rigid insulation installation in vaulted ceiling

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Hi everyone! I am installing some rigid insulation in my vaulted ceiling at the weekend and wondered if anyone could give me some pointers..

the rafters are 8 x 2 so I was going to use 150 or 125mm boards and 30mm insulated plaster board.

foil side to the room or the roof?
I know I need to leave a 50mm (500mm might be a bit big) gap between the membrane and the insulation board, but do I need to fill every tiny gap in the rafter with insulation - and seal it nice and tight with foil tape?

any other little tips to save me time?

thanks for nay time you take to reply, these forums have been invaluable to me in the last few weeks and I really do appreciate everybody's help!
 
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but do I need to fill every tiny gap in the rafter with insulation


Tiny gap = tiny heat-loss, up to you. Not worth it in my opinion.
.
True if there's no air movement, but it's easy to get a situation where the air is going round the insulation and a couple of gaps negate the purpose of the insulation completely. Like the wind blowing through a loose jumper. So either fill all the gaps or make sure your air barrier stops air moving through the insulation.
 
Ah, in which case I withdraw that advice as I have never really been sure about breathable roof-membrane - partly because I'm not at all sure I believe the breathable bit, but that's by the way.
 
True if there's no air movement, but it's easy to get a situation where the air is going round the insulation and a couple of gaps negate the purpose of the insulation completely. Like the wind blowing through a loose jumper. So either fill all the gaps or make sure your air barrier stops air moving through the insulation.
So I should fill every tiny gap and use some foil tape to make the whole underside of the insulation air tight
 
Yes ideally if there are gaps on both sides. That's why cavity insulation has to be hard against the inner leaf, and for board insulation has to be hard against the bottom of the floorboards, but sealing all the joints would also work.
 
ok i'll seal it tight... one question. where the last rafter is close to the wall there is a 50mm gap - what should i do here? try squeeze a sliver in? or use some expanding gap filler.. or something else?
 

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I'd use expanding foam, but the trick will be to keep it in there while it goes off - try to make a former that you can seal up as you go along - out of something like gaffer tape or thin cardboard.
 
I'd use expanding foam, but the trick will be to keep it in there while it goes off - try to make a former that you can seal up as you go along - out of something like gaffer tape or thin cardboard.
would it matter if the gap was completely filled? or does this section also need the same 50mm air gap above it.
 
I wouldn't lose sleep over filling it up, and if it's that well sealed with expanding foam then it shouldn't be possible for too much vapour to get in there to condense anyway. Are you foil taping over your PIR panels? I'm a bit OCD in that respect and would pay particular attention to the detail around there but that's just me.
 
I wouldn't lose sleep over filling it up, and if it's that well sealed with expanding foam then it shouldn't be possible for too much vapour to get in there to condense anyway. Are you foil taping over your PIR panels? I'm a bit OCD in that respect and would pay particular attention to the detail around there but that's just me.

yes I am going to foil tape over the panels, I will cut back the expanding foam flush and tape it to death!
 

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