Roof Insulation Help

[CSP2025]

HI all,

We live in an old cottage (in a conservation area) which has little insulation anywhere and are looking to improve the property in staged approach starting with the roof (85m sq) and MVHR. We have met with the conservation office and he has agreed that a 150mm raise of the roof height would be permitted.

 

Our bedrooms are partly in the roof with the slope of the ceiling stopping 30mm above the windows in some rooms. As such, our architect has proposed the following:

 

- 12.5mm plasterboard with 3mm skim coat finish and emulsion paint decoration

- 100mm timber roof joists.

Frametherm roll 32 mineral wool

- Vapour control layer, SIGA ‘Majrex’, All joints lapped and taped using SIGA

‘Wigluv’ tape

- 120m of Phenolic insulation over rafters fixed using helifix

InSkew

- Siga Majcoat roof membrane SOB, with edges taped.

- 38x25mm SW treated timber counter battens and roofing battens.

- Existing clay plain

 

We are trying to keep a lid on costs and have been quoted around 30k for the above This however, does include replacing the rafters as we currently have 75mm deep rafters.

First of all, does this proposal look sufficient? I was under the impression the VCL should go below the insulation?

 

If we kept our current 75mm rafters and filled with Knauf omnislab (as it comes in 75mm thickness) and replaced 120mm Phenolic with 120mm (or 150?) PIR, would we still achieve a good level of insulation? Are there any alternatives to Siga membranes in this setup as these are very expensive!

 

We had one roofer visit and he was suggesting we use 55mm of PIR and lay Photonair multifoil over the rafters. After reading several posts I’m skeptical with this approach.

 

Any advice would be greatly appreciated.

 

Thanks,

Carl.

[Conor]

Ok, first things first. Do you need to replace the entire roof and raise it?! Or do you mean ceiling? Why can't you just fire in 400mm of loft insulation? Some photos will help.

[CSP2025]

 

No real need to replace the roof. The reason for raising the roof was down to the sloping sections in the bedrooms. Also, if we insulated in the roof we then wouldn't have to insulate the MVHR ducts going through the attic.

 

Currently, we have no insulation on the red lines and minimal on the yellow. We are also limited internally with space due to the windows: