Having just moved into a property with no insulation, I am going to get the loft insulated. I was looking at using environmentally friendly materials... then I saw the cost of sheeps wool! Also I don't really want warmcel as its loose fill. This pretty much leaves glass wool (which I'm told uses recycled glass, so better than rockwool I believe).

I have two questions:
1) how much insulation? Part L says a u-value of 0.16, the Approved Document L1B says 250mm of mineral wool will do that. I want to exceed this though, but my research and calcs seem to show that it'll take 15-20 years to payback an extra 100mm of insulation at the minimum... is this everyone else's findings? Will extra insulation make that much of a difference?

2) Secondly, the roof sits very low on the house, so internally at first floor round the perimeter is a sloping plasterboard piece (see very bad photo attached!). The gap in the roof space from the top side of this plasterboard to the roofing felt is 4inches. I'll need to keep 25mm for ventilation to the roof - so I only have 75mm for insulation around this part. What do you suggest I use? I was thinking some insulating board with better R-value than mineral wool. Any ideas?

Ta!

PS I think I saw a document on the Sheffield Insulation web site that compared U-Values for the different boards they sell.

Thanks for all the help... skeiling - so thats what its called! Insulating plasterboard internally looks a good option as we're redecorating anyway. Calculating it all out roughly (http://www.vesma.com/tutorial/uvalue01/uvalue01.htm) leaves me to think any less than 100mm insulating plasterboard will need some insulation behind the skeiling also which (from the links - useful thanks!) looks hard to put in.

Generally If fitting insulation internally don't forget this moves the cold point. Might need to move the vapour barrier to stop vapour reaching said cold point. Although I think the foil on the insulation can act as the vapour barrier.

How about that Ecowool as a alternative to glass/rock wool ?

BQ are doing a 2 for one, I think

350mm (150mm+200mm laid the otherway) of glasswool should be about £6-7 m2 ( £300 for a average house )

I believe Phenolic board (kingspan) is one of the best insulator for thickness

If it helps Kingspan is approximately twice the thermal performance for the same thickness as mineral wool or glass quilt.
I insulated my roof with kingspan but I could only fit 100mm between the rafters, still allowing an airgap under the felt for ventilation.
This is where my room is open to the pitch. Where we have a conventional loft we boarded over the rafters with kingspan so there is a large ventilated space behind. This keeps the loft warm as we use it for storage.