Insulation Advice/Knowledge ??

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RobH71

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I am currently building an extension and according to my drawings I need 100mm Kingspan between rafters and then 62.5 mm insulated plaster board. It’s not really clear but I guess I fill between the rafters up to the ceiling joists and then fill between joists then insulated plasterboard all over.

my question is could I just place 150 mm PIR between the joists and then stick to the original plan of 100 mm and 62.5 mm over the rafters that form the ceiling ?


upload_2021-2-26_12-5-17.jpeg
 
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Wouldn't you just do 270mm mineral wool between and over the joists and normal plasterboard under?
 
Opinions differ, but since Grenfell I would not want PIR board on or in my house.

Mineral wool needs to be thicker, but does not burn.
 
JohnD said:
Opinions differ, but since Grenfell I would not want PIR board on or in my house.

Mineral wool needs to be thicker, but does not burn.
Click to expand...
Nor kitchen units, furniture, carpets, doors, wallpaper, beds, clothes. Best be safe not sorry.
 
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if that's how you feel, I will not stand in your way.
 
Actually my next question was going to be could I use mineral wool between the joists. I would have to use PIR board for the rafters wall plate to ceiling height but I guess I could do wool above joists.
 
The photo may be deceiving but there is about a meter ish of rafter that leaves the wall plate before meeting the joists and there is only the 150 mm thickness of the joists before the roofing membrane.
 
The flat ceiling needs insulation between and over the joists. The easiest way would be to use quilt insulation to give 300mm. PIR between and under to the rafters as per your plan.
 
Cheers woody do you mean superfoil type stuff or loft rolls
 
Quilt is fibreglass or Rockwool.

The principle is that insulation needs to go between and over joists to prevent cold bridging of the joists.
 
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