Matttttth - roof insulation

[Roz]

Can some smart person answer this? 
 

Plan submitted to building regs for roof make up

60mm celotex ga4060 (lambda 0.022) between rafters plus 72.5mm insulated koolther K18 plasterboard (60mm insulation Lambda 0.018)

 

is it thermally close to

70mm celotex (lambda 0.022) Between rafters plus 72.5mm Celotex insulated plasterboard PL4060 (60mm insulation lambda 0.022) 

 

I don’t know if I have done it right but when I work it out I get a difference of 0.07 in the R value... how relevant is that? 
 

The celotex insulated plasterboard is a lot cheaper than the kooltherm

and yes we have limited head room hence these thickness values, for a barn conversion

[A_L]

2 hours ago, Roz said:

I don’t know if I have done it right but when I work it out I get a difference of 0.07 in the R value... how relevant is that? 

 

The difference is insignificant. However there is  a more serious consideration.

 

Whenever insulation is between rafters allowance must be made for the thermal bridging effect of the timber which is significant. Normally the timber is assumed to be 15% by area and the layers in which the celotex 70/60mm are used between rafters have thermal resistances of 1.832/1.570 respectively.

 

[PeterW]

How deep are the rafters ..?? That seems to indicate you only have 70-100mm Rafter depth for insulation .??

[Roz]

59 minutes ago, A_L said:

 

The difference is insignificant. However there is  a more serious consideration.

 

Whenever insulation is between rafters allowance must be made for the thermal bridging effect of the timber which is significant. Normally the timber is assumed to be 15% by area and the layers in which the celotex 70/60mm are used between rafters have thermal resistances of 1.832/1.570 respectively.

 

 

20 minutes ago, PeterW said:

How deep are the rafters ..?? That seems to indicate you only have 70-100mm Rafter depth for insulation .??

the rafters are 120mm (ish, whatever the closest size is to that) at 400centres. BCO requested 50mm Of ventilation space above the insulation

[PeterW]

So you don’t need 50mm if you’re using a breather membrane - 10mm will do, and you can get 100mm in there which would be better with just 25mm over the top, back filled between battens. Tape the lot and there is your vapour control too.