Insulating rafters

[Brickie]

Hi all,

We relocates just before Xmas to a 1930’s built semi with a generous loft. It has the standard fibre wool insulation between the joists but nothing in the rafters. 
Now,the job I’m on for the next month or two is using the Kingspan 100mm PIR boards,with a LOT of offcuts due to the design. (I kept mine for reuse but no one is bothering so they end up skipped.)

Im wondering how best to make use of these to insulate the rafters,given that their depth is 100mm,as are the boards & (I believe) a 50mm ventilation void is desirable. Is there any risk or signicant loss of performance in simply cutting them in half for two lots of 50mm depth pieces? 
TIA

[PeterW]

Screw 25mm battens to the rafters, then use offcuts to space the insulation off the tiles. Gives you a 25mm air space. 

[Oz07]

you'll struggle to slice offcuts down the middle to split the thickness if I understand you. what size offcuts are you getting compared to rafter spacings?

[nod]

26 minutes ago, PeterW said:

Screw 25mm battens to the rafters, then use offcuts to space the insulation off the tiles. Gives you a 25mm air space. 

Beat me to the punch 

Good advise 

[Brickie]

1 hour ago, PeterW said:

Screw 25mm battens to the rafters, then use offcuts to space the insulation off the tiles. Gives you a 25mm air space. 

Screw them uptight under the roof? Not sure I’m quite with you. 
Off cuts are 400mm plus. 

Edited January 21, 2020 by Brickie

[PeterW]

Nope - screw the battens along the rafters making them 125mm deep. Now use offcuts of batten against the tile battens to hold the insulation off the tiles. Make more sense ..?

[Brickie]

Gotcha!