2. Similar to the above, but fix expamet mesh to the battens and use bonding and skim to form the arched reveal. Same disadvantage.

3.. Use pieces of celotex backed plasterboard, dot/dabbed to the stone work, avoiding battens completely and fix to the original stonework underneath. This is probably the least complex way of rough forming these arches, and I can bond/skim to finish. However, I'm conscious that because of the way ill have to construct the arch in pieces 'Ill have lots of joints that aren't sealed behind the plasterboard surface, so worried about vapour control with this approach. But no cold bridging.

I've got 10 of these arches in total to sort, so ease of building is important to me as although I'm a competent diyer, with only weekends available to me, less complex is generally better!

Thoughts and suggestions welcome. Thanks, Simon

You can get 12mm celotex, which could be persuaded to bend around the arch. Use a few layers stuck to together with low expansion foam. Then get some of the flexible plasterboard and do similar, then skim.
If flexible PB too expensive, you can "bend" normal plasterboard by making cuts in the back.
The whole thing will be held together with expanding foam, which hopefully you are OK about !
 
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