Insulate sloping ceiling from Loft

[aveatry]

Hope this is the best place to post this rather than in my projects. But I had a problem and here is what I did.
I have a sloping ceiling in the bedrooms outer walls because the room sort of gos into the loft space. So when I am laying insulation I am careful not to push it too close to the edge so as to not block the air from the soffett bellow but that leaves an uninsulated section of ceiling. see my diagram here - its not quite right but give an idea

I got some 25mm jablite high performance and with some beefy 5x60mm single thread screws I found that I only needed them to hold the parts together while handling as they are tight when in place I attached 50mm polystyrene and kningspan battens as stand offs that would fit tight against the underfelt and keep the 25mm jablite pushed against the sloping plasterboard picture here
The uninsulated slope section is 13 inch deep and marked with the squiggly marker pen so when I am pushing it down I know when to stop.

Here is one pushed down but the air vents in the white soffett seen at the bottom are out of view due to the angle as its very tight up there to get a picture


Here is the diagram of how it is now - the pink part is the jablite

So now I can add extra loft insulation and push it right up to the jablite knowing that I am also maintaining the air flow
Here is a section in progress

And here it is with the extra insulation

I got the idea from American insulation techniques and they can buy a product called rafter baffles that they slide down so they can add extra loose lay. So for me it does 2 jobs - adds insulation in a hard to reach area and allows extra insulation elsewhere

I hope it helps others or gives ideas

I went for 25mm jablite as there is a shortage of kingspan and celotex. I used 50 mm polystyrene because its what I had been saving because i knew it would come in handy - and it did and same for the 50mm spare off cut of kingspan

 

[jamiegw]

Thanks for writing this up, it is exactly what I need in my loft. Did you make any changes or is everything working nicely?

 

[cdbe]

Whatever you do, don't put the standoffs in the centre, they will comprise the "sag" of the roof membrane.

 

[Mr Chibs]

Could put screws into the sides of the rafters, to act as stops for your insulation, if you are using 50mm insulation, set the screws 50mm from the face of the rafter.

Looking at your pictures, the insulation needs to be tight to the rafters... otherwise there's no point... where the ceiling joists are, just chop a notch out the insulation or add a second piece to fill the gap.
Aluminium duct tape is cheap and good to cover joints etc.