Insulating Garage Ceiling

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I will be insulating and boarding my garage to make it into a more comfortable space for working at home.

I have a pitched roof garage which I want to insulate and board at ceiling level, but how am I best to go about this?
I am also wanting to install downlights which I will need to take into consideration.

Could I place 100mm insulation board between the ceiling joists with a cut out for each downlight. I'd cut a slither from the pieces I have cut out to use above the downlights to close the gap into the roof space.
To prevent any moisture coming through I would fill any gaps with expandable foam
I'd run cables above the insulation and through conduit to the lights in order to prevent contact between the cable and insulation.

In theory should this give me what I need?

It might be that at times there is condensation above the insulation, however if there is enough ventilation in the roof space (which there is), the boards are fully sealed in with expanding foam and the cables are protected from the insulation, then all should be ok?
 
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Its probably cheaper to insulate it like a conventional roof with loft roll between and over the ceiling beams. You can use downlighter with built in hoods to deal with the fire protection and with a loft roll, there is no reason to worry too much about sealing the holes.
 
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Cheers.

Definitely like the sound of cheaper. Would this be as effective as 100mm board?

Assuming any condensation that did form above the loft roll wouldn't be enough for me to worry about it coming through to the board?
 
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As long as you do it like a loft, with between the joists and over the joist insulation - it will exceed the insulation amount you are proposing. Ideally you need 150+100mm or more. I'm assuming your are doing the walls and floor too? otherwise its a waste of time.

If you aren't too fussed about making it compliant then 200mm will be pretty good.

I would highly recommend these with a compatible dimmer for your downlights
https://www.tlc-direct.co.uk/Manufacturers/Aurora/Aurora_DE8/index.html

Also for the walls, don't batten, use plasterboard foam to attach celotex to wall and again plasterboard to celotex. You can also add a few frame fixers, but I never found this was needed. 75-80mm will be enough to hit .19U on the walls. Assuming the current structure is block
 
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Yeh walls and floor will both be done too. Will be framing out the walls leaving a gap between the wall and insulation. Will then place Kingspan between the timbers, foamed in where needed and vapour sealed with foil tape.
Plan is to use 50mm for the walls which will hopefully be enough.

As those downlights are fire rated, would they still need housing over the top?
 
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Re the walls, if you can spare the space go for 100 or more mm slab insulation (the stuff used in cavity walls)- it'll give you better sound insulation as well as being easier and cheaper to fit.
 
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Yeh walls and floor will both be done too. Will be framing out the walls leaving a gap between the wall and insulation. Will then place Kingspan between the timbers, foamed in where needed and vapour sealed with foil tape.
Plan is to use 50mm for the walls which will hopefully be enough.

As those downlights are fire rated, would they still need housing over the top?

no need for housing the LEDs and that method of fixing insulation is what I said NOT to do. Much more likely to be damp. If you do it direct bond it will save you wood and take no more space as well as having a superior U value.

this stuff. https://www.toolstation.com/soudal-genius-gun-plasterboard-adhesive-foam/p99304?

then standard foam in the gaps.

I’d go at least 80mm insulation 50mm on the floor is ok. The floor may well have to be battens
 
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