Open Eaves ventilation

[SBMS]

Our architect designed our roof with open eaves. We are having attic room

roof trusses and will be having the ends of the rafters exposed and open (no soffits or fascia boards). SAP have specified 25mm air gap then 150mm rigid PIR between rafters and insulated plasterboard below. Breathable membrane above with a slight droop between rafters for airflow under tiles. 
 

We are short on details however on how to ventilate with open eaves; wondered if anyone has done this setup and what the recommended approach is?

Edited November 6, 2022 by SBMS

[Temp]

If the membrane is vapour permeable I don't believe the void below it needs to be ventilated. Ventilated voids are normally 50mm. 25mm voids are just to allow the membrane to drape/sag allowing rain blown under the tiles to run down. So I believe you only need ventilation above the membrane.  

 

Tiles or slates? 

 

Hand made plain tiles tend to be twisted/irregular so there are lots of ventilation gaps, i don't think extra ventilation is required above the membrane.

 

Slates and interlocking tiles tend to have no gaps so I think counter battens are required to lift the tile battens up and improve ventilation under them. Ventilation would be required at the ridge. This shows conventional/closed eaves but the ventilation would be similar I think.

 

 

 

Best get Building Control to approve a sketch.

Edited November 6, 2022 by Temp

[SBMS]

Thanks @Temp  so there need to be a vapour control layer on the underside of the insulation/rafters? Ie under the insulated plasterboard? Or does the insulated plasterboard incorporate a vapour control layer on the underside?

 

We are using slates. 

[Temp]

Yeah we have a plastic vapour barrier between insulation and regular plasterboard.

 

Some insulated plasterboard includes a vapour barrier..

 

https://www.kingspan.com/gb/en/products/insulation-boards/wall-insulation-boards/kooltherm-k118-insulated-plasterboard/

 

It might also be possible to use a plastic sheet vapour barrier between your rafters and the insulated plasterboard but it would need a calculation to check its on the warm side of the dew point.

 

[Temp]

I would do a sketch cross section highlighting:

vapour barrier

vapour permeable membrane

 over facial ventilation and rridge ventilation

Thickness of insulation and insulated plasterboar

 

See what Building Control say.

 

You might want to put something on top of the rafters at the eaves so you don't see the underside of the membrane between the exposed rafter ends. Eg WBP plywood or T&G boards?

[SBMS]

We are going to run it past BC, but have decided to go with 125mm between rafters and then counter insulate with rigid insulation over the rafters.  Does the counter battened insulation (just regular 50mm Ecotherm) act as a vapour barrier?