Roof line insulation/ventilation query

kevcos · 20-07-2020 11:29AM #1

I am renovating/extending a house and querying some of the Architects insulation details. Research online gives conflicting options so I'd like to hear opinion on this forum.

Query No1:
Upstairs the ceiling around the perimeter of the house is formed against the back of the rafters (horizontal ceiling joist level is ~400mm above wallplate level).
Here the architect has specified the between rafters to be fully filled with Rockwool insulation. The roof covering is a traditional cold roof build-up (breather membrane, batten & tile).
I know if PIR insulation was used a 50mm air gap between the insulation and breather membrane, but is this not similarly required is using mineral wool?

Query No2:
A new build extension has a typical warm roof build-up (OSB, VCL, 120mm PIR, liquid roof membrane).
WRT U values, here the architect is specifying an additional 50mm Rockwool between the flat roof joists installed tight against the OSB (i.e room side of the VCL). ~400mm wide rips of brich ply will be installed tight underneath that Rockwool with the majority of the joist been exposed as a ceiling.
The space between those joist where the Rockwool is to be installed will be unventilated and I'm concerned about introducing insulation within this space. The architect is terming this a Hybrid roof.

Appreciate any opinions on the suitability of these build-ups.

BryanF · 20-07-2020 11:57AM

1. Yes, unless double battens above felt
2. Seems ok. vcl will be the Important layer.

kevcos · 20-07-2020 12:20PM

BryanF wrote: »

1. Yes, unless double battens above felt
2. Seems ok. vcl will be the Important layer.

Thanks for the response.

Interesting that you say fully filled rafter void works unless double battens above the felt.
The architect has actually drawn double battens (batten on the rafter and then cross battens). To me that seemed sensible as the double batten provides a ventilation gap albeit external side of the breather membrane. So interesting to hear why you say is doesn't work?

In any event the roofer has only install a single batten above the felt,

Calahonda52 · 20-07-2020 12:35PM

1: The 50 mm gap is required if the felt is non breathable.
2: The 50 mm gap is required if the felt is breathable and there is no counter battening

The key component here is making sure there is no air passing through the Rockwell so the eaves detail is crucial.

See Pages 70 or so in SR 45: 2014

Nothing on top of the PIR before the liquid membrane?

Why not add more PIR and lose the 50 mm internal rockwool

kevcos · 20-07-2020 01:48PM

Calahonda52 wrote: »

1: The 50 mm gap is required if the felt is non breathable.
2: The 50 mm gap is required if the felt is breathable and there is no counter battening

The key component here is making sure there is no air passing through the Rockwell so the eaves detail is crucial.

See Pages 70 or so in SR 45: 2014

Nothing on top of the PIR before the liquid membrane?

Why not add more PIR and lose the 50 mm internal rockwool

I think I read ByranF's comment about the air gap incorrect.
Counter battening to provide the air gap makes sense.

For context I'm Irish but this project is in the UK and in my experience architects (and the building industry in general) over here have little interest in air tightness of buildings.

FWIW the flat roof build-up is:

OSB3 T&G VCL primer VCL Fabric backed PIR Carrier membrane (exact same membrane as the VCL) Liquid membrane system
Increasing external PIR thickness is not an option as parapet level cant be increased.

Calahonda52 · 20-07-2020 03:16PM

okay, so why not more PIR underneath, easier to deal with

kevcos · 20-07-2020 04:07PM

Calahonda52 wrote: »

okay, so why not more PIR underneath, easier to deal with

Easy to deal with from an installation POV?

I need to study up on hybrid roof design but I'm guessing that perhaps being more breathable than PIR, mineral wool is the preferred insulation type to use warm side of a VCL?

BryanF · 20-07-2020 05:47PM

Sorry to be clear double battens required above breathable felt of cavity. Below felt is fully filled.

Calahonda52 · 20-07-2020 07:56PM

kevcos wrote: »

Easy to deal with from an installation POV?

I need to study up on hybrid roof design but I'm guessing that perhaps being more breathable than PIR, mineral wool is the preferred insulation type to use warm side of a VCL?

I suppose from a solar drive perspective yes.
From an internal perspective, if the alu faced PIR is well fitted, joints filled with foam and the all taped up with alu tape, the vcl is redundant, which it is as well from the outside by the same logic, as I assume the liquid membrane finish is as porous as glass:D

Roof line insulation/ventilation query

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