Hybrid roof build up - VCL position

[Coyne]

I am building a bungalow with a skillion, low-pitched roof (just 11.5 degrees, so borderline a flat roof by definition).  It will be a hybrid roof with insulation between and above the rafters (more above than between), with decking between the 2 layers of insulation. 

 

When looking at hybrid roof advice online, it seems the recommendation is for the VCL to go above the decking, as it would in a warm roof. So the VCL is essentially sandwiched between the 2 layers of insulation.

 

Is that correct? I can't understand why with a hybrid roof, the VCL would not be below the rafters to prevent vapour entering all of the roof build-up? Does anyone see any problems with the following build-up: 

 

Vapour-control backed plasterboard directly below rafters > rockwool insulation between rafters > structural ply decking above rafters > then 70mm PIR boards above decking > then OSB sheathing on top of insulation boards > and finally EPDM roof covering

 

I am looking at using rockwool insulation between the rafters, and PIR above the decking, to help with interstitial condensation risk where the VCL fails. 

 

[Thorfun]

Get a condensation analysis done by the manufacturer of the insulation. They can also advise on the location of the VCL. 

[Dave Jones]

6 hours ago, Coyne said:

I am building a bungalow with a skillion, low-pitched roof (just 11.5 degrees, so borderline a flat roof by definition).  It will be a hybrid roof with insulation between and above the rafters (more above than between), with decking between the 2 layers of insulation. 

 

When looking at hybrid roof advice online, it seems the recommendation is for the VCL to go above the decking, as it would in a warm roof. So the VCL is essentially sandwiched between the 2 layers of insulation.

 

Is that correct? I can't understand why with a hybrid roof, the VCL would not be below the rafters to prevent vapour entering all of the roof build-up? Does anyone see any problems with the following build-up: 

 

Vapour-control backed plasterboard directly below rafters > rockwool insulation between rafters > structural ply decking above rafters > then 70mm PIR boards above decking > then OSB sheathing on top of insulation boards > and finally EPDM roof covering

 

I am looking at using rockwool insulation between the rafters, and PIR above the decking, to help with interstitial condensation risk where the VCL fails. 

 

 

no not correct.

 

The VCL is there to prevent any moisture in the house migrating into a sealed warm roof where it cannot then get out. VCL backed plasterboard isnt a real VCL either. Assume you are going to making lots of holes it in anyway for lights and cables and its next to useless.

 

As you do not have a warm woof you need ventilation under the roof covering, you can use abutment vents, eaves  vent and mushrooms.

 

Your cold roof (you do not have a  warm roof)can become like this very quickly without ventilation.

 

 

 

 

 

 

[Coyne]

Thank you for this link - I spoke to the specialist who made the video and he's helped with my hybrid roof build up. 

 

 

[Iceverge]

 

That's fine where it is. 

 

 

Bit slack on the U Value. Wouldn't cost much more to go for 100mm Rockwool plus 100mm PIR for a U value of 0.14