Garage conversion - roof insulation. Am I about to screw up?

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Hello All,

I am trying to work out the best was to insulate a pitched garage roof for conversion into a general purpose room.

The roof is currently tialed and felted and made of 5" (130mm) beams.

I am looking as insulating with insulation board (celotex etc)

I understand that I have to leave a 50mm gap between the felt and the insulation so this leaves be 80mm to play with.

My current thinking is to put 50mm celotex between the beams and then cover with 20mm insulated plasterboard (GA450 - 12.5mm plaster and 12.5mm insulation).

The room is 10 foot by 20 so the cost inc. VAT is just shy of £400 at insulationgiant.com with 7 boards of each.

So the obvious questions are 1. Is this enough insulation 2. can I get away with less and 3. am I missing another obvious option?

Any thoughts and advice as greatly appreciated.
 
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exactly. The roof is a cut roof with a single sided slope (with one wall higher than the other).

No too fussed about regs but will follow where I can.
 
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Well you're gonna end up with about half the insulation you would need to comply with the Regs. I kid thee not!

Your 50mm air gap is good but you should be fitting continuous ridge vents too.

In a loft conversion roof (same requirements) you would be looking at about 100 between and 30 beneath + plasterboard.
 
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Jesus - thats a lot of insulation.

Sod regs then. Would I be spending massive amounts on heating with my current plan? Would I actually notice the difference anyway?
 
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It will require more heating to keep the temperature the same as if you had the required insulation so will cost more. You would notice the difference. There's no point worrying about doing it to Regs if you're not actually going through Building Control is there.
 
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I agree however I do not want to detract from the value of the house when I could add to it.

Would you recommend going all out with regs insulation wise then (it is in my own house and I have no immediate plans to sell up).

If I need to need a 50mm air gap then I can only go 75mm between the rafters. Does that mean I then need 55mm below as well?

What would be the normal if regs did not exist?
 
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If I do 75mm between the rafters and 50mm below (including plasterboard) will this be okay?

This comes to £512 so £100 extra in total sounds like it will be well worth while!

I could go for 65mm below which will meet your specifications but totle cost then will be £550. It's beginning to hurt!
 
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There is no 'normal' if regs didn't exist. You either do it to regs or you don't. Yes if 75 between then approx 55 bellow, depends on the rafter spacings. Getting Approval is not just about insulation though, its providing adequate ventilation, insulation, waterproofing, drainage electrical safety etc.
 
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I agree however I do not want to detract from the value of the house when I could add to it.
So you're doing controlled work without approval and which wouldn't get a completion certificate anyway because it doesn't comply with the regulations. Strange way to not detract from the value of the house.
 
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Would that detract from the value or just not add to it?

As you might have guessed, I am 26 on my first house, have only ever worked in an office and am pretty much making it up as I go. Thank god for the forums!!!
 
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Would that detract from the value or just not add to it?
It could be easily argued that by not complying with the regs you will sell for less than than if you had complied with the regs, even after the additional costs of complying. So you are reducing the potential value added. There will also be those who may wish to buy your house who will pay extra for a compliant conversion and there may be people put off by a non compliant conversion.
 
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A point well made. Sounds like I will have to reach a little deeper into my pockets then!

Pretty sure the roof is waterproof. I will get the electrics done to spec by a sparky friend of mine. It look like the roof ventilation can be fitted whenever so I will save that job for later.

Are there any further things I need to take into account with this job if I were to build to regs then?

Thanks
 

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