flexi woodfibre in a pitched roof, insulating from below

[6boys]

Dear all, I've just joined because my son and I are tearing our hair out in frustration trying to do this! My specification is slates, breathable membrane, scottish sarking boards, 200 mm joists fully filled with steico flex batts 60mm balterno tongue and groove woodfibre sarking, battons and variable vapour barrier, plasterboard. The roof is on and being slated (nearly done) and we have started insulating. It is so difficult!

As you may know steico flex is almost impossible to get at the moment but I ordered a lot last July from a well known online retailer which was the cheapest it came in November last year and we have stored it covered inside for months. We managed to insulate the walls of the new dormer (140 steico flex between frame with 80 mm Balterno outside, the flex was nice and springy and held up well with no sagging though quite a lot of "encouragement" was needed to force it into the frame. For the roof I was expecting the same. We are using two layers of 100 mm BUT they are so different in colour and texture than the 140 thick stuff (bought at the same time, same place, stored the same) it is paler and a whole lot softer and just sags and falls out (on top of us!). 

We've tried lots of ways of getting it to stay in but none very successful, we even tried to put the sarking boards up first and pushing the stuff down behind them, it is too flimsy to push down properly. When we do get it up and the sarking boards on the lower layers still seem to sag and I thought the idea was that it fully filled the joist with no air gaps to decrease the chance of mould.

Has anyone any tips, the online videos seem to show something different which like the 140 is nice and springy and firm 

Please help we are hot and frustrated and covered in bits of wood fibre and sneezing! we have a stretch of coombe 15 metres long x 1.8 m and four more rooms still to do!

I don't have money to buy more and it is impossible to get anyway, we would be happy not to see any ever again!

[Russell griffiths]

Buy a roll of packing banding. 
1000m roll is about £30 then a good quality staple gun. 
run a row of banding on face of joists, have others pre cut

put insulation in and staple a band over it. 
I did 355m on my own like this. 

[6boys]

looks impressive, is that fibreglass? the woodfibre is heavier but I may resort to fibreglass in places so the tip is helpful!