Insulation Air Gap for Pitched Roof

[John86]

Hi, about to have a DIY meltdown with my 1890 terrace house. New slate roof with vented ridge and breathable membrane installed. 50mm PIR between rafters on the pitched roof that enters the bedrooms, and then 50mm insulation plasterboard installed. Has all been plastered and decorated. However, I've realised I've only left a 10mm air gap between PIR and membrane.

I would be most grateful for any advice please on what my options are to resolve? I was meant to be moving in next week...

 

[blup]

50mm minimum IIRC, but you could cut bottom ends off to maintain air flow, albeit at the expense of insulation

 

[freddiemercurystwin]

Was the roof membrane draped between the rafters? Assuming the membrane is draped and provided there remains a gap between the membrane and the insulation it will be OK. Or has the membrane been pulled taught with no drape?

 

[John86]

The membrane has been pulled taught with no drape, but you can fit your fingers in-between where the membrane overlaps, so there's some airflow.
Thanks,
John

 

[freddiemercurystwin]

Point is there should be a drape, if there isn't then any rain that might get through the tiles may not be able to trickle down the membrane as it can collect where the battens touch the (undraped) membrane, pool and seep through the membrane.

 

[Mr Chibs]

Looks like my raised collar roof, with insulation applied years ago, minimal air gap, all it still good and dry.
If worried, see if there is room to install felt vents, failing that, perhaps you could install eaves vents.

 

[blup]

The membrane has been pulled taught with no drape, but you can fit your fingers in-between where the membrane overlaps, so there's some airflow.
Thanks,
John

Maybe a case of wait and see over the winter

 

[John86]

Yes, I believe it is a raised collar roof. There's minimal gap by the eaves, and the rafters almost sit flush to the top plate, so minimal airflow. Therefore, that's why I'm thinking I need maximum gap within the rafters, between roof and PIR, to prevent condensation buildup and any moisture if the new membrane fails.

This would mean taking down the new ceiling and PIR, replace with a 15mm PIR and tape seals, then replace with insulated plasterboard and replaster. Nuclear option, but the house is still unoccupied.

Thanks to all your comments so far.