PIR Airflow Gap on New Roof

[John_10]

Hi, about to have a DIY meltdown with my 1890 terrace house. New slate roof with vented ridge and breathable membrane installed (pulled taught). 50mm PIR between rafters on the pitched roof that enters the bedrooms (about 1.4m in length), 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. Minimum airflow already exists in eaves.

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...

[Mr Punter]

I would not worry about it.  If you have a loft inspection hatch, have a look up there every 3 months.  If there is moisture collecting on the timbers and not evaporating you may need to introduce additional ventilation such as vent tiles or soffit vents.

[MikeGrahamT21]

10mm is better than no gap at all, ensure adequate vent openings as above and the air with get through just fine

[Redbeard]

The thickness of PIR does not seem to comply with English or Welsh B Regs ( I am not au fait with Scottish Regs but imagine they are v similar). 150mm would sort it.  I'd be bothered about a 10mm vent gap. 'Conventional wisdom' after 2010 Regs was 25mm (so 50 PIR/50 air-gap in 'typical' 75mm rafter situation) but gov't guidance now says 50mm. I have seen some examples of moisture issues with only 25mm gap but, to be fair, usually whern combined with poor workmanship. Slighly concerned about the taut membrane. If a slate slips does water 'pond' at the first batten?

 

Still a little confused re your 10mm gap. With 50mm PIR that implies rafters are 60mm only...??

[John_10]

Hi, thanks for the replies so far. I've added a few more photos to further illustrate the current situation. As you see, the eaves have also not been left with much air space and size of the rafters.

[John_10]

[Temp]

You said its a breathable membrane (aka vapour permeable) so I don't think its a problem not having a ventilated air gap. Especially if there is a vapour barrier on the inside. Perhaps check what the membrane manufacturer says.

 

Are there counter battens above the membrane? The membrane shouldn't really press up against the underside of the tile/slate battens. Should be allowed to drape.

[Redbeard]

Some manuf'rs allow for full-fill, where others require a minimum 25mm air-gap (and Best Practice suggests 50, as previously mentioned). As @Temp says, check with the manuf'r.

[John_10]

[John_10]

Thanks all. This is how the battens look. Spoke to the building inspector today. He reckons it should be okay, as only a small area of pitch is affected, and it's a breathing membrane with a least some air gap.