Upgrading roof thermal-element,attic room,already limited headroom?

[Neverfinished]

I plan re-slating the roof (currently large cement tile) on my mid-terrace victorian house & Building Control expect me at the same time to upgrade its thermal-element (insulation!) to meet their latest U-value requirements. I have only 75mm roof rafters; also, my (original,existing) attic room has an already limited height (pitched) ceiling so I'm already close to hitting my head as I enter the room from its stairs. A U-value calculation provided by Proctor Insulation suggests 60mm PIR between rafters then a further 65mm PIR + a VCL in 20mm cavity, ie totalling a basicallyunacceptable  additional 85mm on underside of the room's ceiling.

Can anyone suggest a way I could avoid my needing so much insulation on the internal side . . preferably without resorting to my having to add insulation above rafter, ie 'raising the roof' ? . . as this would be relatively impractical - since an adjoining neighbour's roof, 1 side, is 'integrated' with mine, our 2 roofs being as one . . . whilst a 'substantial' bonding gutter could be created at the perimeter with my neighbour to accommodate a higher roof/additional insulation, its likely expensive / might incur resistance from neighbour. Continues below:

 

              Existing attic room quite like this:                                                      Boundry of my roof runs down centre line of chimneys

 

 

This is the provided U-value calc. :-

Name	Depth (mm)	Layer Thermal Conductivity (W/mK)	Layer Thermal Resistance (m2K/W)	Bridge Material	Bridge Percentage (%)	Bridge Thermal Resistance (m2K/W)	Layer Vapour Resistivity (MNs/gm)	Layer Vapour Resistance (MNs/g)
Slates	9	2.198	0.0041				2.5
Unventilated Air Layer - Battens	25	0.1623		Softwood Timber	8.3	0.19		0.05
Proctor Air - draped over rafters. sarking/membrane	0.08
Unventilated Air Layer - Proctor Air draped into rafters	15	0.1623		Softwood Timber	8.3	0.12		0.05
Polyisocyanurate - Between rafters	60	0.022	2.7273	Softwood Timber	8.3	0.46		100
Polyisocyanurate	65	0.022	2.9545					100
Reflectatherm Plus	750
Reflectatherm Cavity (Floors, Roofs)	25	0.4533		Softwood Timber	8.3	0.19		0.05
Plasterboard	12.5	0.21	0.0595					60

Parameter	Value
Total Thermal Resistance	6.0951
Uncorrected U-value	0.1640
Fixings	0.0016
Air Gaps	0.0017
Total Corrections	0.0033
Corrections Not Applied as ΔU is less than 3% of total U
U-value (corrected & rounded)	0.16

 

Any suggestions & thoughts would be welcome, thanks!

 

I'll further refer to the 2 membranes in the roof build-up, as in the U-value calc. They're Proctor's top range stuff & I plan using them :-

The Proctor Air membrane is described as air & vapour pearmeable; their Reflectatherm as vapour & air resistant VCL, no specific ventilation required.

The permeability of the Proctor Air membrane perhaps raises the question as to whether the use of PIR, especially as being foil faced, is far less suitable here than say mineral wools, in regard to vapour permeability, since the latter would work well with the permeability of my chosen roof membrane. Any comments regards this welcome too. It would certainly require raising the roof quite an amount, perhaps undesirably so.

[Temp]

I believe there is an exemption if its not practical to insulate to current standards. If really stuck I would try proposing something you find acceptable over the stairs an possibly thicker elsewhere?

 

23 minutes ago, Neverfinished said:

A U-value calculation provided by Proctor Insulation suggests 60mm PIR between rafters then a further 65mm PIR + a VCL in 20mm cavity

 

Do you need the 20mm cavity for services? Perhaps look at insulated plasterboard for the inner layer saving 20mm? But check it includes a vapour barrier.

 

Various thicknesses...

 

https://www.insulationuk.co.uk/products/tekwarm-thermal-laminate-pir-plasterboard-thermal-laminate-pir-plasterboard?variant=43041559249077&currency=GBP&utm_medium=cpc&utm_source=google&utm_campaign=&gad_source=1&gad_campaignid=17325772288&gbraid=0AAAAAoNNCLSLOHgQm4MFd1KJft-3MU8Hp&gclid=Cj0KCQjw_dbABhC5ARIsAAh2Z-QakNYw1edDI7RmZF3flqz89MIRtUIzqLXOB7coP8qIIgOsnww3PqQaApJcEALw_wcB

Edited May 3 by Temp