Is extra insulation worth it?

[Jaygti]

With the new warm roof on our 1 1/2 story house we need to fully fill between the rafters and add 60mm of wood fibre board on top. 
this will give us a u-value of 0.17, sound insulation of 49Db and 9.3 hrs of decrement delay. 
 

80mm above  gives us

u-value 0.16

49Db sound insulation 

10.7hrs decrement delay

 

100mm above gives us

u-value 0.15

50Db sound insulation 

12.1hrs decrement delay

Our heating Is by a log burner, and we have an unlimited supply of free wood, so we won’t save much money on heating with a thicker roof. 
 

my question is, is it still worth going for a thicker roof, for reasons of comfort etc especially in the summer, or am I unlikely to notice much difference ?

The downsides are the thicker boards are more expensive, and it will eat into a bit of headroom upstairs ,although not too much. 
 

thanks in advance

Edited February 13, 2020 by Jaygti

[Dreadnaught]

Fabric-first approach: I would go for 100mm of insulation.

 

In my opinion, it is as much about internal comfort as anything to do with energy loss. The higher level of insulation will provide a marginal improvement in the uniformity and stability of the temperature within your home. And the marginally warmer walls will marginally improve the infra-red emittance, so giving a feeling of warmth. As @Ed Davies mentioned recently in another post, your feeling of warmth comes approximately half from the air temperature but half from the invisible heat-glow from the surfaces around you (I have simplified quite a lot but you get the point). That is why I would go for 100mm insulation.

 

If you have ever been inside a full passive house, that feeling of comfort comes from a combination of: (i) the stability and uniformity of the internal climate (which for example extinguishes drafts); and (ii) the invisible heat-glow from the walls.

 

Just my opinions.

[A_L]

5 hours ago, Jaygti said:

my question is, is it still worth going for a thicker roof, for reasons of comfort etc especially in the summer, or am I unlikely to notice much difference ?

 

In Central England the 100mm option will save around 1kWh/m²/yr of heat loss. You already have some decrement delay so I doubt it would be significant.

Edited February 13, 2020 by A_L
correct two typo's