Roof insulation: How thk K18 do I need?

[nanuuk]

I have 100mm of insulation between rafters as per drawing. Now I need Kooltherm K18 to the underside of the rafters but the drawing doesn't say what thickness to use?

It says next I will need 12mm thk foil backed plasterboard, skimmed. I don't understand. Why do I need that as well? As far as I know the K18 = an insulation and 12.5mm plasterboard itself. How thick should be the insulation layer then in my case if I already have 100mm?

 

[freddiemercurystwin]

Beacaus to meet Building Regs you need about 120mm Kingspan in total so about a 20mm continuous layer between the bottom of the rafters and the plasterboard. Perfectly normal construction for a lofty. Ask whoever did the drawings exactly how thick it needs to be as rafter spacings can alter the overall thickness required.
eg

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• freddiemercurystwin
• 8 Jun 2010
• 20

 

[nanuuk]

Thanks FMT. The rafters are approximately at 400 centres. The area of sloping roof is 13.7m2 for front and the same for the back. Loft size is 3.7m x 5.2m x 2.5m(height in the middle). Do you think 32.5mm K18 is still enough for that?

 

[freddiemercurystwin]

Sounds about right. May be worth a quick ring to your designer or your BCO beforehand as they'll be approving it.